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Posted by: dvandorn Apr 3 2007, 05:12 PM

Now that we're finally about to do a detailed inspection of the darkest of the dark streaks emanating from the north-northeast portion of the crater rim, it's time for final speculations before we know the truth of the matter.

I'm in the clean-sweep camp. The large-scale orbital observations make these streaks appear almost definitely of aeolian origin -- the manner in which the streaks feather along the edges, and the way in which they curve off as they extend out from the crater, are all consistent with wind/ground interactions.

Observations of the lighter, western streak seem to show more visible concretions right up on the surface. If this holds true of the darker streak, I think that proves the clean-sweep theory.

Think of it this way -- if you packed pebbles and dry dust as a pavement and then let the wind strip away at this surface, the dust would blow off and the pebbles would remain. What dust remained would sit in the lee of the pebbles.

This seems to be exactly what we're seeing in the first dark streak -- the lighter soil component has been blown away entirely, and the darker component (probably eroded concretion material) has been mostly blown away but its remnants sit in the lee of the concretions.

I would expect that any depositional streak would appear as dust or fine-grained soils which cover over the materials we see on the surface outside of the streaks. That's *not* what we're seeing.

In addition, I'd have to treat any suggestion that the blueberries themselves are being blown out of the crater to form the streaks with an awful lot of skepticism. Martian winds aren't strong enough to move the relatively large-and-heavy concretions along level ground -- it would be absolutely impossible for these thin-air winds to have blown them entirely out of the crater and up to a crater diameter's distance away.

Now, if the MIs in the darker streak show that dark dust is consistently filleted on the upwind side of the concretions, and shadowed with less dust downwind of the concretions, *that* would be an indication that the streaks are depositional. But, so far, that's not what we're seeing.

-the other Doug

Posted by: BrianL Apr 3 2007, 06:20 PM

Ooh, I love adding my completely geologically untrained, marginally informed opinion to any question that divides us into camps. I anxiously await the appearance of the mascots.

Bad news for you, Doug. I'm adding the clean sweep camp to my already dodgy resume that includes being a member of the far side through the keyhole beacon camp, the Oppy should lay down and die at Victoria camp, and the (admittedly unannounced) gosh homeplate is really kind of boring let's get on to the promised land camp.

On the plus side, I've never been in the abyss camp.

Brian

Posted by: Shaka Apr 3 2007, 07:23 PM

I certainly hope the plan codes show an MI sequence of the surface at this location, so that we will be able to make side-by-side comparisons with MIs taken in the streak ahead. Some overhead pans will also be useful for close comparisons of berry abundance and distribution. Codebreakers? What's the plan?

Posted by: imipak Apr 3 2007, 08:02 PM

Me too!

Actually, ("I'm glad you asked me that, Brian"), my view of this controversy is clear and unequivocal. I believe I can say, without fear of contradiction, that I don't know enough to decide yet, and that further research is indicated. "Send pictures!"

Posted by: ElkGroveDan Apr 3 2007, 08:07 PM

This one's a no-brainer. The dark streaks are caused by a steady removal of the finer ambient dust/silt in the region (indeed coating the entire planet).

Posted by: Juramike Apr 3 2007, 08:35 PM

What do you think is particularly special about the dark streak areas?

Is the local windspeed slower (due to turbulence effects?) or do you think the local windspeed is faster in the dark streak area?(again due to turbulence effects?).

Why are the dark streaks on the North side of the crater, but not on the South side? Where would you expect to find other localized dark streaks in Victoria crater?

For that matter, do other craters nearby show a similar dark streak pattern?

-Mike

Posted by: helvick Apr 3 2007, 09:03 PM

They extend from bays that act as turbulence causing funnels given the predominantly ~SSE to ~NNW prevailing wind direction.

Faster and more turbulent. Like wind in urban canyon zones.The reason only some exhibit this is because the effect depends strongly on the shape of the specific bay ramps and their surrounding cape bluffs.
If this was dark material being lofted out of Victoria then my gut feeling is that there would be streaks emanating from many more of the bays and in more directions. Bays like Bahia Blanca and the Bay of Toil in particular.

Prevailing wind direction.

Nowhere.

Yes

Posted by: Juramike Apr 3 2007, 09:18 PM

I'll buy that.

So...the faster windspeeds are picking up lighter colored material and leaving behind the darker material (from the "gray" sands around the apron of Victoria). We would then expect to see deposition of the lighter stuff around the darker streaks - which we do.

Would this explanation also work for the lighter dark streaks seen in the SE corner of Victoria trending towards the SE?
And the even lighter dark streaks seen in the W?

So the streaks around Victoria are telling the preferred wind vectors: from SSE dominant, then from NW, and finally from E as the minor component?

-Mike

Posted by: fredk Apr 3 2007, 09:30 PM

I've been leaning into the dark deposit camp. Another point just occured to me, and I think I've now toppled head first and completely into Deposit Camp. Why are the streaks emerging from bays D2 and VwP much darker than any others?

Look at the orbital view. I hope we can all agree that the winds in the streaks are roughly from the SSE. Why don't the other bays on the north rim, such as Blanca, Toil, and Bottomless, have similarly dark streaks to the two darkest? If anything, I'd expect a darker streak from Bottomless, eg, since the winds should approach and funnel through it more directly than D2 and VwP. See attached image:

Basically, if the streaks were due to clean sweep, I can't see why the other northern bays wouldn't produce dark streaks. If instead the streaks are depositional, it's easy to imagine compositional differences between east and west sides of Victoria that result in dark material eroding out preferentially on the east side. Indeed, looking at the orbital view again, there does appear to be different material inside the NE rim in the form of dunes similarly dark in appearance to the streaks.

The winds are amplified as they funnel through the bays, and then slow down again as they spread across the plains. As they slow down, first the heaviest, then lighter and lighter dark particles settle out.

Edit: I see that while writing this helvick posted making the same point but coming to a different conclusion! Intriguing.

Posted by: fredk Apr 3 2007, 09:47 PM

Could the effect really be so sensitive to the shape of the bays that none of the bays between and including Matias and Duck show streaks nearly as dark as the two darkest? Doesn't it seem more likely, since the few darkest streaks are clumped together, that the clumping is due to compositional differences across the crater?

Posted by: helvick Apr 3 2007, 09:56 PM

I think it can be argued either way. _If_ cleaning by turbulent air is the explanation then the shape of the funnel is absolutely relevant. If it is some darker zonal material only present on the eastern side of Victoria then then that could work too but apart from the Blueberries have we seen any material that is dark enough to produce this? Personally I'm unconvinced by the black dust idea but we'll find out soon enough I hope.

Posted by: fredk Apr 3 2007, 10:05 PM

There was the dark dust piled up beside rocks on the edge of VwP that I mentioned in the VwP thread:
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1131/1P228587931EFF8190P2395L2M1.JPG
(see bottom of frame.) And there's the dark dune material inside the NE rim.

Posted by: MarsIsImportant Apr 3 2007, 10:08 PM

I don't buy it completely, because of the preponderance of dark dust directly below the cliffs of Tierra del Fuego. The prevailing wind direction doesn't explain these clearly depositional dune like features. This dark material is trapped just below the cliffs. A wind direction from the North instead would explain such a trap. But the wind direction is not generally from the North, is it? If it came from the North, then that would destroy the wind channeling idea.

I doubt that the current prevailing wind has remained the same over these hundreds of million of years. There should have been a lot more mixing than appears evident given enough time involved. So these features must be fairly young. Also, the dark nature of the material suggests they are probably related. The dark material below the cliffs and inside the crater is probably debris from the erosion of blueberries (just a guess that would need to be confirmed). That would suggest that the whole area where the streaks are is saturated with blueberries beyond normal concentration for Meridiani. That means there is a much higher concentration both above and below ground at these locations. I wonder if the streaks maybe related to underground fractures of the type SS talked about near Soup Dragon. If water plays a role, it could help explain things nicely because of the higher concentration of the berries. Wind directions change with time. I would expect it to do so on Mars too, especially with the time frames we are talking about. So the wind channeling process might have helped clear the surface with the streaked areas. Yet, with probable change in wind direction over large time frames--why are they situated in one general direction? Fractures might help explain them with subsequent seeping (if they are deep below ground)--just a thought.

Yes, wind must have had some role to play here. But it cannot be as simple some have explained so far. There must be a lot more to the story. It's the 'a lot more to the story' that I'm interested in.

Posted by: Juramike Apr 3 2007, 10:14 PM

Hmmm. Both very good hypotheses. There's no way I'd put hard money down on either.

If you look very closely at the boundary of the dunes vs. apron to the N of Victoria, it "seems" that the dunes/apron boundary is closer to the crater in the dark area and further out in the brighter zones. The dark ray coming off "the bay 4 to the E of Valley without Peril" seems the shortest of all. The dunes are more visible closer to Victoria. The dark stuff appears to obscure less of the dune field. (At least when I stare at it).



This makes me suspect that the dark rays are not deposited or if they are they don't go very far, but that the "regular" whitish stuff goes further. So I go with the dark stuff being darker and heavier component and left behind.

Throw me in the "swept clean" camp, but a good shred of evidence to the contrary would flip me in a heartbeat.


So why no streaks near Bottomless Bay? Great question. Local topography? Could the topography from deposits from a SSE wind cause a new deposition pattern when the wind shifts to a NW wind? Could this erase the effect on the W side first, but set up larger turbulences when it finally reaches the NE part of Victoria? The effect could be real subtle, but just might be enough to make an effect.

(Man, I need a sandbox, dark volcanic sand, light sugar sand, and a fan and a few hours to play).

-Mike

Posted by: Edward Schmitz Apr 3 2007, 10:27 PM

- There IS dark matterial at the base of the cliffs below the streaks. In fact, the entire floor of victoria is much darker than the planes. But the stuff at the base of the streak cliffs are forming dunes and are darker than the rest of VC.
- The streaks are all grouped together. There are two bays that don't have streaks in the middle of the group. But these clearly have very steep walls. Much harder to blow stuff out of.
- The bays on the other side of the crater display the same variety of shape as the streak bays. Don't see why they would be different, wind wise.
- faster, not turbulent, air will clean most effectively. The bays along the line of the wind should have the darkest streaks.

Clearly, I'm in the depositional camp.

Posted by: Shaka Apr 3 2007, 10:36 PM

I've searched in vain for a nice clear discussion (with diagrams) of the effect on prevailing winds by impact craters. No doubt the effects would vary significantly with a wide assortment of variables: The diameter and depth of the crater, the height of the rim, its profile and planview shape, the prevailing wind direction and speed, its seasonal and longer term variations, the sediment substrate, its grain sizes, density etc. etc.
Somebody should write one. It's not what I would term a "no-brainer" (as the beacon was ), but a lot of brain power has already been devoted to the issue (Search at adsabs.harvard.edu for crater wind trails Mars .)
There is a fairly broad consensus that wind streaks are downwind of the crater, that they are light or dark in albedo depending whether they are depositional or erosional in nature, that one crater can produce both types of tail, and that the local wind speed across the surface will govern whether deposition or erosion of dust occurs. We may be confident of the prevailing winds in this area of Meridiani (and there are lots of MSSS images showing dark trails extending NNW from Meridiani craters, but we know much less about what the unique topography of Victoria Crater (no raised rim, cape-and-bay planview) will do to the velocities of those winds. Both the pattern of ripples on the bottom, and the traces of trails around the crater show that the wind patterns are complex over the short to medium term.

What we learn here at VC will comprise an important contribution to the field.

Posted by: atomoid Apr 4 2007, 01:08 AM

Since the dark streaks only seem to be emanating from the steepest cliffs, which also happen to have the most boulders at their base, the dark streaks must be evidence of erosion, and thats why there are no streaks coming out of the other downwind bays (simply because there isnt much erosion there due to either compositional or prevailing wind factors). So we have the dark dust having blown up and out (with some of it getting stuck behind at the base of the cliff) and the dust deposits in the streak pattern as the dust-lofting turbulence subsides.
This can be seen in the attachment by fredk:

(its odd that i can see the image when i 'Preview Post' this attachemnt in the message composer but it doesnt display when i 'Submit Modified Post', so http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87495)

It all seems that simple, but then theres the tentative observation that the dark streaks are actually composed of the blueberries themselves, which is the only thing that is threatening to make me bail out of the dark-dirt camp. So if thats true i'll be camping on clean-streaks, if not im stuck in the dust, even though its where i wanted to camp all along. so i guess i have to commit now or it would be cheating, so im officially camping in the dirt.

Posted by: Shaka Apr 4 2007, 02:21 AM

O.K., Atom, this is possibly a very astute observation, but I would like to see your data on the steepness of all the Victoria bays so I can convince myself that steepness correlates with streak darkness.
If this is true, however, I don't see how it follows that the streaks are therefore dustier. I could as easily conclude that greater erosion and steepness correlate with wind strength through the bays, and thus greater likelihood that light dust will be removed from both the bays and the streaks beyond.
Why the bays to the northeast of the prevailing area wind axis have the greatest wind velocity may result from a complex rotary eddy set up within the crater as a function of its complex topography. (Who is going to be first to set up a model of VC with dark sand and light dust and hit it with a hair dryer? )


Suit yourself, but please wipe your feet before you come in the house .

Posted by: fredk Apr 4 2007, 03:13 AM

This kind of thing occured to me too. But then you still have to explain why the mirror image of the effect doesn't happen with the bays on the opposite side of the wind axis from VwP, such as Duck Bay and it's neighbours. In fact we see no dark streaks at all from Duck bay and neighbours.

Do not resist, Luke. Come over to the Dark Side...

Posted by: MarsIsImportant Apr 4 2007, 03:31 AM

Notice how the whole area north and east of Victoria is darker, beginning with those streaks. It's not just about the streaks when we talk about the dark material! Tierra del Fuego has the dark material accumulating under the cliffs there, but it doesn't appear to accumulate under others. Why? (I don't think it has anything to do with how steep they are).

Although there is a tendancy for us to see what we want in these images, IMO we must be looking at slight compositional differences in the ground itself, not just the surface. So when Tierra Del Fuego erodes, it creates a lot of dark material dust. It then is blown out over the surface from the bays where the streaks start. Yet, the erosion of those particular cliffs are creating the dark dust faster than it can be blown away.

My guess is that the accumulation under Tierra del Fuego could be from erosion of the higher concentration of blueberries from within those cliffs. So that might be a source from which the darkness of the streaks is enhanced because of the winds. But since the whole northeast side is much darker (not just the streaks), then there probably are other processes involved.

Notice also, that the top of St. Vincent is very clean. There may be a combination of clean-sweep and depositional processes at work in this area of the crater. We should not look at this in isolation. If you look closer at other bays, there are fainter streaks in other areas of the crater. These on the northside are merely much darker. I hope I'm not just seeing what I want to see.

Posted by: CosmicRocker Apr 4 2007, 05:27 AM

The fun thing about this discussion is that I think I could make some good arguments in favor of each of the two prevailing theories. But I had previously cast my vote on this phenomenon a year or maybe two ago, and I remain in the clean sweep camp. If for no other reason, Occam's razor is pretty convincing. Light-colored, lee deposits are usually depositional, dark-colored streaks on the lee side are usually due to the removal of the ubiquitous light-colored dust. The wind flow through this crater is surely a complex subject. My intuitive suspicions lead me to expect a more laminar flow through the center, but interactions with the roughness on the east and west sides might create turbulent vortices that would exit the NNE and NNW bays. You could use that to support either argument, though. I see some patchy dark dunes inside the SSW side. The only way we'll likely be able to convincingly conclude the debate is with empirical observations of the surface.

You really should consider a career in politics, if you haven't already.
The problem I see with the dark dune material inside the crater is that it may simply be dark material that has had the coating of light-colored dust removed from it. It might also be a reworked accumulation of dark sediment that fell from a layer above. I've speculated earlier about a pre-impact residual soil that might exist in patches between the bedrock and the ejecta layer, to explain the unexpected dark stuff that was observed in some of the bays.
I don't know where this idea of supersaturated blueberries came from, but I went back and looked at the surface all around Victoria and I pretty much saw the same concentration of concretions everywhere. They looked pretty dusty on the west side, and less so as Opportunity moved to the north side.
There's no doubt in my mind about that.

Posted by: MarsIsImportant Apr 4 2007, 01:06 PM

I see a huge difference in concentration. But perhaps we should replace the word blueberry for dark material. Blueberries may only be one kind of dark material. Here are some images from different time periods representing vastly different locations.

The first one was taken in July while still on the plain approaching Victoria.

There is some dark material, but the distribution is vastly different than this image taken in December.

And yet that distribution is very different from this recent image on the edge of one of the streak features. Look in the foreground at the base of the images, to get an idea of what I'm talking about.

You could easily take a shovel and grab nothing but concretions on that last image. They are piled together as if somebody was storing them. Granted, the appearance of the concretions is different, but so is the concentration of them.

Posted by: antoniseb Apr 4 2007, 02:24 PM

I'm in the deposit camp (as opposed to clean sweep)

I've spent some time wondering about these streaks, and a couple weeks ago someone pointed out the loose dark material just under the NNE cliffs (such as Tierra del Fuego). It occurred to me at it could be that prior to the event that formed Victoria Crater, during the depositing of the salt layer, that this particular area had some feature that lead to an increased density of blueberries (or other hematite concretions) compared to the rest of the area that is now this crater.

While it IS possible that the clean sweep model is part of the process (if the crater formation threw out extra blueberries at these NNE locations), I imagine that the slumping of the material leading to the hematite drapes under the cliff also resulted in abrasions creating enough hematite dust to create the streaks.

Side note: if the wind is really amplified at the bays, perhaps Opportunity should loiter at one for a Spring cleaning of the solar panels.

Posted by: MarsIsImportant Apr 4 2007, 02:46 PM

Impact deposit could very well be part of the explanation. In the last image that I showed in my previous post, there are a lot of sparkle like features--specks everywhere. I first thought that they might be some sort of image artifact; but soon realized that they are pretty evenly distributed with NONE in the sky. A look at the base of the image suggests that some of the concretions are split open. Perhaps the splitting of the blueberries on impact created reflective surfaces, and those are the sparkles we see in that image!

Posted by: CosmicRocker Apr 4 2007, 02:50 PM

MII: You really can't compare the rippled plains to Victoria's annulus. According to the hypothesis SS has described, the annulus is a special case. Downward looking pancams are the best images to use for these kinds of comparisons.

Posted by: Juramike Apr 4 2007, 03:01 PM

I agree with both Antoniseb and MarsIsImportant. The simpler explanation is that there is a slight compositional difference across Victoria crater, with a larger concentration of blueberries (or other dark concretion) in the NE.

But this also fits the "clean sweep" hypothesis. The darker concretion-bearing material was blown out during impact in a smaller tighter annulus. Over time, prevailing winds have blown out lighter colored (and physically lighter also) material from the crater to form the brighter streaks which are covering the darker material.

This should be fairly easy to check. By going slightly to the E of one of the darker rays and doing a few wheel scuffs, we should get down to an area that would have more darker concretions than a corresponding are in the NW.

Has anybody got MI images from set locations around Victoria that they could easily post for comparison?
('Course this would only be the surface pix, we would need to dig to provide more evidence for the "clean sweep" vs. depositional theory.)

-Mike

Posted by: MarsIsImportant Apr 4 2007, 03:07 PM

I'm not comparing the rippled plains to Victoria's annulus. I simply put that there to show there are all kinds of different distributions of the dark material. Maybe I did not make that clear.

The comparison is with the last two images. That is a very fair comparison. And the concentration difference is Huge. I hope that clears up any misunderstanding.

Posted by: fredk Apr 4 2007, 05:37 PM

Antoniseb makes a good point. I've thought that the biggest nail in the coffin of the clean sweep idea was the existence of a clump of a few very dark streaks in the NNE and essentially none in the NW. It would require a very contrived and sensitive dependence of strength of turbulence (or whatever) on local topography to explain the complete lack of dark streak from Duck Bay and neighbours. This is the kind of explanation that could make Occam roll over in his grave, CosmicRocker!

This I see as very strong evidence of compositional differences from Victoria's west to east side. This idea has support in the morphological differences we see across the crater, for example with the "linear fracture" features inside the NE to E rim.

However, as antoniseb pointed out, this could still be consistent with clean sweep. If there are many more blueberries outside the NNE rim than the NW rim, then it's possible that the winds are equally strong out of Duck Bay and VwP, eg, but that outside of VwP there are many more blueberries to be cleaned of light dust, so that area looks darker.

This is possible, but already we have evidence against it. Compare this pancam view from sol 1128, very close to the first dark streak:
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1128/1P228318003EFF8100P2588L7M1.JPG
with this pancam from sol 995, near the Beacon:
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/995/1P216514426ESF76WUP2461L7M1.JPG
The concentration of blueberries is similar.

It's still possible that it's not the blueberries that vary in concentration from east to west and make the dark streaks dark after clean sweep, but another finer dust/sand component.

Regardless, we should know soon. Notice http://qt.exploratorium.edu/mars/opportunity/micro_imager/2007-04-04/1M228942542EFF81D2P2936M2M1.JPG taken from tosol's between-streak location. I suspect with one more drive (anyone know the plans?) we'll be sitting on the darkest streak and then take another MI for comparison with today's image. Hopefully the answer will be clear then!

Posted by: fredk Apr 4 2007, 05:59 PM

MII, those specks are noise in the CCDs. The noise tends to be more noticeable in the L7 images, since they need long exposures with that deep blue filter. You can see the specks in other images, like this one where you can see it's not split berries:
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1124/1P227966074ESF80LOP2587L7M1.JPG

Posted by: MarsIsImportant Apr 4 2007, 06:12 PM

Well then, my first guess was correct. The specks are artifacts. Thanks for pointing that out.

Like I said before, sometimes we see what we want to see. Split berries would explained a lot of things. Now we still have a great deal of mystery here! Yet some of the berries could still be split--we just need a closer look.

Posted by: MarsIsImportant Apr 4 2007, 06:56 PM

Well, the color of the depositional dust would depend upon its source. Perhaps the compositional differences within Victoria create different sources of the dust deposited over the surrounding area. If the dust in the east is the same basic color as the blueberries, then it would be difficult to distinguish the two from a distance. So you have light colored dust deposits in the west covering the berries, and dark colored deposits covering the east. So the difference would only be the east to west difference in the composition of the underlying rock or soil, in which the impact that created Victoria occurred.

This all adds up to something...more things for Oppy to investigate. HURRY up Oppy! I'm getting impatient.

Posted by: MarsIsImportant Apr 4 2007, 09:37 PM

Does anybody know what day this image was taken? It must be fairly recent because it was posted today. It's important because it shows split and mangled blueberries along with sand or dust particles. If this is from within that first dark streak, then it supports an impact deposit origin. If not, then I'd still like to know exactly where it was taken.

http://qt.exploratorium.edu/mars/opportunity/micro_imager/2007-04-04/1M228942542EFF81D2P2936M2M1.JPG

Posted by: fredk Apr 4 2007, 09:53 PM

http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87555, that's 1135, between streaks.

Posted by: atomoid Apr 4 2007, 10:05 PM

Ever notice how the only area in Victoria with the dark streaks is also associated with what is pretty much a linear feature that is the cliff boundary layer.
again, here is http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=9947

I'm not sure if anyone here has already pointed this out, but this might be a very important detail, as it could be that this is a fault boundary, meaning one (or perhaps both (or neither)) of two things:

1) being a fault, its is composed of compositionally different (that is, water-modified) materials.

2) it could have experienced more geologically recent rockfall so there is relatively new dark material to blow out of the cliff face. elsewhere in the crater these exposures have long depleted.

Posted by: MarsIsImportant Apr 4 2007, 11:30 PM

I've mentioned that...not nearly as well put as you though.

Posted by: MarsIsImportant Apr 4 2007, 11:35 PM

Sorry, I guess I didn't click on all of your link in that post.

Posted by: Steve Apr 5 2007, 01:29 AM

I just wonder if a CRISM image wouldn't tell us a lot about the characteristics of the dark streaks. Given CRISM's 19m/pixel (38m resolution) they should be able to get a nice spectrum of the streaks.

On an off-topic tangent, it just seems unlikely that there are no CRISM images of Gusev or Meridiani in the system somewhere. (in mild conspiratorial mode)

Steve

Posted by: Jeff7 Apr 5 2007, 03:10 AM

Put me in the deposit camp. El Dorado was also a dark region, and it was clearly deposition. True, this could be an entirely different material, but it just seems to me like it'd be deposition. The wind could come in, hit the cliff faces, cleaning them off, and bringing dust along with. It loses energy as it climbs the crater wall, but maintains some speed because it's being constricted by the funnel-like shapes in the scalloped edge. When the wind finally makes it over the edge and into the open, its speed drops as it dissipates, and it's no longer able to keep the dust airborne. Plop, down it falls = dark streaks.

Posted by: fredk Apr 5 2007, 05:08 AM

Just a reminder to people (like myself!) who thought they had the streaks all figured out, that things are perhaps more complicated than you thought:

It looks like there's a broad light coloured streak trailing to the SE of Victoria. That's consistent with light streaks from other craters in the area, such as Endurance. I'm guessing that's due to bright dust eroding off of the bright evaporite along the rim. We're obviously dealing with varying wind directions and distinct processes.

Posted by: CosmicRocker Apr 5 2007, 06:57 AM

That's interesting, isn't it? I've played with various stretches of the HiRise imagery, too. I am not yet certain what to make of that. I have been concentrating on the ground with Opportunity's eyes.

There is a series of foreground pancams taken on many sols that gives a nice sampling of the near surface views. Setting MMB panoramas to a rover relative view and skimming through the sites, one can quickly notice the places where useful imagery exists. I really don't think complicated theories are needed.

Here is one of the quite early images from a relatively light area from sols 1016-1019. To me, it looks like berries buried in dust. You really need to look through the whole series of such images to notice the changes. If some of you don't look at all the images, you might not see the larger picture. I'm sticking with that razor guy.

Posted by: Edward Schmitz Apr 5 2007, 04:41 PM

It doesn't strike me as possible to have such a long thin jet of air coming off of a feature that has virtually no relief above the plain.

On the other hand...

If a small "fountain" of dark dust were being thown into an otherwise laminar flow, this is the effect you would see. The "fountain" would be a jet of air coming up one of the bays carrying dust particals from those dunes at the base of the cliff.

Fred's enhansement seals it for me. There are streaks off of ever bay (some more than one). They're all about the same length. That indicates that the phenomenon is effecting the whole crater. There is simply more dust on that one side.

Posted by: CosmicRocker Apr 5 2007, 07:18 PM

I should apologize up front for posting a montage that is kind of a mess, but I am tired of adjusting the layers to make it prettier. I've assembled a sampling of 10 false color pancam images from light and dark areas around the crater and then drew arrows to their approximate locations on Tesheiner's route map. It seems to me, at least qualitatively, that the surface appears generally dustier in the light areas, and generally cleaner in the dark areas.

Posted by: Shaka Apr 5 2007, 07:31 PM

That's a nice start at organizing the data for systematic comparison, Tom. But with only one view from a truly dark streak, it's still a bit early to draw conclusions. The next few days of pans and MIs should tell the tale. I for one need the close-ups to be confident of what I'm seeing.

Posted by: fredk Apr 5 2007, 07:35 PM

Thanks for the post, Rocker. I think it's very hard to judge dustiness based on uncalibrated colour images, though. Different channels may have been stretched differently at different times.

I am really looking forward to an MI from the dark streak to compare with the sol 1135 one. Without access to minites data that'll be about the best we can do ourselves. I'm afraid we may have to wait till after the long weekend, though.

Posted by: CosmicRocker Apr 5 2007, 08:14 PM

That's right, guys. I wasn't trying to say anything too definitive; that's why I used words like qualitatively and generally, etc. Apparently I find the trend displayed by this set of images more convincing than you. We should get other imagery in the darkest streak eventually, but I'm not sure that uncalibrated imagery is a huge problem. The colors may be wrong, but dust can usually be identified by its texture. Anyway, I have some other stuff in the works, but I'm still waiting for a few images.

Posted by: MarsIsImportant Apr 5 2007, 09:42 PM

That's good work Rocker. Thank you very much.

But my interpretation of those same images is very different. What you show is that the light colored areas are dusty with light colored dust; and the dark colored areas appear to have dark colored dust mixed in with the blueberries. IMO, this suggests the depositional theory is more correct--the one that takes into account the possibility of slightly different composition of the soil layer underneath different parts of Victoria.

I don't see the darker areas any cleaner at all; they are simply darker because of the dark dust. The fact that they are darker can create an illusion that they are somehow cleaner, but they are not. So, there appears to be deposition across the entire annulus of Victoria. Some streaks are light colored, while others are dark or darker still.

Posted by: atomoid Apr 5 2007, 11:36 PM

...and gosh i hate it how quoting removes the image attachments, http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87584

yeah, wow, is that a 'contrast enhanced' image? regardless, i'm stumped again, confusing the issue could also be the possibility that the dark streaks are laid down in episodic periods and are now dormant... the light streaks go against the current wind direction, whatever that is im no longer sure, did we rush to judgement and infer that by looking at the dark streaks? who knows what the wind direction is lately?

Posted by: fredk Apr 6 2007, 12:23 AM

That image I posted was straight from http://www.msss.com/moc_gallery/r10_r15/images/R14/R1400021.html; I did no processing of my own.

My guess is that there may be seasonal wind direction changes, or temporary changes associated with major dust storms.

Posted by: CosmicRocker Apr 6 2007, 07:11 AM

That's interesting, fredk. If I had looked at your image a bit more carefully I might have realized it was from the MOC. I had been playing with some contrast enhancements of HiRise imagery and that MOC resembled one of those.

You encouraged me to look again at some of the other orbital imagery available. I'm not certain I appreciate all of the differences that are taking place over time and between the various cameras. I think I'll stick with the close-up views from the rover's eyes that I am more familiar with.

I think the wind direction has been said to reverse every year, with the seasons.

Posted by: fredk Apr 6 2007, 04:28 PM

To add one more layer of complexity, here's an animated gif with two MOC frames: The first was taken http://www.msss.com/moc_gallery/r10_r15/images/R14/R1400021.html, and the second http://www.msss.com/moc_gallery/r22_s04/images/R22/R2200640.html

In the second frame the dark streaks in the NNE of Victoria appear much darker. The images may differ in their calibration, but clearly there's a real change since the darkest streaks darken substantially relative to the fainter dark streaks. Eg, look at the east-most of the 3 darkest streaks, which comes out of Bay D5. There's a much fainter and shorter dark streak coming out of the same bay towards the ENE. The main, NNW directed streak gets much darker relative to the shorter streak between the two frames.

The illumination and imaging angles are different, but why would that cause a difference between the two streaks emerging from D5?

I don't think this is surprizing, since I could certainly imagine seasonal wind differences, and while the wind is blowing towards the SE, the main dark streaks should get covered in the ubiquitous light dust. It does mean though that the darkness of the streaks may have changed even since the hirise images.

Posted by: fredk Apr 6 2007, 04:45 PM

Welcome to the Darkness!
Compare tosol's http://qt.exploratorium.edu/mars/opportunity/rear_hazcam/2007-04-06/1R229125096EFF81HQP1301R0M1.JPG with http://qt.exploratorium.edu/mars/opportunity/rear_hazcam/2007-04-04/1R228859304EFF81D2P1312L0M1.JPG

This navcam is downright spooky looking:
http://qt.exploratorium.edu/mars/opportunity/navcam/2007-04-06/1N229125982EFF81IBP0642R0M1.JPG

Posted by: MarsIsImportant Apr 6 2007, 04:58 PM

...clearly depositional. You can barely see the evidence of blueberries underneath the dark dust.

Posted by: Juramike Apr 6 2007, 06:19 PM

Thank you very much Cosmic Rocker! That montage was excellent!

IMHO it appears that the WHITE streaks or rays around Victoria are depositional.

The apron around Victoria is composed of blueberries and dark dust. The dark dust is mostly in the NE quadrant, and blueberries seem to be of the same concentration everywhere.

After the initial apron formation - which would have been darker in the NE, the winds have blown lighter toned material on top of the darker area. Except in a few places where the winds were stronger due to local topography (deep crater bays). There you see that the light stuff was not deposited, so we see the underlying blueberries and dark (presumably heavier) dust.

Look closely at the attached image. How come there are no dark streaks in the middle two shallower bays? If dark dust is being deposited, there should be some dark dust lifted out of the crater and deposited beyond these two bays.

Instead, we see lighter colored material.

-Mike

Posted by: Stu Apr 6 2007, 06:41 PM

(a hand goes up hesitatantly at the back of the class)

Seems to me that there might be a mass of darker material just beneath the apron to the upper right here... maybe as bays erode they eventually eat into this subsurface layer and the dark streaks are blown out...? Thought about this after noting how, if you look at the bays which are the sources of the dark streaks, they all go back to a line I've marked in blue on the pic... the two streak-free bays haven't reached that far back yet... maybe when they do, through more erosion, they'll produce their own streaks, too...?

Posted by: Juramike Apr 6 2007, 07:30 PM

[Lecturer's eyes glaze over, turns and stares at the blackboard, realizes alternate hypothesis fits observations. Shrugs resigningly as wave of desperation washes over him.]

Yup. That seems also to be a possibility.

I'm thinkin' a couple of wheel scuffs by Opportunity with an MI inspection would be nail it.

-Mike

Posted by: Juramike Apr 6 2007, 07:48 PM

Here is what might be a sequence of events leading to what we observe (need to click to enlarge, sorry!):

-Mike

Posted by: Edward Schmitz Apr 6 2007, 08:10 PM

Why wouldn't I say the same thing about the "clean sweep" theory? If there are winds sweeping out of the bays and cleaning the ground, why not in those two bays?

The answer is the same in both cases. The walls of those bays are much steeper than the ones with streaks.

Those in the clean sweep camp... Can someone explain to me how these tiny little bays are creating these powerful dust-cleaning jets?

If the bay was acting as a nozzle, it should (at least initially) shoot straight out along the center line of the bay. It doesn't. As soon as it enters the wind stream, it is swept down wind. There is very little difference in the vector of the wind leaving the bay and the wind stream itself.

If we suppose that the bay is instead causing turbulance, there should be some kind of swirling pattern. That clearly doesn't exist. As further evidence that this is not formed by turbulence, the streaks form at the tips of the bays. Turbulance would form at the tips of the capes and flow down wind. The capes that would form the most turbulance are the ones that extend perpendicular to the wind flow. The darkest streaks form off capes that are almost parallel to the wind flow.

I look forward to more lively debate on this subject. For those who think this will resolve with MI images, I am skeptical...

Posted by: Juramike Apr 6 2007, 08:28 PM

What type of experiment/observation do you think would be required to provide evidence?

What would the result be for "clean sweep" theory support?
Then what would be the result expected for the "dust deposition" theory?

-Mike

Posted by: atomoid Apr 7 2007, 12:02 AM

Deposition should be easily apparent, shoudnt it? we should see less blooberries and darker dust burying many of them, we should be able to scruff a wheel and get an MI to determine a depth of the depositional layer.

As for clean sweep, we should see more blueberries (even though bloobs are darker!) since they should be forming more of a desert pavement, if indeed there is soil removal going on and everywhere else we see blueberries at the top. there's no reason to assume differing blueberry concentrations in the initial apron deposits before these dark streak patterns emerged. Again a wheel scruff and MI will presumably be done and we'd be able to tell whats going on under the surface, if anything.

Posted by: fredk Apr 7 2007, 03:25 AM

While we wait for MIs, here are two hazcam views, from sol 1134 (between streaks) and 1137 (on the darkest streak). The local times of the images differed by only 5 minutes, so the lighting is identical, and the rover was pointing in almost the same direction in both. The exposures appear to be quite similar, based on the sky and rover parts, but of course they're not calibrated.
Off streak:

On streak:

On streak is of course darker, but to my eye the berries look less distinct on streak as well. It's possible this is due to a coating of dust, but I could easily be wrong. I think the difference in coatings (extra light dust off streak or dark dust on streak) would be very thin, so I think we really need the MIs.

Posted by: CosmicRocker Apr 7 2007, 07:03 AM

It's tough, fredk. The hazcams, navcams, and then pancams all seem to leave us with the same lousy resolution of the critical details at their respective distances. More MI's will be nice, but so will pancam closeups.

atomoid: Unless it's something exotic, it should become apparent if we get the appropriate resolutions in a variety of filters aimed downward. We're right in the middle of the darkest swath according to the map.

Posted by: antoniseb Apr 7 2007, 10:50 AM

I agree both that the MIs will help AND that it looks like in the streak there is a dust coating on everything including the berries. This is kind of the opposite of the clean sweep idea, which suggested that the berries themselves provided the darker shade.

Posted by: BrianL Apr 7 2007, 06:00 PM

I don't necessarily consider berries as the source of the darkness. When I threw my vote into the clean sweep camp, I simply took the position that the underlying layer of the apron is dark, with lighter dust on top of it. Whether the winds are keeping these areas dark, or the winds are depositing light dust everywhere but these areas, it amounts to the same thing. Light material on top of a dark apron is my clean sweep position.

If I was in the depositional camp, I would be arguing that the apron has a light base, with dark material being deposited on top of that in these areas. I'm beginning to think this position might be correct. The wheel tracks seem to show lighter areas, as if the wheels were picking up the dark deposits revealing the lighter base beneath. However, I'll stick with my original vote and wait to see how JPL calls it.

Maybe it's more complicated than either position suggests.

Brian

Posted by: Juramike Apr 7 2007, 07:25 PM

It looks like the blueberry count is approximately the same in either the dark or light zones around Victoria.

So either the coating (light or dark) is very thin and draping over the already present blueberries or the blueberries themselves are also being wind deposited at the same uniform rate all around Victoria. I have a really tough time understanding how the wind could be able to sort light and dark sands yet not be able to sort the larger and heavier blueberries.

To do list for Oppy:

1) Look at MI of surface in dark zone.
2) Scuff soil at this location: take another MI.
3) Move to light area (just over the "border" with the dark zone).
4) Look at MI of surface in light zone
5) Scuff soil at this light zone: take another MI


Hypothesis: White streak deposition (light overlaying dark):
Expected observations:
MI dark surface: blueberries + dark material
MI dark scuff: blueberries + dark material
MI light surface: blueberries + light material
MI light scuff: blueberries + dark material

Hypothesis: Dark dust depostion (dark overlaying light)
Expected observations:
MI dark surface: blueberries + dark material
MI dark scuff: blueberries + light material
MI light surface: blueberries + light material
MI light scuff: blueberries + light material

Hypothesis: Side by side depostion (Stu's hypothesis):
Expected observations:
MI dark surface: blueberries + dark material
MI dark scuff: blueberries + dark material
MI light surface: blueberries + light material
MI light scuff: blueberries + light material

It is assumed that the MI concentrations of blueberries would remain constant, both on the surface and after scuffing. If blueberry concentration varies significantly in either area (when scuffed), then that implies differential depostion of blueberries as well and the overall picture becomes a little more complicated.

From the Pancam image from fredk's post, it initially appears that the blueberries in the dark zone image have a thin coating of dark dust. So I would have to politely leave the "clean sweep" camp and cross over to the Dark Side. At least until MI results come in, at which point I reserve the right to waffle further.

-Mike

Posted by: CosmicRocker Apr 8 2007, 04:05 AM

BrianL: Yeah, I think the berries are mostly light in many of the filters, and that the clean-sweep hypothesis is best understood as the exposure of the dark, underlying apron soil. But there is the "berry shadow factor" which would play a variable role.

I was looking for some HRSC imagery that might show some changes over time. The resolution of what I found was not impressive, and the changes might be more related to lighting, atmospheric conditions, or something else; but there are large differences between the two images in this animated gif. The image from orbit 1183 was taken on December 19, 2004, and that from orbit 2064 was captured on August 23, 2005. I'm not sure that these help us understand very much, though.

One thing I found interesting about these was the variable location of lat/long between the two. I locked the animation on Victoria, and you can see a slight shift in the longitude axis and a more noticeable shift in the latitude axis.

Posted by: Stu Apr 8 2007, 09:53 AM

Just to clarify my own thoughts here...

What I think is happening is that as the crater erodes away there comes a point where a buried layer of darker material <very fine-grained, powdery material, laid down in this region of Meridiani in the distant past. This material could be either 1) particles excavated from beneath the surface by an impact that created one of the very large craters in Victoria's area, or 2) fine, dark dust grains dumped nearby during a long ago large dust storm> - is suddenly exposed to the air and therefore to the winds that move across the crater and/or whip up out of it. This material is then blown away from Victoria, falling back to the surface to leave dark streaks that cover the existing "apron" material.

Because this underlying layer of darker material is not uniform, but is clumped and has areas of greater density etc embedded within it, the erosion of the crater occasionally exposes patches of much denser material which then creates much darker streaks: if conditions are right - strong winds in the right direction, a dense patch of dark material exposed - then a very dark streak can be produced. At other times, when there's not so much dark material being exposed, the streaks are less dramatic looking, or vanish altogether.

Just thinking aloud...

Posted by: fredk Apr 8 2007, 06:14 PM

Dark streak MIs are in: http://qt.exploratorium.edu/mars/opportunity/micro_imager/2007-04-08/
It's not clear to me what's going on, though the berries look quite clean on the dark streak. We may have to wait for official word on this.

Posted by: Shaka Apr 8 2007, 06:35 PM

OK it's official (IMHO)! The Clean Streakers rule! Shiny, glossy concretions and well sorted, coarse sand.
QED

Posted by: dvandorn Apr 8 2007, 06:40 PM

Boy, it really is hard to tell from these two MIs, isn't it? I notice a strong specular reflection from the concretions in the sunlight, which I don't recall seeing (at least not in such a pronounced fashion) on blueberries in other areas.

I wonder if perhaps that wouldn't indicate that the berries have been sandblasted here, by very small-grained dust particles? Polished, as it were? I find it hard to believe that the relative darkening of the berries (if that is, indeed, what we're seeing) is due to a dust coating. That would have to be *awfully* fine dust, to simply darken the surface of the concretions without any individual grains being visible.

Some of the concretions also seem to sit in shallow bowls of the fine-grained soil. I would think that would be indicative to deflation rather than deposition, but I could be wrong...

-the other Doug

Posted by: Shaka Apr 8 2007, 06:45 PM

Hold onto your hat, Oppy, so it doesn't blow away!

Posted by: Floyd Apr 8 2007, 06:49 PM

I think it is deposition. Unlike most other MI (I have gone back to sol 750) the blue berries seem to be almost alone in the fine sand without lots of small light colored fragments. For example, compare this MI from the http://qt.exploratorium.edu/mars/opportunity/micro_imager/2007-04-08/1M229297319EFF81IBP2936M2M1.JPG with this from http://marsrovers.jpl.nasa.gov/gallery/all/1/m/1135/1M228942542EFF81D2P2936M2M1.JPG. Seems like more dark fine grain material burying light color fragments.

Floyd

Posted by: fredk Apr 8 2007, 07:07 PM

Floyd scooped me, I was just about to post this image, comparing between-streak (top) and on-streak (bottom):

Clearly, as he says, there are much fewer small berry-like fragments visible on streak. It looks to me like deposition of dark sand/dust which has buried the small fragments, but somehow the dark deposit doesn't stick to the large berries or has blown off due to wind.

Also, dvandorn, I don't think most of the darkening on streak is due to darker berries. Looking at the hazcams I posted yesterday, I think the darkening is between berries. That's consistent with what we're seeing here.

Posted by: Shaka Apr 8 2007, 07:50 PM

It's like this, guys, dusty surfaces scatter incident light in a wide ~hemispherical arc. So significant light reaches the eye regardless of the viewing or illumination angle. Shiny surfaces (like a mirror) reflect most of the light in one direction which matches the incident angle. So the bright (specular) reflection reaches an eye at the same angle as the source of illumination, but almost no light reaches eyes at other viewing angles, so the surface generally looks dark (at least much darker than a dusty surface). So a surface of clean concretions and clean rounded sand grains will look darker than a dusty one, even if the material composing the substrate has the same albedo as the dust. In this case the hematite concretions and basalt sandgrains may actually be darker (lower albedo) than the oxidized, pink dust, which accentuates the effect. There may also be a higher density of the berries in the streaks, though I haven't yet seen clear evidence that there is a significant difference. Someone needs to do the patient counting.
Basically, the clean, rounded sand grains at El Dorado were dark from high viewing angles (and from orbit) for the same reasons. Clean Martian sand dunes look dark, dusty ones look lighter.
Trust me.

Posted by: fredk Apr 8 2007, 08:20 PM

I don't think anyone can argue with the fact that a dusty coating can lighten the surface. But we also know that dark dust can certainly exist. So the issue has to be settled by observation, not by a priori arguments.

Looking at the MIs I posted two messages up, there are clearly fewer berries visible on streak, not more, and it's the smallest ones that are lacking. How do you explain this with the removal of light dust on the streaks?

Posted by: Shaka Apr 8 2007, 08:47 PM

Sorry, Freddo, it will take a lot more data than that to convince me that there is any significant difference in the size-frequency distribution of berries and berry fragments inside vs outside of streaks. Keep counting.

Posted by: fredk Apr 8 2007, 10:33 PM

Thanks for this, Cosmic! I think I'm more optimistic than you, though. If you look at these two images, the two MOC images I posted earlier, and the Hirise images, I think there is a pattern:

My first and your second frames were both taken during southern summer, and the streaks in the NNE of Victoria are not very dark in these frames relative to other features. My second, your first, and the Hirise images were all taken during southern winter, and all show the NNE streaks as very dark.

My picture is that during southern autumn or winter, winds blow towards the NNW, and gather dark sand/dust from inside the NE rim and deposit it outside in the few darkest streaks. Then in southern spring/summer, winds blow towards the SE, and the darkest streaks are coated with the ubiquitous light dust.

If this is anything like correct, we're lucky we got here in time since we're now well into southern spring and the dark streaks may be getting covered in light dust. This may help explain why the first streak appeared less distinct from the ground than we may have expected, though viewing geometry was probably the biggest factor.

A windsock, a windsock, my kingdom for a windsock!

Posted by: CosmicRocker Apr 8 2007, 11:41 PM

That's interesting, fredk. I meant to eventually compare them carefully, but I got caught up in this new project. It would be interesting to verify which direction the seasonal winds blow. I could swear I found that information once. I'll try to chase it down again. Unfortunately, it seems that those directions work just as well for the dust cleaning hypothesis.

Here's another montage using MIs from sols 924 to 1139. I tried to choose the best focused ones. They are reduced to 50% of their size to make this thing manageable, so you might want to look at the full-sized images for the best clarity.

I think most would agree that the MIs from sols 924 and 1069 are noticeably dusty. They are from areas well away from the main dark streaks. The images from sols 1103, 1135, and 1139 all appear to show significantly less dust. They are all from the general area of streaks, and even though two are from inter-streak locations, one might expect them to be cleaner than areas where no or faint streaks occur. That observation fits well with the clean-streak idea. I'm pretty convinced that the sol 1139 MIs are the least dusty of all. The very well sorted sand in those really clinched it for me.

1001 KB

Posted by: Shaka Apr 8 2007, 11:50 PM

The next best thing to a windsock is our trusty RAT; the grindings are sometimes blown out in a dust trail. So I would propose a sampling program as follows: (Of course, our PIs may well dismiss the streak issue as trivial and non-controversial, and so be unwilling to spend the time.)
1. Make at least 2 more MIs at different spots here.
2. Make at least 2 overhead PANs for counting berries and fragments.
3. Then proceed to another location (or two) in mid-streak to repeat the MI and PAN routine,
4. Proceed to the area of exposed rock close to the rim and do a RAT, observing the dust plume.
5. Eventually repeat steps 1 and 2 at locations outside of the streak to the east.
That should provide statistically conclusive evidence to resolve the issue for even the most devout Depositionist.

Posted by: fredk Apr 9 2007, 12:39 AM

Thanks again, CR! I think though that it is tough to compare dust levels from these images, especially when some have bad focus or are taken under very different lighting conditions (crucially, sol 1139 is under full sunlight). I am much more confident that 1139 contains many fewer small berry pieces than 1135, and given those two sols' proximity, and surface homogeneity on metre scales based on hazcam and pancam navcam imagery, this observation carries much more weight.

I have to ask you though what exactly do you mean by "the very well sorted sand in those really clinched it for me" - how does the sorted character support sweep vs deposit? If the darkness is deposited, then as the air jet emerging from the bay moves northward out over the plain and slows down, it should drop the heaviest dark particles first, and then lighter and lighter. Couldn't that lead to the "well sorted sand"? How would sweep give you sorted sand?

Posted by: fredk Apr 9 2007, 12:49 AM

Given the drive we've taken out here solely for the purpose of examining the Darkness, I think they can't dismiss this issue as trivial! I'd sure love to know their thoughts on this now, though. Remember too that they've got more to work with (minites etc).

I had thought the MIs might conclusively settle this, but it seems that both devout Depositionists and Sweepers are maintaining their positions! The fun continues...

Posted by: Floyd Apr 9 2007, 01:28 AM

I agree with fredk--I think it is deposition and that the ratio of sand to berries is critical. But it is even more complex. Both light dust and dark material from Victoria are being deposited as well as removed (possibly at different times of year).
If
dark deposition rate = dd/dt
light deposition rate = ld/dt
dark sweeping rate = ds/dt
light sweeping rate = ls/dt
You get a dark band when dd/dt+ls/dt > ld/dt+ds/dt
but if dd/dt much bigger than all the others, then you get a buildup of dark sand covering the little light fragments and all the dust---just what we see in 1139

Posted by: CosmicRocker Apr 9 2007, 05:51 AM

In all fairness, Shaka was the first to note the well sorted appearance of the surface grains. If this doesn't pan out in the end, I'll blame it on him. I might have difficulty expressing this efficiently, but here goes.

You could come up with a dark depositional scenario that employs well sorted grains, but we really haven't seen evidence of sand grains of the the size that are apparent in the sol 1139 MIs that have undergone much transport recently. That sand was blown to where it now is a long time ago, and it seems to not be affected much by modern winds. That sand, along with the berries and other larger fragments aren't going anywhere these days, and I see them as comprising the base soil or surface regolith here, and it has usually been been darker in appearance when we've seen it elsewhere.

I think I see tinier, fluffier dust grains in the MIs from earlier sols that are lying on top of and between the stuff that is too large to move in the current wind regime. They seem to be rarer in the most recent MIs. That is why I described the surface sand as very well sorted. I see fewer smaller grains.

Posted by: Shaka Apr 9 2007, 06:39 AM

In all fairness, Freddo, it was you who first announced the cleanliness of the berries (post #67). I simply extended your observation to the sand grains. To me the trick is to look for the circular outline of the grains. The more clearly you can resolve them, the more dust-free (i.e. well-sorted) they are.

Posted by: Juramike Apr 9 2007, 02:22 PM

"Dig we must."

Posted by: centsworth_II Apr 9 2007, 03:41 PM

I, like many, saw the dark deposits below the rim and the dark fans
extending from the rim above them and thought: Ah, yes, dark material
from the deposits is being blown out of the crater.

But if, like you say, the dark areas blow Victoria's rim are dark for
the same reason that El Dorado is dark (blown free of light dust). Then
the same wind that blows the patches below the rim clean also blows
the areas above the rim at those spots clean.

Posted by: centsworth_II Apr 9 2007, 04:00 PM

Maybe he's referring to this sort of process described by Steve Squyres:
"El Dorado is a spectacular dune field.... We think that, in terms of how this thing got
here, because of the configuration of this terrain with respect to the prevailing winds,
it may be an aeolian cul-de-sac.... What you see in the Microscopic Imager is a sand
that is very well sorted, very well rounded.... This is very clean stuff."
http://www.planetary.org/blog/article/00000497/

Dark areas below Victoria's rim could present this same "cul-de-sac" situation.
Above the rim? Certainly not a cul-de-sac. But still blown clean of light-colored
dust (and fine sand?).

Posted by: fredk Apr 9 2007, 05:28 PM

Does anyone know if El Dorado was thought to be deposited dark sand or dark sand swept clean of light dust? "Aeolian cul-de-sac" sounds like a place where winds stagnate and drop deposits, rather than where winds are fast and lift off light dust. If El Dorado is depositional, then that supports the streaks being so too, since the grain size in the streaks is smaller than at El Dorado.

Posted by: Edward Schmitz Apr 9 2007, 05:41 PM

In all four frames that are not on a streak, one can see the outline of the larger berries. One can also see their shadows. On the dark streak frame, no outline - no shadows.

The smaller grains are almost non-existant. One can only see the very top poking out of the soil.

The clear interpretation is that the fine grains are burying the larger ones. How can that be anything but deposition?

The dark streaks are dark grains being deposited on the lighter apron.

Posted by: Floyd Apr 9 2007, 06:26 PM

Seems like the 1139 berries are more deeply buried. Most seem to be more than half way, which is more rare on other images.

Posted by: Juramike Apr 9 2007, 06:50 PM

Assuming that the dark fine grains are burying the berries (and presumably overlying any lighter-colored sands) as the initial obervations suggest:

Are the fine particles heavier (denser) or less dense than the lighter-colored "normal" sands?

What does this say about the local wind speed? Is it higher or lower?

How did the dark streak form? What is the sequence of events for the deposition?

-Mike

Posted by: Juramike Apr 9 2007, 07:36 PM

Here is a side by side comparison of two wheel images as Oppy did a little shuffle in both a dark zone (sol 1137) in a light zone (sol 1126).

Even with the differences in lighting, there is really very little, if any, differences between the two. If I try really hard, I can almost imagine that the lines in the dark zone (1137) images show a little bit more fine structure due to smaller grain size. (The "X marks the spot" part of the wheel tread left a very detailed impression in this picture). But I may be really trying to read the tea leaves here. I would also like to say that there are hints of lighter tracks underneath as well, but that's more wishful thinking.

I desperately want Oppy to do a burnout on Mars and look at the underlayer. I fully expect to see a more "normal" sand grains and coloration.

This would make great sand to write a message with a stick....like "Send more robots" or something like that.

-Mike

Posted by: Shaka Apr 9 2007, 07:54 PM

Both. It is depositional for large basaltic sand grains, and erosional for smaller, lighter grains of dust. In a past period of strong prevailing winds from a southerly direction, sediment of all sizes (including particles eroded from the vesicular basalts) were blown up against Husband Hill. The wind was strong enough to blow the fines up and over the hill (some got dropped up behind the summit in dunes), but the heavier basalt grains a significant fraction of a millimeter in diameter didn't make it. They blew partly up the hill in gusts and then rolled back, up and back, over and over until they got beautifully round and clean and sculpted into a dune field that could not climb over Husband Hill, and was such an effective light trap that it fooled a guy from Portugal into thinking it was an abyss.

Sure, you can say the Victoria streaks are both depositional and erosional in that sense, as well. Obviously there is sand and concretions in the streak and they had to get there some way. In the past they must have been blown there by some powerful winds - and deposited - but we have been debating why the streaks look darker than other parts of the annulus now. Every Martian year or three dust storms spread light pink dust over everything. The VC streaks are obliterated. They only reappear gradually over subsequent weeks as the prevailing southerly winds (winds less powerful than the storm winds) blow through the crater, circulate in some complex pattern molded by the crater topography, and funnel or rotate up and out through The Valley without Peril and adjacent bays, lifting and carrying the pink dust with them. Thus the streaks reappear through an erosional process, a process that cleans the dust from the underlying berries and sand. This dusty wind polishes the berries to a high sheen.

Why don't the winds funnel through and clean Bottomless Bay and Bay of Toil as well? Well, they do to a minor degree - you can see faint streaks if you look hard - but the winds probably aren't as strong through the western bays because the crater is higher here (remember Beacon?) or because of some other topographic asymmetry.

Parsimony favors the Clean Streakers because they don't have to postulate a discrete source of dark material being eroded out of the NE cliffs. Where is the source? What is it? I can't see it. I'm still waiting to see a Pancam shot with an arrow pointing to an outcrop of basalt or coal or god-knows-what black stuff. I'm not holding my breath.

Posted by: Edward Schmitz Apr 9 2007, 08:15 PM

How about these...

I spent about two minutes looking for these. There are more and better.

Posted by: Shaka Apr 9 2007, 08:38 PM

I'm sure you can point to a place near the end of a dark sediment streak where it would be appropriate to go look for an outcrop of basalt or coal or whatever, but none of your arrows actually shows such an outcrop. I would be glad to have Oppy visit some of these locations for a close-up search, but I am very doubtful that she will actually find anything other than more sulfate-rich sandstones. We have looked at a lot of rock on Meridiani Planum, and not seen any such outcrops. I don't see any yet in Victoria.

Posted by: Edward Schmitz Apr 9 2007, 09:00 PM

Is it your assertion that because we haven't seen these layers, the assumtion is they don't exist. The evidence points to some dark deposit in the north northeast part of VC. It may, in fact, have an origin far from VC. But the dark, fine-grained matterial is there now. The orbital shots show these desposits in the crater. The ground frames show that it is mobile. I agree that the frames pointed to earlier may not be a source. But they are clearly not being uncovered. Those deposits high on the wall moved to that location if they did not originate there. The dunes, the deposits on the crater wall and the dark streaks are all too conspicuous to be coinsidence.

Posted by: WindyT Apr 9 2007, 09:21 PM

I take it the possibility that this dark material might be originally caused by groundwater coming out in a point source fashion has been addressed? Because from your image on the left, that's certainly what it appears like. There's not so much as an outcrop as there is an old flow of dark material from point sources in the wall.

I'm sure this will be eventually settled with closer looks, but my first guess is water borne deposits into the NE section of the crater, and then that material blown up onto the annulus from the winds out of the South.

My big question is _when_ this dark material appeared.

Here's the final query of mine... more of a tease. We've already seen that in at least a couple of places under camera surveillance, groundwater has come to the surface. Say that these dark spots are some indication of old groundwater flows... wouldn't it be neat to think of this happening under our noses?

Posted by: Juramike Apr 9 2007, 09:39 PM

I'm wondering about the composition of the dark stuff.

Could we get clues via APXS and mini-TES and subtract out the known lighter material background?

Or is the dark layer too thin?

-Mike

Posted by: fredk Apr 9 2007, 11:19 PM

That's an interesting picture for the formation of El Dorado, Shaka - do you have references for this or is this your own?I think this is a major weakness with Sweep. I see it as very unparsimonious to postulate an unusual extremely sensitive dependence of wind speed on subtle topography. Yeah, the NW rim is higher, but only by a couple of metres! (Remember we had to stretch those images of the rim during the approach to Vicky by a big factor!) That compares with a total depth of 70 odd metres.I don't think we need an entirely black outcrop as such - as long as it contains the dark material as a component which separates out due to differential erosion. Also remember we can see some difference inside the NE rim, namely the linear "fracture" features. There could be an association.
Still, I think the long arrow in Edward's first image is fairly convincing as a dark piece of outcrop. Of course lighting might be a factor. Here's another view of it from sol 1021:

In fact, Shaka, the more I look at this image the more certain I become of this. The dark feature looks, quite incredibly, just like your avatar! I think we can only interpret this as a direct message to you, Shak, to open your eyes and see the Dark!

Posted by: fredk Apr 9 2007, 11:34 PM

Posted by: Shaka Apr 10 2007, 01:06 AM

You guys trying to test my religious convictions?
My tiki knows the depths of my devotion.

Seriously, why was I so naive as to imagine one good MI from the streak would settle the argument.
You dirty streakers are nothing if not resilient. I'm sure we could continue the fun into the indefinite future, but I'm tempted to let the issue simmer now until we get at least a little more data from Oppy. I've gotten the uncomfortable feeling, though, that no amount of data will sway some of you. I will at least make the following promise: If Oppy ever discovers a stratum of black material of whatever genesis, in whatever location, I will abjure my tiki, and grovel at the feet of whatever unwashed dieties you all follow.

Posted by: centsworth_II Apr 10 2007, 01:28 AM

Why can it not be that the inner slopes of Victoria are composed, all around, of
exactly the same material and the evidence shows that most of this material has
a coating of light-colored dust while at some spots the dust has been blown away?

Posted by: CosmicRocker Apr 10 2007, 05:25 AM

While I might not be so brave as to propose as detailed a model as Shaka described, I agree with his assessment, that it is "both." I can't provide current references, but this kind of picture is so in line with basic geological concepts that the most casually observing geo-type would almost assume these processes were taking place. We clearly have evidence that the El Dorado drifts are dark through and through, but they do have a light coating of dust that we have somewhat recently seen partially and repeatedly removed by some kind of wind turbulence. I see it as a bunch of dark, very well rounded and sorted larger grains that were deposited in an earlier wind regime that are being covered by light dust, and then uncovered when the winds become more vigorous.

Might some of us be surprised by a particularly Martian process? You bet. But I am still thinking that some variation of the "light colored dust settles when the winds die down, but is selectively removed when they are more vigorous" theory holds at Meridiani, as it apparently does in Gusev.

centsworth II: That's exactly how I am interpreting the dark areas inside the crater. There are even some smaller, dark areas inside the southwest side of the crater that I assume were cleaned by winds curling over the edge as they entered the crater. I can picture winds setting up a general, crater-bound, counter-clockwise circulation that pulled air into the SW side of the depression and encouraged it to exit on the NNE side.

Posted by: fredk Apr 10 2007, 04:43 PM

I'm sure there are lots of less obvious explanations for the streaks, involving multiple processes. Some have been discussed already. One that interpolates between the extreme depositionist view (darkness is currently, or seasonally, being deposited in streaks due to localized sources inside NE rim) and the extreme sweep view (darkness underlies all of the apron, and is swept clean only at the streaks due to localized winds) is the following:

At some time in the past, an event, such as a collapse of a section of cliff, or even just gradual erosion, exposes a dark source which erodes quickly and spreads dark sand around the NE of Victoria. The source is now depleted, but there are still today more/darker/closer-to-surface dark deposits in the NE. Then we don't need to postulate strong wind variations from E to W to explain the darker streaks in the NE, and we also explain the dark patches inside the NE rim. This might also explain the apparent lack of small berry pieces we see on-streak in the MIs - they were covered by dark deposits, but some time ago. To sum up, localized deposition in the past, and sweeping today.

Does this scenario sit any better with the devout sweepers? It does remove the need for an extra process (deposition of darkness) occuring today, but instead pushes it somewhere into the (relatively recent?) past.

A variant of this would be that the variations from E to W across the crater are due to pre-Victoria-impact variations, but I have no clue how likely it would be that such variations could survive impact so close to the crater. Also this would have trouble explaining the apparent difference in MIs on- and off-streak.

Posted by: Edward Schmitz Apr 10 2007, 04:49 PM

Well that's the thing about evidence. It points toward an answer. In this case, there is dark dust/sand moving in this area.

If there were no dark matterial moving here, I don't know what effect the winds moving through the bays would be. Maybe a light coating is being removed at the same time the heavier dark matterial is being deposited. But I agree with http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87907Here's my best argument for deposition of dark grained matterial. Barring further evidence, Iwill leave it at this...

- the MI's show buried large grains. http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87826
- the color of the dunes in the crater matches the color of the streaks. http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87901
- there are dark deposits on the crater walls. http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87896
- the proximity of the dunes to the streaks is beyond coincidence.

I have a particularly strong belief in the desposition theory. But it's all good. I hope y'all are enjoying this mystery as much as I am...

Posted by: tedcochran Apr 10 2007, 06:58 PM

I'm not sure what to believe, but I do notice that on the original MRO strip, the blackness extends far to the north, and apparently well into the lighter dunefields beyond the apron.

If the wind is/was from the south, and encounters a higher crater wall to the NW than to the NE, an eddy would form. This subtle change in topography might be enough to cause a local wind speed increase sufficient to lift dust over the NE crater rim, for deposit downwind (where "downwind" very quickly reverts to "toward the north"), but I have a harder time believing that it can cause velocities far enough northward to sustain sweeping at ground level.

On the other hand, the ground appears darker south of the crater, and after the lighter dune fields north of the crater's dark streaks comes more of the darker ground further north.

I decimated the original 132 MB strip into the attached 91K version for those with lower bandwidth. The only other small version I can find on the forum cut off the area to the south.

--tc

Posted by: atomoid Apr 10 2007, 11:27 PM

As a Depositionist, and to build on http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87960 above, i also have to reconcile my creed with the observation that "light colored dust blows away first, leaving the dark", so it should follow that there is light dust blowing away from inside the crater, so i'd guess it should leave dark areas inside the crater. however, the http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=10020 look clearly depositional and dont appear to be evident of any sort of wind dynamic when viewed at this range. (well unless of course the streak and crater dark deposits are formed by different processes).

It also follows that the dark streaks emanate from what seems to be the area of most ongoing debris removal, given by the number of boulders and the offset (closer to north) placement of the inner dune field. Whether the boulders are merely evidence of deflation in the north side or are just recent rockfall debris, both processes would suggest that there is more debris transport from the north side where the dark streaks emanate from. Since the debris has to be deposited somewhere, i expect it to be pinned up against the highest areas of the cliff, especially if that is their source, as well as the dark streaks themselves.

If the dark streaks are to be interpreted as the prevailing wind direction, i can imagine the prevailing wind setting up a Clockwise rotation tendency inside the crater due to the preferential exits of Duck bay and bottomless bay. This should tend to nudge the dark deposits to drift to the NNE, right where we see them. While much of it should remain stuck beneath the steep parts the upper cliffs right where we see it, some of this should eventually spill over the top, which is in itself a strong sorting mechanism, and deposit the highly regular grains seen in teh dark streak MIs.

I have an easier time thinking of talc-size dust blowing around and burying the blueberies like this rather than the larger sand grains as appear to be in the http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=10010 that have been assembled here, but even if thats also going on, perhaps there wouldn't be much of a streak to look at, since it would have blown away. the larger gains can stay around, whereas the light dust may be too ephemeral.

Posted by: Jeff7 Apr 11 2007, 12:26 AM

Something I notice too in the "http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=10010" picture: the berries in the streaks are shinier. Dust carried by wind could easily provide this polishing service.

Posted by: CosmicRocker Apr 11 2007, 05:23 AM

Thanks folks, for your "excellent" comments on the picture, if not on my interpretation of it. Does anyone think that a few berries in the recent MIs appear to have been pushed into the soil by an instrument on the IDD, or perhaps a rover wheel. The missing images will hopefully clarify that.

Without a doubt, fredk. To clarify, when I made my "most casual observer" comment I was referring to the processes taking place at El Dorado. It is not nearly as obvious to me here at Victoria as it is on the other side of the planet. The darker stuff inside the crater, in the vicinity of the dark streaks outside the crater, really does open the door to other and multiple interpretations.

We kid around with terms like "believer," "devout," "depositionist," and other words that remind us of certain religion versus science debates, but there is no room for beliefs in planetary exploration. I'm having fun, and I feel as if I am exploring another planet in a way I have never previously experienced. It seems that most of us have a similar mindset, and that enhances the experience for me. Thanks to everyone for that.

Until new imagery or other blessed information is delivered unto us, we can only continue to speculate, pray, or go back and dig through available information for scraps we might have missed on the first go-around.

Posted by: fredk Apr 11 2007, 02:53 PM

Amen! (You asked for it, Rocker! )

More seriously, the latest http://marsrovers.jpl.nasa.gov/mission/status_opportunityAll.html#sol1131 tells us what Oppy's been up to in characterizing off/on-streak soils. Sounds pretty extensive:

Posted by: centsworth_II Apr 11 2007, 03:11 PM

I think the point is, they are shiny because they are
clean of dust, which has been blown away. The matte
appearence of berries in the bright areas between dark
streaks is possibly due to a coating of dust.

Posted by: BrianL Apr 11 2007, 05:27 PM

Well, the JPL people are clearly devout Depositionists. In the status update they speak of "dark wind streak material which emanates from Victoria Crater". Of course, they prematurely put the beacon on the far side, too.

Brian

Posted by: Jeff7 Apr 11 2007, 06:12 PM

I interpret the shiny as being a result of polishing action, not so much the removal of dust. They are also spherical, so dust would be more prone to fall off, especially with gentle wind flowing over.

Hopefully we'll have an official explanation soon.

Posted by: fredk Apr 11 2007, 06:59 PM

A drive today roughly southwards towards the next (unnamed) bay. We still seem to be "in the dark". This may be the closest view we get of the fresh looking rockpile crater we've seen from a long way off:
http://qt.exploratorium.edu/mars/opportunity/navcam/2007-04-11/1N229567449EFF81NEP1977R0M1.JPG

Posted by: atomoid Apr 11 2007, 10:06 PM

I had always interpreted the 'pushed-in' blueberries and being pushed in by the MI (perhaps as some sort of setup for getting a focus distance measurement). I can't see those spaces as preexisting since they would long be eroded away or filled in. ive always assumed they were made in the process of taking the MI.


Regarding the "polished" look of those dark-streak blueberries, if its a dust coating, then it has been somewhat modified (by humidity?) mainly since its shiny and not diffuse, you can see the bottom area where its not polished and this is supposed due to slower grain motion at ground level at this scale. I can see some sort of polishing going on, but i dont think thats just due oto the polishing ffc removal of blueberry material, i think it has added something, i might be reading too much into it, but it looks as if the dark material has imparted some of itself, in a sort of lacquer coating to each blueberry.

Posted by: djellison Apr 11 2007, 10:12 PM

The MI has a probe on a spring as a contact sensor - if it touched, you'd just get a little hole where the end of the probe hit the soil.

http://marswatch.astro.cornell.edu/jpeg/astro03/IDD_Inst_Set_1.jpg

Posted by: atomoid Apr 11 2007, 11:58 PM

un-needed quote removed. - doug

so thats the little pin with mid-spring at the lower right of the assembly? im not sure how this could be pushing in the blueberries like we see, so something else must be going on, i thought they might be first pressing down with the other tool (at upper right in the image) as we've seen in many other MIs before (the circular ring imprint in the dust), but now im newly mystified whats causing the pushed in berries without compressed dust ring, i wonder what theyre doing to cause this...

Posted by: fredk Apr 12 2007, 02:04 AM

According to the latest update they've also been doing apxs measurements - could that result in pushed-in berries? Does the apxs give the circular outline? A circular outline might be hard to make out if it only contacted the berries and not the surface between them.

Posted by: CosmicRocker Apr 12 2007, 03:21 AM

The Mössbauer and APXS both have circular contact plates. I suppose if pushing the berries into the soil encountered enough resistance, the IDD could stop before making a circular impression in the soil.
APXSMössbauer

Posted by: Edward Schmitz Apr 12 2007, 04:54 PM

I noticed for a long time that the apxs will make a deep impression to a bearly visible impression and everything in between. I think it's safe to assume any pushing of berrys is due to one of the instruments. If its just one berry, it could be a direct hit with the MI probe on a previous frame.

Posted by: fredk Apr 12 2007, 09:47 PM

Oppy has made a slight shift to study a rock nearby. This was followed by a humourous shadow puppet performance which was well received by all present:
http://qt.exploratorium.edu/mars/opportunity/forward_hazcam/2007-04-12/1F229656979EDN81O7F0006L0M1.JPG

Posted by: Shaka Apr 12 2007, 10:14 PM

Big Bird and Gonzo?

Posted by: Pavel Apr 12 2007, 10:42 PM

Soup Dragon and a Clanger!

Posted by: BrianL Apr 13 2007, 12:20 AM

Curiously, I see a leprechaun on the rock, being attacked by a dodo bird.



Brian

Posted by: atomoid Apr 13 2007, 12:49 AM

http://www.swanshadow.com/images/TennesseeTuxedo.jpg

Posted by: MarsIsImportant Apr 13 2007, 04:20 AM

I was thinking more of "Mystery Science Theater".

Posted by: CosmicRocker Apr 13 2007, 04:51 AM

Something like this? That was great. I saw that image come down today, but I totally missed the shadow puppet show. I've got to stop looking at rocks.

This became a topic on irc today, and a guy from Argentina noted that it reminded him of Mystery Science Theater 3000. There was something familiar in those shadows, but I couldn't pin it down without his clue. This is my small attempt at humor, but there might be other potential captions.

Posted by: Reckless Apr 13 2007, 07:41 AM

Someone playing golf with their friend the T-Rex.
Roy F

Posted by: Astro0 Apr 13 2007, 07:53 AM

Sorry, all I could think of was this when I saw that image.
The flamingo strikes!!!!
Enjoy
Astro0

Posted by: centsworth_II Apr 13 2007, 02:34 PM

Florida Stowaway Finally Emerges from Hiding

Posted by: Shaka Apr 13 2007, 08:11 PM

Could there be a causal relationship between this and your avatar, Brian?
Obviously there is a connection with our generational cartoon Weltanshauung.


P.S. Lots of new images of surfaces in and out of streaks. More to come. Our statisticians can get to work counting and measuring blueberries.

Posted by: fredk Apr 14 2007, 03:32 PM

Thanks for the show, Oppy!

Getting back to business, one consistent difference between the new pancams off- and on-streak is in the wheel tracks. Compare off-streak, sol 1136:
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1136/1P229038150EFF81D2P2401L2M1.JPG
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1136/1P229038213EFF81D2P2401L7M1.JPG
with on-streak, sol 1139:
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1139/1P229299607EFF81IBP2403L2M1.JPG
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1139/1P229299654EFF81IBP2403L7M1.JPG
The local times are over an hour apart for these images, but it's pretty clear there is a difference in the tracks. Off-streak, the tracks are dark and smooth, with no berries showing through. On-streak, the tracks are much lighter in L2, and many berries show through. As I said, this is consistent across all the new pancam imagery.

I can think of two interpretations at the moment: one is that the fine soil on-streak is shallower than off-streak before a harder layer is reached, so the berries can't be pushed in as far. The other is that the soil off-streak is "softer" than on-streak, so berries are more easily pushed in.

Regardless, the off-streak behaviour has been the norm at meridiani, I believe, so something new is clearly going on on-streak.

Posted by: mhoward Apr 14 2007, 05:03 PM

Going off what fredk points out, here is a little photo essay, off-streak vs. on-streak. I have no idea what any of it means, I'm just in it for the pretty pictures.

http://www.flickr.com/photo_zoom.gne?id=458870099&size=l http://www.flickr.com/photo_zoom.gne?id=458846556&size=l

http://www.flickr.com/photo_zoom.gne?id=458873753&size=l http://www.flickr.com/photo_zoom.gne?id=458860842&size=l

I will say it doesn't seem like the first time we've seen tracks like these. I bet there are comparisons around Endurance. Just speculating, though.

Posted by: MarsIsImportant Apr 14 2007, 05:24 PM

Fred, I think I see what you are talking about. What chemical action would cause the soil to be relatively stiff on streak? Moisture of some sort in the recent or distant past? Is this observation real?

Howard, we need closer close-ups for a good comparison in those images.

Posted by: Floyd Apr 15 2007, 01:18 AM

The dark dust is on the down wind side of the rock. Case for deposition.

Posted by: dvandorn Apr 15 2007, 04:27 AM

Not necessarily, though I've moved from smack in the middle of the clean-sweep camp to somewhere on the fence, at the moment.

On the side of clean-sweep, it seems to me that if dust was being removed from the streak in general, a well-sorted amount of that dust would tend to fall out behind uneven, jutting protrusions (like rocks). That sorting might well favor heavier, larger-grained (and darker) particles, while the lighter dust (both in mass and in hue) blows further away.

If the dark dust is blowing out of the crater and being deposited, I'd expect it to pile up on the upwind side of the rock, not the downwind side. If it takes a rock to make the dark dust pile up, I'd argue you're seeing the minor movement of dark soil components, not dust blowing out from the crater.

Of course, without a good model of the wind dynamics in and around this crater, it's hard to tell just what's happening, here...

-the other Doug

Posted by: centsworth_II Apr 15 2007, 05:05 AM

Any sign of djellison there?

Posted by: CosmicRocker Apr 15 2007, 05:29 AM

fredk: That's very interesting. Really interesting, actually. Is this driving you mad, or what? It seems that so many raw observations can be interpreted favorably for either camp. What's with the uncommon L1 pan we saw today?

Floyd: That sol 1143 image was taken through an L2 filter, right on the edge of the near infrared band. The views through other filters appear quite differently. Take a look at the L7. http://qt.exploratorium.edu/mars/opportunity/pancam/2007-04-14/1P229654197ESF81NEP2599L7M1.JPG

More importantly though, that tiny patch of extra darkness would be very hard pressed to explain the overall darkness of the streak in the HiRise images. Just guessing, but it might be enough to influence a pixel, if that bright rock was not nearby.

Nice imagery has been coming out of the queue in the past two days. Can't wait to see some more.

dvandorn: I'm still leaning clean, but this isn't the slam-dunk I thought it would be.

No signs of that other Doug on this end, either, centsworth_II. We really could use a tie-breaker at this point.

Posted by: djellison Apr 15 2007, 07:56 AM

They are a fairly regular event, an albedo observation of the surrounding terrain usually used to tie in with an observation by an orbiter.

Doug

Posted by: Bill Harris Apr 15 2007, 09:09 AM

You've missed the boat entirely. There is no dark streak, only a bluish area that appears darkish when viewed with a red filter. The source material is the blue-toned talus at the base of the cliff.

Posted by: Stu Apr 15 2007, 09:22 AM

Nice colour sets down today...

Posted by: helvick Apr 15 2007, 09:52 AM

I'm still firmly in the cleaned streak camp folks - we have a harder sorted crust layer with a higher visibility of (slightly cleaner) blueberries => stiffer surface as per mhowards demonstration and an effect that is only significanlty "visible" in a filter that highlights blueberries.

Posted by: AndyG Apr 15 2007, 10:44 AM

I love the first picture, Stu. And it made me wonder about the effort required to shove the blueberries into the soil. A few minutes with Excel later, and I see that it's straightforward. With minimal wheel sinkage the ground pressure on the rover can be quite high: about 40kPa (<6psi) for a tween-cleat's area-worth of support. This is about 2/3rds of the pressure of a human foot on the ground.

It's no surprise that when the aluminium wheel is initially contacting, say, the tops of 50 of these berries per tween-cleat-area, the pressure rises: the load is the same, but the surface area is reduced. Pressure on the berries as they're forced down is around ~320kPa (46psi). Say about a pound (Earth pound!) per 3mm berry. No wonder they disappear!

Thanks for the mathematical wandering!

Andy G

Posted by: sattrackpro Apr 15 2007, 02:44 PM

I'm late to this dust-up... mostly because I didn't think there was much doubt about what causes the dark wind tracks at Victoria. However, obviously there's a lot of people thinking 'deposition' versus 'swept.'

When you look at the same light-dark phenomenon at other craters, it is easy to see that it is wind-caused - almost always in the prevailing wind direction, and often extended by vortices ('dust-devils') ranging from small eddies to well-formed powerful whirlwinds that form in craters and travel some distance before collapse.

Whirlwind formation happens best at craters of the right size and depth for a given wind speed and time of year. Below are two pictures showing whirlwind tracks that begin at craters and travel various distances away - note that all of these craters are not known to have dark material inside them to provide 'deposition.'

Vortices cause the 'deposition' folks a problem because they don't pick up dust in a column until they are well formed which occurs after exiting the lee side of a crater, then they carry it away dropping it across a large area down-wind after the column disintegrates.

At larger or smaller craters whirlwinds of any strength are rarely well formed and their 'tracks' are shorter - as at Victoria, and the famous 'double crater' below.



At Victoria as at any crater, there are times of the year, and times of the day, that these tracks appear darker due primarily to dust removal, and the effects of lighting and reflection of light. Below is a picture of very dark wind-tracks at Victoria - and a picture of Oppy's tracks taken shortly after leaving ‘Jason' on Sol 382 that appear light.

Bottom line, when these wind tracks at Victoria are the darkest, they are more likely caused by dust removal than highly unlikely greater deposition of dark material from within the crater - and Oppy's tracks can appear light or dark depending on were the sun is more often than what the material under the wheels is.




I don't believe the presence or absence of 'blue-berries' has any effect or plays any part in the appearance of the wind (most likely numerous small whirlwind) tracks at Victoria.

Posted by: sattrackpro Apr 15 2007, 03:39 PM

One last point - I don't know if there is likely to be wind at Victoria at this time of year, but if there is any likelihood of wind - Oppy should stand by where she is for a day or two in the hope of getting several nice cleanup jobs done on her solar panels.

It would have been most interesting had the operators taken photos of the panels before heading into this likely place to get a cleanup job done - and some after photos afterward, if there is any wind...

Posted by: fredk Apr 15 2007, 03:40 PM

Regarding the L1 albedo pan, I don't recall one in the past which was 2 frames high. Am I wrong?

Bill: I'm sure you meant "bluer than the rest of the apron" - we haven't seen anything genuinely blue-hued yet. The streaks have been essentially invisible in L7. But since reddish hues dominate here, it makes sense to call the streak "dark". It's certainly worth pointing out, though, that this "darkness" is more than just albedo and presumably that tells us something about composition. Regardless, there's something to explain here!

dvandorn: why do you expect depositional dark dust to pile up on the upwind side, but dust swept off to pile up on the downwind side? By postulating that sweep picks up dark as well as light dust, but drops only the dark, you are http://en.wikipedia.org/wiki/Epicycles#Slang_for_Bad_Science

Isn't it just a matter of wind speed, and I'd expect that to be slower in the lee. Therefore deposition of dark dust from inside Vicky would be more likely in the lee. If this was sweep, we should see less sweeping in the slower wind speed lee, and hence a lighter surface in the lee.

A reminder that we've seen these lee deposits on the rim at VwP:
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1133/1P228765407EFF81ARP2396L2M1.JPG
This means that there is dark dust originating upwind of the rim, which sounds like Deposition to me!

Rocker: it's the red bands where the streak is visible - it's invisible in L7. This "tiny patch" certainly doesn't cause the entire streak to be dark, but it does show that deposition of dark dust does occur as a process.

Yeah, this is driving me mad! But I think we're closing in on Depositionism here. The good news is the planners are clearly taking this very seriously, judging from all the pancam/MI observations.

Posted by: fredk Apr 15 2007, 04:11 PM

Sattrackpro, I don't beleive we've seen any individual dd trails in meridiani, and certainly no active dd's. Also, the dark streaks from Vicky and Double don't look like dd trails. Why wouldn't we see individual dd tracks coming from smaller craters in Meridiani?

The Double crater image is interesting, but I can see dark deposits (not just shadows) inside the rim. Likely the same thing is going on here as in Vicky.

Posted by: fredk Apr 15 2007, 04:32 PM

Here's another very interesting observation that I interpret as evidence for Deposition (heck, puts the final nail in the coffin of Sweep!), but no doubt Sweepers will claim it supports them.

These frames were taken from the sol 1137 location, looking WSW at the rover tracks:
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1141/1P229474974EDN81IBP2298L1M1.JPG
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1141/1P229474997EDN81IBP2298L1M1.JPG
http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1140/1P229387205ESF81IBP2598L2M1.JPG
They clearly show that light coloured dust has blown towards the left out of the rover tracks since they were formed.

Sweepers: where does this light dust come from? Deposition predicts that there is light dust under the dark deposits, which could be exhumed by the wheels.

It's interesting to see that the wind direction is towards the rim. I speculated before that winds vary seasonally here, so it seems that deposition from the crater is not occuring now.

Posted by: centsworth_II Apr 15 2007, 04:36 PM

No need for photos, just keep your eye on that energy dial.
This is how previous "cleaning events" were documented.

Posted by: centsworth_II Apr 15 2007, 04:42 PM

Where is the dark dust that should be swept onto the tracks from the right
as the light dust is being swept off the tracks?

I get the impression that "all" dust on Mars is light and darker materials
come in larger, sand-sized grains. maybe this has to do with optics and
not composition, or is dark basalt harder and less likely to be ground
into smaller than sand size particles?

Posted by: centsworth_II Apr 15 2007, 04:46 PM

Are you saying that the dark deposits in Victoria can not possibly be explained
by the same process used to explain the dark deposit that is El Dorado?

Posted by: ElkGroveDan Apr 15 2007, 04:58 PM

The entire planet is coated with it. The frequent global dust storms are very efficient at circulating the stuff and depositing it everywhere.

Posted by: dvandorn Apr 15 2007, 05:14 PM

That's my impression, too -- and a good part of my rationale to answer Fred's questions. Though the question of basalt isn't the right one, I think -- I believe that the ubiquitous red dust is actually oxidized basalt.

In regard to "clean sweeping," though, I'm thinking that you're right, the dark soil components on Mars are, in general, composed of larger grains than the brighter, redder dust. They move primarily by saltation and secondarily, for grains in the small end of the size range, by aeolian transport. Martian winds pick up the lighter, smaller-grained redder dust, and keep it in atmospheric suspension, more easily than they can pick up or keep suspended the larger, heavier, darker dust grains.

That has two different consequences:

First, a wind across the Martian surface will tend to sweep away the brighter, redder dust and deposit it thinly downwind. For a given set of wind conditions (i.e., wind speed and direction), this will set up a dynamic equilibrium after a certain time, during which areas that are more exposed to the winds are swept of bright dust to a greater degree than upwind sweeping re-deposits similar red dust. These areas slowly become darker, creating the wind-generated darkening events that have been visible even from Earthly telescopes for centuries. When wind patterns change (which they do, seasonally), portions of the terrain that are darkened may brighten, and other areas may darken. But the thing to remember is that, under each type of prevailing wind pattern that a given spot on Mars sees in a year, an equilibrium is reached and given areas are swept clean and darkened more than they are re-brightened by re-deposition. But, and this is the important point, *nowhere* on Mars is seeing *just* dust deflation or deposition. Both processes are always happening, everywhere -- it's all a question of which process, deflation or deposition, is dominant over a given landform at a given time.

Second, each type of dust, the bright red stuff and the darker gray stuff, exhibits a range of grain sizes. While bright red dust can be swept off from the darker gray dust, the smaller grains of the dark gray stuff will also be picked up by the winds. Because these darker grains are larger and heavier than the lighter red grains, they fall out of the air much faster and more easily. They don't stay suspended in the air for nearly as long, and aren't swept as high off the ground. So, in an area that is being swept, you'd see the smallest grains of the dark dust (which are still larger and heavier than the largest grains of bright red dust) piled up in aelian forms, in the lee of rocks and protrusions where turbulence degrades the wind's ability to keep them suspended. That's why it's important to see how well sorted the grains are in the dark aeolian forms -- if they're very well-sorted, it indicates that the dust that's forming these dark aeolian forms is dropping out from suspension because it all becomes too heavy to be supported when the wind hits the lee of the protrusion. In other words, it's not being deposited from very far away, this material is being picked up from the ground inches or feet away from the dark dune, moved the very short distance the wind can carry it, and being dropped as soon as the conditions keep it from being suspended in the air.

That's also probably why Victoria's dark streak seem to feather out from the center -- the bright dust that gets removed is deposited hundreds of meters, if not kilometers, away, while the small amount of dark dust that the same winds can transport only gets a few meters away from where it was picked up. So, along the edges, the darker dust actually does get deposited over the brighter dust at the margins, giving it that feathered look.

All of this explaining of what I think I'm seeing is moving me back towards the clean sweep side of the fence...

-the other Doug

Posted by: fredk Apr 15 2007, 11:15 PM

Not at all. I and others here have pointed out that what's happening here is almost certainly a mix of deposition and sweeping, as I think we agree has been the case at El Dorado. The question is "is there a dominant process that causes the darkness in the streaks?"

Posted by: fredk Apr 15 2007, 11:20 PM

What I meant was where does this light dust come from, in these wheel tracks. If the light dust has been removed from the dark streaks by wind sweeping, why is there light dust in the wheel tracks? If the light dust has been coated by dark deposits, I could see the wheels exhuming the light dust.

Posted by: fredk Apr 15 2007, 11:35 PM

Dvandorn, your description of varying particle sizes fits beautifully with my view of deposition! We have some source inside the rim. The strongest winds are able to lift the smallest dark particles out of the crater through the bay wind funnels. As the wind spreads over the plain it slows, dropping the largest dark guys first, then smaller, etc. At any distance from the rim there is a certain size of dark particle just barely in suspension, so any obstacle like the rock will cause them to drop in its lee, leaving a well-sorted deposit.

This leads to a definite prediction: the dark particles in the streaks should be largest near the rim and smaller farther out. Perhaps we'll be able to test this as we move towards the rim.

Of course these winds can also clean light dust if it's there to clean. But the Depositionist claim is that the Darkness is primarily due to deposited dark stuff.

Posted by: Steve Apr 16 2007, 01:30 AM

Thanks for the excellent photos of dust devils downwind from craters. When I first saw pictures like that I wondered whether in those cases vortices were picking up darker soil from the crater and depositing it downwind, depositing less dark soil -- and hence becoming fainter -- as they got further from the crater.

Your model implies vortices only form downwind of craters. Mine has dust devils more widely distributed, but those that pick up dark soil from craters leave darker tracks. In that view most vortices are sweepers, but some (after passing through craters with exposed dark soil) are depositional for a while.

I'm convinced by the dark soil in the NE corner of Victoria that the streaks downwind of that corner are depositional.

Steve

Posted by: Juramike Apr 16 2007, 01:45 AM

Fredk, do you think this prediction will also hold for the drift behind the rock?

Would you predict that the size of the particle grains behind the rock be smaller than those on the ground? The lower wind speed should help drop smaller particles than normally found in this location. (Can a quick MI at the drift boundary help solve the debate?)

Would a sweep scenario also predict smaller particles in the drift?

-Mike

Posted by: Juramike Apr 16 2007, 01:49 AM

(Oh. I see from the Hazcam images they already took the MI pictures. Remind me to check these things BEFORE I post.)

-Mike

Posted by: CosmicRocker Apr 16 2007, 05:12 AM

I'm sorry folks, but I am growing tired of this debate. I really am trying to keep an open mind, but most of what I see seems to be cleaning. Obviously, the depositionists are also confident in their theory. All I can say is that deposition seems to require a more complicated set of conditions than sweeping does. According to the MIs, we are seeing down to where the coarser grains last moved, when the winds were strong enough to move them. That's clean enough for me.

Posted by: djellison Apr 16 2007, 06:47 AM

Dust devils form on sun-facing slopes, then migrate with prevailing winds picking up and redistributing the lighter fines, revealing cleaned, darker soil underneath. It's well documented process - and one that we've seen upclose and personal within Gusev crater many times.

As for Victoria, darkness and the wheel tracks....

• Dark soil with Berrys on top
• Dust falls out the sky as it does everywhere, lands on top of the berries and the darker soil underneath.
• Wind comes along, blows dust off the top of the berries, but doesnt 'touch' the soil underneath because the berries cause a wind shadow.
• Thus you have darker berries visible from above with a lighter soil underneath
• Wheel comes along, pushes berries into the soil - and now you see the dark soil again but with a lighter dust coating on top, thus appearing a little lighter.

The tracks here are VERY reminiscent of Eagle craters airbag imprints.

Doug

Posted by: Ant103 Apr 16 2007, 01:25 PM

Nice rocks there
(Sol 1045)

Posted by: centsworth_II Apr 16 2007, 03:37 PM

So you are officially a clean sweeper?

Posted by: Tesheiner Apr 16 2007, 03:38 PM

> Nice rocks there

Remember we were trying to pinpoint the location of those ones when they were barely visible on the horizon (on sol 1096 perhaps?)

BTW, this rock pile was named http://en.wikipedia.org/wiki/Granada. OT: Those who have been there should know http://en.wikipedia.org/wiki/Alhambra.

Posted by: djellison Apr 16 2007, 03:56 PM

Regulars will know that I don't side on these (frankly a bit silly) 'With us or against us' arguments. I'm just commenting on what I see

And I've visited the Alhambra, it's utterly breathtaking.

Doug

Posted by: Edward Schmitz Apr 16 2007, 04:20 PM

You answered your own question. The darker matterial is larger grains. Harder to move than the light dust (but not impossiblie).

Posted by: Ant103 Apr 16 2007, 04:22 PM

Tesh' : You mean http://marsrovers.nasa.gov/gallery/all/1/p/1096/1P225490535EFF79T0P2373L7M1.JPG

Posted by: Edward Schmitz Apr 16 2007, 04:54 PM

Clean sweepers are not addressing the deposits on the walls of the crater.

http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87896

This dark matterial is clearly being deposited and not being uncovered.

There are convincing arguments on both sides - in theory. When the evidence is taken in its totality, deposition is the dominate process.

The one thing I would point out here is that the wind outside the crater is not moving substantially faster than on the plains. The reason for the dark desposits is that there is a fountain of dark particles coming out of the bays. The smaller particles are lofted high than the larger ones. Once they enter the prevailing winds, they are swept down wind and dropped. The smaller particles going farther. This is evident in that the dark streak does not protrud straight out of the bay. It is immediately swept down wind. Talk of vortexs or wirl winds are not supported. This has been stated by steve. If you don't remember this, I will look up the reference.

I appreciate you carrying the ball here, fredk...

Posted by: djellison Apr 16 2007, 05:06 PM

Yes - but darker than what? Darker than very bright rock, sure - but the same 'darker' than the dark streaks against the surrounding terrain?

There are convincing arguments on both sides....full stop.

Doug

Posted by: Edward Schmitz Apr 16 2007, 05:51 PM

The orbital view clearly shows the dark matterial in the streaks to have the same http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87901 (at least within a limited part of the spectrum) as the deposits around the north side of the rim.

So to answer your question - Yes, the same dark as the streaks...

Posted by: CosmicRocker Apr 16 2007, 07:34 PM

In case anyone missed it, that "rock pile" is a rather fresh-looking crater. It's not very apparent unless viewed in 3D.

Posted by: Shaka Apr 16 2007, 07:49 PM

I'll hazard a guess that it's another secondary (or even tertiary!) chunk of ejecta from Beagle impact. It's very reminiscent of the rubble pile we passed on the way south to Beagle.

Posted by: fredk Apr 16 2007, 08:10 PM

I'm not sure exactly what silliness you're referring to here, but my interest is to try to understand the nature of these streaks given what (limited) information we have publicly available. Of course jpl has all the data and will no doubt come to a conclusion and (hopefully) we'll hear it (indeed the fact they've driven out here highlights the importance they place on this issue!). But what's extraordinary about this mission is that we all have the opportunity to take part in this process in real time! We can observe some feature, speculate about its origin, and then look at what further observations do to support or refute our ideas. I find this extremely fun, but then science is my thing.

This "taking sides" business is a normal part of real science. You provisionally take a position based on what you see, then debate with people taking different stances, and in the end (hopefully) come to a conclusion. Nothing silly in that.

Admittedly things can get out of hand in these public discussion groups. But you do a pretty darn good job of keeping things under control!

Posted by: fredk Apr 16 2007, 08:19 PM

Indeed!

Posted by: Stu Apr 16 2007, 09:17 PM

Agreed. If we all just sat here waiting for Steve S to tell us what every single feature was, well, the yawns would be deafening. Remember how much fun we all had discussing / debating / arguing about Beacon's location and nature? That was one of the highlights of the whole mission so far for me, even tho I turned out to be on the wrong side of the fence crater, simply because it drew everyone here together and set us a challenge, which I think we rose to. I still think the streaks are deposits, but if I'm wrong then I'm wrong, and at least I'll have had fun joining in the discussion and having the opportunity to talk with people who know a helluva lot more about this stuff than I do. ( Which is pretty much everyone here, I know, but that's ok too. )

Posted by: climber Apr 16 2007, 09:36 PM

I'm NOT convinced yet. I'm sure Oppy will be heading soon in Beacon's direction to double check this
Seriously, I hope they'll stop there this time.
BTW, I agree on the need to debate even if the subject could be futile. If we don't, the Rovers will last more than ourselfs

Posted by: Stu Apr 16 2007, 09:44 PM

Hmmm.... fresh-looking crater...

Posted by: fredk Apr 16 2007, 10:50 PM

And again, in 3D:

Posted by: atomoid Apr 17 2007, 02:39 AM

Debate over?
After reading all these great thought-provoking arguments in the last few pages, its apparent that the same points are being argued by both camps, mainly:
--> Of all the dust being deposited on the 'dark streaks', it is primarily dark dust.
--> Of all the dust being 'swept away' from the dark streaks, it is primarily light dust.

Simply because the light dust is too ephemeral and blows away based on the preponderance of wind speeds whereas the dark dust just tends to tumble up and along slowly, and both process occur in a somewhat simultaneous and overlapping-offset fashion based on seasonal wind differences, a tortoise and hare co-evolution.

- I'd characterize the streaks as "depositional" (i hope i dont sound like im massaging the semantics to fit) because the dark streaks contain extra dark particles over and above that of the rest of the apron and consist of material that was not part of the original apron material. Perhaps a centimeter or so in the thickest areas, with the blueberries setting some sort of upper limit due to their wind interaction and can only be partially buried (or maybe this is due to generally chaotic air movement exiting VC which doesnt allow blueburying drifts to build under such conditions). The real 'depositional' clincher here is if there exists a build-up of dark material on the bare bedrock at the mouth of the bay, which looks to be the case from the HiRise images, although even then it looks to be a thin veneer.

Sweepers and Depositors can live together in peace and harmony and indeed need each other to survive(!):
--> The dark streaks wouldn't be visible if it werent for the clean sweep activity going on to reveal them.
--> The clean sweeped areas wouldnt be apparent if there werent a preponderance of dark dust beneath them to give them contrast with the rest of the apron.

Posted by: nprev Apr 17 2007, 03:06 AM

Late to the party, but here are some thoughts.

As I'm sure has been noted previously, the blueberries are obviously much less wind-transportable than dust. Given the observed density of them across Meridiani by Oppy, the key discriminator between deposition & erosion seems to be whether the berries are darker or lighter than the substrate (I personally can't tell from Oppy's raw B&W imagery what the reflectance/absorption characteristics of the berries are, much less how they would appear to MRO) . Therefore, a primary investigation strategy would be to determine if ground-level obs of the 'plumes' reveal more or less blueberries per m^2 of terrain sampled.

Probably just re-invented the wheel there, but made it square thiis time...

Posted by: WindyT Apr 17 2007, 04:19 AM

I'm not sure what Martian wind velocity would be required to move significant numbers of those blueberries.

I wonder if the wind was strong enough to roll a few of them off the northern apron of Victoria's annulus.

Posted by: CosmicRocker Apr 17 2007, 04:55 AM

I know I said I was tired of this, but I might not be. I came across an image from sol 1109, when Opportunity was in an inter-streak area on the Cape of Good Hope that I must have missed earlier. In my mind,this pancam target pretty much condenses most of the differences between the light and dark surfaces into one pancam subframe for me.

The attached image is a montage showing the subframe image in context. From the rover's eyes it seems that this area is one that is generally lighter, but which contains some darker patches. The pancam overlapped the navcams displaying the darker patches in context, and provides a more detailed view. To me it appears that the light area in the bottom of that subframe is coated in light dust, and that the top half of the image is mostly swept clean of light dust.

The transitional boundary between the two displays partially buried berries. I swear, this is my last comment.

Posted by: Juramike Apr 17 2007, 05:58 AM

Thanks for the interesting montage, CosmicRocker.

[And yes, I'm going to try to tease more comments from you...]

That's an impressive pile o' blueberries in the upper part of the subframe in your montage. How do you think this compares with the images from the dark streak zones? Can you explain the differences in blueberry density?

I think that the the montage you made gives really good clues about deposition/ablation of material at the subframe location, but I'm not sure I understand if it formed exactly the same way as the dark streak zones or if it is yet another part of the story at Victoria.

What do you think was the sequence of events that explains the image from the subframe image location?

-Mike

Posted by: Juramike Apr 17 2007, 06:14 AM

I totally agree. But it all really boils down to focusing on the right question:

"What key observation/experiment would help decide between the different scenarios?"

-Mike



I still preserve my right to waffle as new evidence arrives.

Posted by: Tesheiner Apr 17 2007, 01:52 PM

Yup.



... named "Huesca" (spanish city near the Pirenees).

Posted by: djellison Apr 17 2007, 02:24 PM

Somehow I can't imagine features getting names after British cities...just not quite romantic enough

Milton Keynes
Bradford
Sutton Colefield
Slough
or even Leicester

Posted by: Sunspot Apr 17 2007, 02:49 PM

There's Robin Hoods Bay
Lulworth Cove.

Posted by: centsworth_II Apr 17 2007, 02:58 PM

I don't see why the same arguments about the dark deposits lower on Victoria's slopes
can't apply to deposits higher up on the wall. They are the same material that occurs
all around the rim, but swept clean of light dust, OR they are a different, dark material
that has been blown up from deposits of this dark, mystery material from below.

Posted by: centsworth_II Apr 17 2007, 03:24 PM

What a great demonstration of clean and dusty berries!

And going back to details of MIs from sol 1136 and 1139, the off streak
berries (top) look dusty while the on streak berries look clean.
Also, I think it would be hard to argue that there is more dark fine material
seen in the lower image than in the upper. The main difference is the
cleanliness of the berries. Perhaps this is the only difference between on
and off streak areas -- the cleanliness of the berries.

Note the mini Iapetuses (Iapeti?) in the bottom image .

Posted by: djellison Apr 17 2007, 03:57 PM

Just for a bit of fun - I botched a little animation of the timeline for a cleaning hypothesis using grey ground, a grey-blue single berry, and salmoney dust.

Dust falls onto ground and onto berries. Makes everything look a little brighter. Then - along comes the wind, but the ground is sheltered by the berries - so only the top of the berries get cleared off - thus you see a whole lot more berry (dark) and less soil + berry ( light )

A similar sort of process also happened to an extent up at the year old tracks they observed on the way out of Endurance. For eons the berries sat on the surface, the wind blowing over the top...grains of soil underneath not really feeling the wind. Then, along comes a wheel - pushes the berries into the ground, and thus the soil within the track is re-exposed. the wind then works away at the soil and you end up with the pushed-in-berries being re-exposed at the bottom of the wheel tracks.

All just speculation of course..just a bit of fun

Posted by: centsworth_II Apr 17 2007, 04:32 PM

Posted by: djellison Apr 17 2007, 04:34 PM

Do you see me saying that it isn't deposition or it has to be cleaning? Nope. I'm just trying to visualise what the MI's seem to be showing.

Posted by: fredk Apr 17 2007, 06:56 PM

It's very hard to compare those two MIs since one is in shade and the other in complete sunlight. Much of what may appear to be dust on the top (shade) frame looks like it may be jpeg artifacts.

Posted by: climber Apr 17 2007, 09:01 PM

So am I (see avatar) ... but on the other side of the rim

Posted by: BrianL Apr 18 2007, 03:25 AM

I don't know. I would love to see homage paid to such memorable locations as Amlwch, Swanage, Corriemuchloch and of course, Clonmult, Sadberge and Buldoo.

Brian

Posted by: CosmicRocker Apr 18 2007, 05:15 AM

I knew I'd be drawn back into this discussion, even though I swore I had made my last comment. I am not sure why the berries are sometimes aggregated into local surface concentrations. There are other forces acting on them that may or may not be relevant to the dark streak question. We have often seen the berries piled up into local concentrations on various scales, and in areas where the streaks were weak or absent. I am not yet convinced that the dark streaks mean simply that more berries are visible. I think the cleaning extends down to the sand-sized grains below them.

When I first proposed that these were streaks were clean, I really thought it was a slam-dunk situation. In some ways I still do. But when you get down down in the dirt with the imagery we have to work with, it seems to me that there is quite a lot going on. I might never have seriously considered the dark deposition idea were it not for the fact that there are accumulations of dark sediment inside the crater's north side, as Edward has reminded us. I explain that away by proposing that those internal dark deposits exist all around the crater, and that they have been selectively cleaned of light dust on the north side of the crater by the same winds that made the external streaks. Without other supporting information, that leaves me with a somewhat empty feeling, though.

Another interesting observation would be fredk's images of light dust blowing south from the wheel tracks. I'm still working on that. I think this is going to make one heck of an interesting paper when they eventually publish it.

Posted by: Stu Apr 18 2007, 07:28 AM

Another little "Berry Bowl"...?

Posted by: Gray Apr 18 2007, 01:33 PM

Bigfoot!!

Posted by: centsworth_II Apr 18 2007, 02:22 PM

Keep in mind that, per Doug's model, the amount of light
colored dust in the on and off streak areas is "the same",
only the distribution is different, with the dust in the streak
having been blown off the berries but not out of the streak.
This dusty layer, even in the streak area, becomes
prominent when the rover pushes the berries below.

Posted by: centsworth_II Apr 18 2007, 03:44 PM

Yes, the lighting and focus variables make it difficult. But won't
stop me from trying.

In The latest, sol 1148 on streak MIs (right), the berries have the same stoney (not shiny)
look as the off streak berries from sol 1135. (Both images are crops of the originals)
Maybe it's my bias, but the right image (on streak) has a cleaner, less dusty look.
There could be an argument made for more dark fines though, and what is the explaination
for the lack of intermediate fragments in the on streak image?

Posted by: fredk Apr 18 2007, 03:47 PM

But the MIs on-streak appear to show clean, well-sorted dark sand/dust between the berries. It looks like either the light dust has completely blown away, or has been covered by the dark sand/dust. Or some combination of both!

Posted by: centsworth_II Apr 18 2007, 03:53 PM

I wonder if there will be any MI, or other study done of the exposed
rock layers in off and on streak areas? It would be interesting to see
the darkness of fine grains caught in cracks and voids in the rock.

For example, what would MIs from the areas circled in red reveal?
(This is a crop from Tesheiner's Opportunity Route Map)

Posted by: centsworth_II Apr 18 2007, 03:58 PM

It sure is hard to make all the idea pieces fit!

Posted by: fredk Apr 18 2007, 04:00 PM

Deposition, of course!

Recall our http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87812

Posted by: Juramike Apr 18 2007, 04:01 PM

In the "subframe image" CosmicRocker posted, it looks like the brecciated rock pavement may lie pretty close to the surface. I suspect because of the shallow depth of the sand layer, that sand/dust was removed, and the revealed berries were gently blown (or just rolled) into little local hollows in the rock topography. The berries are simply lying on the cracks and dips.

After this, there was a deposition event(s) that emplaced a cute little local sand drift right in front of the bowl o' berries (was this dune downwind or upwind? where would you expect a similar cute drift near the rock in the on-streak image). Then another layer of fine dust was put over everything, since the pavement bedrock doesn't seem directly visible in this part of the image.

So in this "subframe image" I am interpreting this as evidence for both ablation and deposition at work at this "subframe image" location. I don't know if the nearness of the bedrock (shallowness of the soil layer) makes this processes behave different compared to the dark streak area. (e.g. deposition might work better on soft sand than hard rock, less bounce).

-Mike

Posted by: centsworth_II Apr 18 2007, 04:10 PM

To my "... what is the explaination for the lack of intermediate fragments in the on streak image?"

But is it deposition of a dark material unique to the streaks, or of the same sand
found throughout Victoria simply rendered dust-free, and thus darker? In other
words, would the sand of the off streak areas look the same as that of the on
streak areas if the dust were blown from it, without the addition of some new,
dark material?

Posted by: fredk Apr 18 2007, 04:21 PM

Glad you've been drawn back, Rocker!

That patch of berries from sol 1109 is clearly tied to a change in slope - look at this 3D image. But the berries don't appear to be in a bowl, instead on a slope. Probably wind is involved as well.

Posted by: Juramike Apr 18 2007, 04:31 PM

Former mini-dunes o' berries?

-Mike

Posted by: dvandorn Apr 18 2007, 05:24 PM

Here's an explanation for the lack of the smaller, what people are calling intermediate grains, on-streak.

The intermediate grains (smaller than the concretions, larger than the soil grains) are likely *not* hematitic concretions. They may well be busted-up pieces of the evaporite.

Perhaps the higher wind conditions in the streak areas have eroded these small grains away to nothing, while the winds just aren't strong enough to erode them down further outside of the streak areas?

I think we need to look at compositional answers for why these smaller grains are missing on the streaks...

-the other Doug

Posted by: Edward Schmitz Apr 18 2007, 06:14 PM

The smaller grains are buried. You can see them in the study that http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87826 did...

Compare the last image with the others. There are very few partially covered pebbles or berries in the other frames. In the last one, all of the pebbles and berries are partially covered. By inferrence, the missing ones are buried.

Posted by: atomoid Apr 18 2007, 09:36 PM

To figure out what is behind all these little berry-drifts, and we see them everywhere, it might help to consider to what depth has the bedrock been ablated here, what, maybe a half meter or so with high variability? The initial apron was surely more of a chaotic jumble of evarporite chunks than it is now after having been sanded down over the eons to a very smooth level overall, and if we consider that the berries dont move unless they are on bedrock to roll away or the sand beneath them moves, then it would be possible that these little berry drifts are whats left behind after a chunk of bedrock gets ablated and all the berries drop out and roll down what was then the existing side of the rock and pile up at the base of their parent rock's eroding surface, which is now long gone, but the berries in most cases dont move, they exist as sort of chalkmarks outlining the deaths of old bedrock chunks, some are more pronounced than others due to the topography of the rocks they exited and further erosional factors.

Posted by: centsworth_II Apr 19 2007, 02:37 AM

If so, it looks like they are buried, in the dark streak
by the same sand material that lies around them in
the off streak areas.

Looking at the four off streak and one on streak
image, I am not convinced that there is additional
dark material in the on streak sand. The only
obvious difference is that the on streak sand appears
clean and dust free compared to the sand in the
other images.

So if deposition is going on, it is of the same sand
that occurs off streak and not of some different,
dark material. The streak areas look dark because
they are free of light dust (on the surface). IMHO

I hope the Mössbauer and APXS results clear this up!

Posted by: CosmicRocker Apr 19 2007, 05:24 AM

I really like that model, atomoid. We've seen quite a few areas were berries have been concentrated along the edges of eroded blocks. Even before Opportunity climbed onto Victoria's apron we saw similar features near some bedrock fractures all along the journey from Eagle crater. I'll be keeping that process in mind. That's what I call thinking outside of the box.

As for the "smaller grains/fragments," I'm not sure how much weight to give them. They are difficult to impossible to resolve in the pancams, so we can't say much about their appearance over larger areas of the surface. We can clearly see them in the MIs, but since the few MIs that have been captured cover such a tiny total area, it is hard to imagine that they provide us with a representative sampling of the surface.

Even if we assume that the relative distributions of the larger berries and the smaller fragments are homogeneous over the entire area and that both are being buried by finer, dark grains, I could simply postulate that the winds here have been high enough in the recent past to not only blow away the lightest weight and lightest colored dust, but also high enough to remobilize some of the sand-sized grains that seem to have been mostly dormant most everywhere else Opportunity has looked. Unfortunately, that brings us back to square one. I am still strongly swayed by the apparent cleanliness of the sand within the streak.

I didn't notice anyone mention it, but I thought it was interesting that the IDD reached out to that little dark ripple to the north of that likely meteorite chunk in front of the rover. Is that rock Alicante?

Posted by: Juramike Apr 19 2007, 05:52 AM

Thanks, Atomoid!

So given the average blueberry density in native bedrock, and the average amount of blueberries we see on the surface, how many meters of rock has been ablated? (Does 0.5 m still work out?)

Given the blueberry pile seen in the "subframe image", and the average blueberry density seen in the surrounding of the blueberry pile, does this still work out to the same value?

How many m3 of rock had to be concentrated to make the blueberry pile in the subframe image?

All this should give the minimum total ablation amount.

(I'm still assuming that the average blueberry count is the same on the on-streak and off-streak areas and that the ablation rates on both are similar.)

-Mike

Posted by: Juramike Apr 19 2007, 02:51 PM

[This is probably obvious to all y’all, but I’m still trying to crunch through this and get constraints on the various scenarios.]

I will assume that berries were emplaced where they eroded and were not significantly blown around. Berries are only being exhumed and not freshly blown in on the apron around Victoria.

The key observation is that the berry count is roughly the same both on-streak and off-streak. This implies that after the big erosion (of 0.5 m? of rock) that no major net overall deposition has occurred after the maximum exumation level was reached. The berries aren’t buried.

I struggle to imagine how after the removal of several cm of rock volume has occurred, that a “magic puff” occurred at the end of the ablation that would be able to selectively enrich native sand to only have dark material all around the apron. (After emplacement of a lighter layer all around the apron and perfect removal of the light sand layer would reveal the original lower layer in the streaks – this scenario doesn’t seem likely).
[Predicted w/this scenario (Scenario 0): on-streak: v. thin “native-enriched” darker layer, at surface, then “native apron” deep layers; off-streak: light deposited layer at surface, then v. thin “native-enriched” darker layer, then “native apron” deep layers).

More likely is that if after original emplacement level was reached, a new layer of lighter sand was added and this could be enriched slowly towards darker sands in the on-streak area. Because the berries aren’t buried, the net addition of light material off-streak cannot be more than a berry width deep. [Predicted w/this scenario (Scenario 1): on-streak: v. thin “enriched-deposited” darker layer at surface, then “native apron” deep layers; off streak: light deposited layer at surface, then “native apron” deep layers.]


Here is a possible sequence of events for the apron of Victoria:

Scenario 1:
1. Rubble breccia pile emplaced on impact.
2. Net Ablation of rock (? m). Blueberries emplaced where they drop.
3. Net deposition of sand (w light/dark particles) < berry widths ( we still see berries at surface).
4. On streak: Removal-of lighter sand deposit to give “enriched-deposited” darker layer at surface.


Here is another possible sequence of events for the apron of Victoria:

Scenario 2:
1. Rubble breccia pile emplaced on impact.
2. Net Ablation of rock (? m). Blueberries emplaced where they drop.
3. Net deposition of sand (w light/dark particles) < berry width ( we still see berries at surface).
4. Net deposition on-streak of v. thin layer of dark material over the sand deposition layer.


Another scenario that also should be considered (and eliminated though observation) is differential depostion of both materials:

Scenario 3:
1. Rubble breccia pile emplaced on impact.
2. Net Ablation of rock (? m). Blueberries emplaced where they drop.
3. (off-streak only) Net deposition of sand (w light/dark particles) < berry widths ( we still see berries at surface) off-streak only.
3. (on streak only) Net deposition on-streak of v. thin layer of dark material over the “native layer”.


For Scenario 1, the dark material comes from the deposition of wind-deposited sands w/light/dark particles. We should be able to get a better constraint on this hypothesis by measuring the amount of dark grains in the wind deposited sands (# particles/cm3), then observe the amount of enrichment in the dark region (# particles/cm2).

That should tell us the minimum amount of original sand wind-deposit needed to be present prior to enrichment.

Any imagemeisters up to the challenge?

-Mike

Posted by: centsworth_II Apr 19 2007, 03:09 PM

Why is the word "dust" conspicuously absent from all of your senerios?
Dust is famous Mars-wide as a primary affector of surface color/brightness
and is a primary element in many of the theories presented in this thread.
While the relative abundances of sand, fragments, and berries in MIs is
being considered, the relative abundance of dust cannot be ignored.

Posted by: Juramike Apr 19 2007, 03:19 PM

I'm assuming dust is "in the mix" as well. I was using the all-encompassing term sand to include anything that is rolled in, blown in, wind dropped, or settled out from the atmosphere that then ends up on Victoria's apron. Martian dust fits all these categores and is included in all the deposition scenarios.

My only exclusion is that blueberries are NOT in the deposition mix.

-Mike

Posted by: centsworth_II Apr 19 2007, 03:21 PM

A reality check for those of us that might be inclined
to explain all dark streaks on Mars in terms of dust
removal. Although the situations at the south pole and
on the rim of Victoria Crater are certainly very different.

From Emily Lakdawalla's Planetary Society Weblog:
Varying dust fans on Mars' defrosting south polar cap

http://www.planetary.org/blog/article/00000949/

Posted by: centsworth_II Apr 19 2007, 03:32 PM

I think this is a dangerous generalization. I suspect that size wise
there is as big a difference between dust and sand as between sand
and berries, with similar differences in their mobilities. If sand is
moved within an area while dust is removed from the area, these
are two different effects based on two different catagories of material.
I don't think a theory that lumps dust and sand together can explain
the dark streaks at Victoria.

Posted by: Juramike Apr 19 2007, 03:55 PM

You are right: I may have made a fatal generalization. I was trying to come up with a simple (AKA "niaive") explanation for possible scenarios.

In order to keep a selective-removal hypothesis possible (Scenario 1), I needed a mechanism to deposit a mix of different-colored particles, then selectively remove the lighter-colored components. I was assuming that the darker components would be heavier and thus left behind. I was trying to keep to a 3 component problem (light-white particles/heavy-dark particles, mix of the two) rather than a multi-body problem (light sand/dark sand/light dust/dark dust, and all the various combinations).

But the reality is probably more complex....

-Mike

Posted by: Marz Apr 19 2007, 03:55 PM

While berries are on topic, I wanted to ask if we're able, given a surface berry density count, to make an estimate on how many "ghost" sulfate layers were deflated to concentrate the berries?

Does this assume that berry distribution is the same for all sulfate layers?

Related question: does the diameter of a berry give any clues where it formed in the layers? i.e. berries at the top of a sequence would be smaller than those at the bottom? (I realize on a debris apron of a crater, that the material was exhumed from the crater and probably jumbled, but thought we might see evidence of this in the crater wall when we finally get a closer look? Was this seen at Endurance?)

Bonus question: why is hematite the only form of iron we see at Meridiani? I'd think with iron and sulfates there'd be gobs of pyrite too, or does this hint at the acidity of the solution the hematite formed in?

Apologies in advance if I'm re-hashing questions from previous discussions.

Posted by: Edward Schmitz Apr 19 2007, 04:31 PM

I saw that too and was tempted to bring it in. There is a remarkable resemblamce to the dark streaks at VC. There are certainly diffenences in the process. The similarity that I would put out there is the fountain of particles and how they deposit down wind. Other places where there is turbulance induced cleaning has a very different appearance. http://themis-data.asu.edu/img/browse/V00881003?band=3&stretch=S2 should be familiar to all of us. That is turbulance.

Compare the two images. Especially the bay at the 10 o'clock position. These are both examples of the particle fountain where the wind has shifted.

Posted by: Shaka Apr 19 2007, 07:20 PM

Comparisons between Meridiani and South Pole patterns are of course interesting and, perhaps, informative, but we need to keep in mind the apparent albedo contrasts in images, as distinct from absolute albedo levels of the materials present. The light background in the Polar image is either water ice or solid CO2, and therefore brilliantly white compared to the light dust covering ice-free areas of Mars, including VC. The dust fans at the Pole which look black in contrast to the ice may, in fact, match the albedo of the light, dusty apron around most of VC.

There is no doubt that wind streaks can be depositional or erosional in origin. They can be relatively light or dark in contrast to the background. The question we are dealing with here is: What is the immediate cause of the dark streaks on the Victoria apron? We should not have to invoke the entire history of Meridiani Planum to answer this. The "dirty" deposition hypothesis requires a discrete source of mineral in the northeast rim of VC that is dark in contrast to the common evaporite and concretion material composing the rest of the region. That deposit of dark mineral must be identified if the deposition hypothesis is to prevail.

Posted by: Edward Schmitz Apr 19 2007, 07:59 PM

two things...

1) The pattern of the streaks are not erosional. Those are the patterns of a substance being tossed in the wind stream and falling out down wind.

2) We don't need to find the ultimate source of the matterial for the deposition threory to prevail. There is mobile matterial in the north side of the crater that matches the streaks in both http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=10035. That's a fact. How it got there may never be known.

Posted by: Shaka Apr 19 2007, 08:09 PM

Neither of these "two things" are facts, except in your own mind.

Posted by: centsworth_II Apr 19 2007, 08:10 PM

In two crops (below) from the image you link, it's interesting to note that only craters
that have dust devil trails leading from them have dark "deposits" within them. Of
course these are not deposits but areas swept clean of dust by the same dust devils
that went on to leave a dust free trail leading from the crater in which they were born.



Concerning "...examples of the particle fountain where the wind has shifted.",
streaks caused by dust removal can also be laid down in different directions as
the wind forming them shifts.

Posted by: centsworth_II Apr 19 2007, 08:26 PM

The pattern is dictated by the wind. It may dictate that the cause was not
turbulent dust devils, but does not rule against a constant, stiff breeze.

Wind would be involved in dust sweeping or in deposition... or both at
the same time. How would the pattern be different for either sweeping
or deposition if the same wind is involved?


Maybe not, but we do need to see the deposited material. If you argue that MIs in the streak areas
show a deposition of the same sandy material that is found in off streak areas I could agree. But
if you say there is some different, dark material that has been deposited in the streaks, I would say
that the MIs do not show any such material to be present. What they do show is a surface
swept clean of dust.

Posted by: Edward Schmitz Apr 19 2007, 08:30 PM

There are varying amounts of streaks coming off of the bays. If you look at each bay there is a strong corrolation between the amount of dark matterial in and under the bay with the darkness of the streak.

If anyone is thinking - "that just means the wind is cleaning the area below the bay too." The whole clean sweep theory is based on how the wind exits the bay. There needs to be another mechanism to clean the area below the bay.

Posted by: Shaka Apr 19 2007, 08:37 PM

Why....precisely?

Posted by: centsworth_II Apr 19 2007, 08:55 PM

Wind patterns on the rim clear dust from certain spots. Wind patterns
within the crater clear dust from certain spots.

Another point: the dust removal theory for the streaks is
Occam's razor friendy. The deposition theory, not so friendly.

Posted by: Stu Apr 19 2007, 09:01 PM

I know my suggestion waaaaaaaaay up the page didn't get much interest or response, but I still wonder if there's some kind of dark mass beneath the crater to the north and north east... something that is eaten into when erosion of the bays eats into it, briefly releasing fine dark material that then 1. spills down into the crater, and 2. puffs up and then away from the crater. Only a few bays show this dark material. Maybe the areas beneath the bays aren't cleaned at all but the dark material is covered by lighter dust blowing into the crater from the surrounding terrain, I dunno...

Posted by: centsworth_II Apr 19 2007, 09:30 PM

The invention of this mass to explain is a good example of why
the dark matter deposition theory is not so Occam's razor friendly.
The simplest explaination does not always end up being the
correct one, but it is the one that needs to be ruled out first.
Does anyone here think the most detailed evidence to date --
the MIs -- rule out the sweep theory?

Posted by: Stu Apr 19 2007, 09:38 PM

I haven't 'invented' it; I'm basing my suggestion on actual observations... There are only rays coming out of a handful of bays, which are bays that cut quite deep into the surrounding terrain... there are large craters in Victoria's area that could have deposited material nearby when they were formed... there are layers visible in the cliffs on numerous pancam images... the rippled dust dunes on Victoria's floor are proof that winds move material around down there, at least suggesting winds blow on the features higher up the crater too...

The slash of Occam's Razor doesn't murder my suggestion, does it?

Posted by: Edward Schmitz Apr 19 2007, 09:41 PM

If the jet of air exiting the crater is required to clean the surface, how is the area below the cliff going to get cleaned.

Did you look at the pictures that I posted?

Did you see the difference between streaking bays and non-streaking bays?

Can we at least agree that there are dark deposits there?



Occam's razor is not proof for anything. It is only a reminder to put aside far-fetched explainations UNTIL you have eliminated the more obvious ones. besides - deposition is simpler in that there is obviously dark mobile matterial there just waiting to be blown up on to the apron.

Clean sweep requires an as yet undefined reason why some bays cause air blasts while others don't. How is that simple.

Stu,

I concure... There may well be that mass of matterial there. Chances are, we've gotten our closest look as those cliffs already. We may never know where it came from...

Posted by: Shaka Apr 19 2007, 09:44 PM

Sooner or later the depositionists are going to have to get into specifics about the "dark mass", namely "What is it made of? and "Where can we see the original mass?" and "Point to the dark material in an MI." and "Why haven't we seen this dark mass before, either from Oppy or from orbit?"
If those challenges cannot be dealt with, the hypothesis will remain entirely speculative.
Numerous workers in the field recognize the process of bright dust removal to produce darker streaks. The erosional hypothesis requires only the wind.

Posted by: Edward Schmitz Apr 19 2007, 09:47 PM

If the entire area were uniform in composition, why even traverse around the crater at all? Just sample one spot and you're done...

As Stu points out, it is a theory based on observation. The totality of observations will lead to the most complete explaination.

So, I'll ask again. Can we at least agree that there are dark deposits at the base of those cliffs?

Posted by: centsworth_II Apr 19 2007, 09:50 PM

Let's just say it's a close shave.

Posted by: Stu Apr 19 2007, 09:54 PM

Ok, I'll take that mic...

What is it made of? Well, it might be a big, dirty, dusty chunk of Phobos that splatted down here and is now being eaten into... more likely, it could be material from farther below the surface than we've seen so far; it's not a huge leap to suggest that the impacts that blasted the larger, nearby craters out of Mars could have excavated deeper lying material and vomited it in VC's direction, laying it down as a localised deposit of dark material in the north/north-east area that was later covered by dust and VC's own ejecta. Just like grass and dirt covers a cow pat.

Where can we see the original mass? We can't, it's beneath the surface, beneath VC's apron.

Point to the dark material in an MI. Hands up, I can't. Yet.

Why haven't we seen this dark mass before...? Because it's beneath the surface, and the material from it is only exposed when the erosion of a bay "breaks into" it, exposing it to the air and releasing it. But we have seen the streaks, on MOC and MRO images, haven't we?






Like your style cents!

Posted by: Edward Schmitz Apr 19 2007, 10:03 PM

What is it made of? Irrelevant - it's there.
Where can we see the original mass? Irrelevant - it's there.
Point to the dark matterial in an MI? How - the MI's don't provide that kind of information.
Why haven't we seen this dark mass before, either from Oppy or from orbit? I cirlced it in my previous posts.

Entirely speculative... Are there or are there not, dune that appear to be the same color and darkness as the streaks right below the cliffs?

Posted by: centsworth_II Apr 19 2007, 10:03 PM

Are those dark deposits in the craters on the left?
I say no, that they are just the same as the craters
on the right but have had light colored dust swept
from them. Do you say that the craters on the left
have actual dark material present in them that is
not present in the craters on the right albeit beneath
a layer of dust?

Posted by: Edward Schmitz Apr 19 2007, 10:07 PM

Mr. Centworth,

Your interpretation of those images are the same as mine. Which don't look anything like what we see at VC. That is dust removal. What we see at VC is deposition.

Posted by: centsworth_II Apr 19 2007, 10:11 PM

Well. I can't argue with that.

Posted by: Stu Apr 19 2007, 10:11 PM

Cents,

I actually think you've helped our case with your pics... the rays coming out of VC broaden, while the "rays" and marks coming away from the craters in your left pic taper to points, suggesting two very different processes.

Thanks!

Posted by: centsworth_II Apr 19 2007, 10:15 PM

The overall process: dust removal by wind, is the same.
What is different is the type of wind: dust devils vs. breezes or gusts.

Posted by: Edward Schmitz Apr 19 2007, 10:17 PM

And, the images from the south pole are definitly deposition and match the patterns seen at VC.

Posted by: Shaka Apr 19 2007, 10:25 PM

O.K. I get the growing feeling that I've fallen down a wormhole and been transported into a forum of another color. I've decided to leave the bickering and to wait until the MER team announces its findings.

Posted by: Stu Apr 19 2007, 10:30 PM

Ok... so I'm standing on the edge of Victoria Crater, just above Soup Dragon, looking over the crater towards where Oppy is now, Occam's Razor clutched tightly in my hand. It's a gorgeous martian day - but nippy, maybe, but the sky is clear and pink and the Sun is shining brightly. I feel a wind on my back, a wind that wafts across the crater and passes over the other side heading north. As it passes over the far cliffs and bays, I'm puzzled... why does it only clear dust away from the ground behind two or three of the bays, and not every bay? Maybe because this particular wind was focussed quite narrowly, and only passed over a few bays? Ok, but there must be winds blowing across Victoria all the time, over every bay, so why aren't they all the sources of dark streaks..? Very curious now, I walk around the crater, towards the streaks... doesn't take long, not as long as those old rovers did anyway... and as I walk up one of the streaks, away from the crater, I wonder how a passing wind could clear away more dust farther away from the bay than it did closer to the bay... shouldn't the wind's strength have diminished? Or did the wind gather strength after passing over the crater somehow?

Standing there I feel a slight tremor run through the ground beneath my feet and I feel an urge to turn around, and do so just in time to see a puff of dark dust billowing up into the air above one of the nearby bays. Maybe, I wonder, there's been a landslide or a rockfall? But then a billow of darker, finer dust puffs up out of the bay, quite violently, and, caught by the wind, the material blows towards me and over me, the fine particles falling softly to the ground, and onto me.

What the frak was that?

A mystery indeed...

Posted by: Stu Apr 19 2007, 10:32 PM

Oh Shaka, this isn't bickering, I think this is pretty good-natured debate

Posted by: fredk Apr 19 2007, 10:46 PM

I agree things are getting a bit out of hand. Based on the evidence we have we clearly can't convince each other on the nature of the Darkness. Both sides feel strongly that their views are the simplest, most economical ideas consistent with what we see.

It would be great if we could find more ideas to rigourously discriminate the opposing models. A while ago I posted that Deposition predicts larger dark grain size the closer you get to the rim. Would sweepers be convinced by such an observation? What else might it take to convince both sides that Deposition is the better model? I could add one more discriminator: the observation with the other instruments of a clear compositional difference between off- and on-streak soil, ie a clear signal of a Darkness of different composition from the rest of the apron. I'd say this is sufficient, but not necessary for Deposition.

I don't recall hearing any such prediction for the Sweep model. Is there a clear prediction of Sweep that would distinguish it from Deposition? What would it take for someone from either camp to be convinced Sweep is essentially correct?

Posted by: Stu Apr 19 2007, 10:52 PM

I don't see any evidence that things are "getting out of hand"... from where I'm sitting this is just a good ol' to-and-fro debate... no name-calling or gloves being slapped across faces... but oh well, I guess I'll just shut up and try and get this sleeping cat off my keyboard... not joking...

Posted by: Edward Schmitz Apr 19 2007, 11:04 PM

Hey Fredk,

I believe the grain size hypothosis would be a strong indicator of deposition. The MI's don't provide brightness or color. Composition via the other instruments is no help. That can occur with either model. I also don't see any mechanism for partial to full burial of the larger berrys and pebbles in the clean sweep model. If more evidence of burial were to surface (no pun intended), would that sway anyone?

I agree with Stu. I don't think this is out of hand. I do agree with Fredk that it seems rather futile without more information. I've made the best case I can given the data at hand.

Posted by: fredk Apr 19 2007, 11:08 PM

Wow, Stu, I felt like I was there! Beautiful description!

It reminds me that, according to the Depositionist model, I don't see why the Darkness needs to have a different compostion than a dark component of the soil we've seen all over the apron, ie the dark stuff that the Sweepers claim is revealed by the wind. That's why I wrote sufficient but not necessary above. Perhaps the Darkness is the same old stuff, but just available inside the NE rim in enough quantity (due to exposure after rockfalls, perhaps? We do see more boulders in the NE of Vicky) or in small enough grain size to be wafted up by wind into the streaks, as Stu described so well.

Would this ease the mind of a Sweeper at all? It certainly sounds economical. I had the impression some here thought that according to the Depositionists the Darkness had to be some unique, bizarre, previously unseen type of material. Shaka wrote above: "Point to the dark material in an MI." Well, in any MI on-streak, it's the stuff between the berries! We all know that. The question is, what is it, and was it deposited or revealed by sweeping.

Posted by: Stu Apr 19 2007, 11:09 PM

One of the (many) reasons I love it here is because it gives me the opportunity to take part in these volleying debates, without fear of being insulted or slapped down. I've enjoyed tonight's discussion, thanks to everyone who stuck their head above the trench!

Posted by: fredk Apr 19 2007, 11:17 PM

I certainly didn't mean to shut anyone up! I only meant we seemed to be going in circles as far as the arguments presented so far. That's why I'd like to hear some new ideas, in particular, what would it take to convince both sides that either model is correct?

Man, I wish I had a cat...

Posted by: Stu Apr 19 2007, 11:36 PM

Here, have this one...

Posted by: centsworth_II Apr 20 2007, 12:55 AM

I expected the smoke monster from "LOST" to appear.

"Standing there I feel a slight tremor run through the ground
beneath my feet and I feel an urge to turn around, and do so just
in time to see a puff of dark dust billowing up..." -- Stu

http://www.lostpedia.com/wiki/Image:23psalmmonster2.jpg

Posted by: centsworth_II Apr 20 2007, 01:33 AM

Maybe I'm not that far from (some) depositionists. My main problem
with deposition is when it invents some new, isolated, dark mass.

It is obvious that the sand consists of dark and light grains. I can believe
that some persistant wind pattern could selectively sort the grains resulting
in areas with an enhanced amount of dark grains. This, combined with the
removal of light dust by the same winds could create both the dark areas
below and on the rim of Victoria. [But] If there is an increase in the percentage
of dark sand in the streak, it is not enough to be obvious to me in the MIs.

Going back to Steve Squyres' description of El Dorado:
"...chemistry very similar to average Gusev soil though somewhat higher in olivine.
This is very clean stuff. Mössbauer mineralogy: lots of olivine, pyroxene, essentially
unaltered, low Fe3+, very clean." http://www.planetary.org/blog/article/00000497/

Note that even the obvious dark deposit of El Dorado had "...chemistry very similar to
average Gusev soil". No new mystery substance.

Posted by: atomoid Apr 20 2007, 01:58 AM

Another way to discern between deposition vs clean-sweep is to look at the mouth of the bays where the bedrock is exposed:

* If there is 'dark material' lying on the bedrock that is not on adjacent bedrock areas that arent dark, then we could bolster the case for depostion, since the bedrock couldn't be hiding lighter dust under a surface layer of dark grains.

* if the adjacent bedrock areas have more dust on them (presumably on top of the dark material), then you could make a better case that the darker areas are swept clean.

...and http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=10166 that the mouths of the bays look just a little bit of 'darkened' so presumably they have a slight peppering of dark material that has accumulated on them. Yes, its hard to tell, and each camp could take it either way, but its the next waypoint as far as im aware, oppy's toe-dip isnt it? so we shoudl have our results soon.

Posted by: centsworth_II Apr 20 2007, 02:01 AM

Why yes, it would, as you can see from my post above.
I'm still not sure if there really is a significant increase in the percentage of dark grains in the streaks
or if the darkness just results from lack of dust. There does look to be and increase in sand -- both light
and dark -- in the streak, so I do see clean (dust free) sand being blown up and over the rim. The dust
having been blown away while the sand still sat below the rim.


Well, some depositionist comments did give this impression.

Posted by: centsworth_II Apr 20 2007, 02:12 AM

I think this is a good idea. A look at grains caught in the the nooks and cracks of the
bedrock at the rim edge, both in streak and non streak areas, could be instructive.

Here's my take on possible sample sites.

(This is a crop from Tesheiner's Opportunity Route Map)

Posted by: dvandorn Apr 20 2007, 03:24 AM

Yeah, that would likely convince me (though, while I still lean towards clean-sweep, I'm not comfortable that it explains the apparent medium-grained cover over the intermediate-sized soil grains in the on-streak soils). If deposition is the correct theory, I would also expect to see a similar relationship in larger dark grain size from the sides of the streaks into mid-streak. Considering the feathered appearance of the boundaries, if the streak comes from the wind's dispersal of a fount of dark material coming out of the crater, then deposition ought to peter out at a predictable rate as you move to the edges in any direction.

That wouldn't convince me as much, since both deposition and clean-sweep could result in clear compositional differences -- especially at the resolution available. If you could prove to me that there are compositional *matches* between the streak soil and the occasional darker spots within the crater, that would convince me more -- but not entirely, since both could be areas where similar underlying material is being exposed by deflation.

Problem is, not even the APXS and Mossbauser are capable of resolving the composition of individual soil factors. And mini-TES is nearly useless for such a fine-scale resolution. We get good data, but there's a limit to its resolution. And composition of individual soil factors is beyond that limit.

I fear that, without being able to say "yes, the soil here has 12% more grains of this mineralogical characterization," or somesuch, the averaged compositional data we get for soil will only fuel speculation, not provide definitive answers.



-the other Doug

Posted by: CosmicRocker Apr 20 2007, 05:22 AM

fredk: Your suggestion that we agree on certain observational tests of each hypothesis is a good idea. I think it is difficult to come up with a simple observation that would satisfy everyone. For example, I thought the simplest way to convince people was to show MIs along the trek where the difficult-to-move sand grains between the berries essentially remained the same, while the smaller dust fragments became sparser as the rover moved from light to dark areas. I thought that was a good test of clean sweep.

Your proposal that "Deposition predicts larger dark grain size the closer you get to the rim" would be very convincing to me, if that process was the only one to operate on the soil grains in the past. I don't think sand-sized grains are being moved much in the current wind regime. Sand-sized and even larger clasts obviously were moved by the wind at some time in the past, but that period in Martian history seems to predate these streaks. For the several years that this rover has been observing the surface, it has only seen evidence of relatively fine material moving with the wind. At some time in the past, however, I can imagine stronger gusts of turbulent wind blowing up the northern bays and moving the sand grains, resulting in a size sorting where a greater population of larger grains remains near the rim, and smaller grains are distributed further out.

If you folks could find evidence of very small, dark grains accumulating widely in the dark streaks, that would be convincing.

Posted by: Stu Apr 20 2007, 07:29 AM

Well, some depositionist comments did give this impression.

That would be me. I'm not sure if the dark material is coming from a vein of familiar stuff, or from a vein of new stuff from elsewhere, very dark subsurface ejecta laid down after an impact yonks and yonks ago. Just thinking outside the crater box. Mind, I really don't think it's that crazy an idea, for reasons given in a previous post. Guess we'll find out soon when Oppy reaches the crater end of one of the streaks. But I do think it's significant that the streaks are all found jabbing away from bays in the area of the crater where (to my untrained eye) we see the most dynamic erosion features - the most eroded cliff walls, the most rockfalls, the most disturbed dust deposits on the slopes, etc...

Posted by: AndyG Apr 20 2007, 09:12 AM

Looking around, you see me standing on the lip of the crater, one bay along, in the act of launching a model glider into the wind. It's an odd-looking thing, with large but low aspect ratio wings - something to do with the Reynold's Number, you think.



I'll be tacking the plane in front of the slope - to the left of the diagram, where the thin air is forced upwards - in order to gain height.

For fun, and with enough altitude, I'll sometimes fly it away from the crater, turn back into the wind, and do a fast pass over our suited figures. But what I won't do, is fly low and into the rotor you get just behind the lip of the crater. This is an area of sink: it can be unpredictable and fatal.

You can tell where the sink is, as it's where the wind-borne dust gets preferentially dumped on the ground. Behind it, away from the lip, the airflow returns to something a little more laminar, where the dust can be cleared.

Andy

Posted by: Stu Apr 20 2007, 09:20 AM

Now THIS is fun! Flying gliders on the edge of a crater on Mars!

I LOVE this place!

Seriously, that made a lot of sense and cleared some things up for me, so thanks for that Andy.

Posted by: climber Apr 20 2007, 10:44 AM

No sign of solar panel clean up yet ? This will be the definitive answer.

Posted by: djellison Apr 20 2007, 11:23 AM

Gliding isn't easy on Mars ( as anyone who's flown X-Plane will tell you ) - but I have flown RC gliders off terrain not that different to the Bays and Capes of Victoria. You have to fly SO fast through the thin Martian atmosphere to get the lift to stay up - but your motor (gravity) is so weak. It's an interesting engineering challenge though. Planes on Mars might look a lot like the U2 Spy plane.

Doug

Posted by: Juramike Apr 20 2007, 03:47 PM

Could small vortex (not a dust devil) form in the bays and help lift dust?

This may explain the dark patch inside the NE bays. Maybe little vortexes form (not large or self-sustaining enough to be a dust devil) in these particular bays due to local topography.

This gives a localized cleaning event on the rocks of the bays, as the vortex lifts out it disperses into downwind gusts which gives a cleaning event on downwind of the bay mouths.

So the darkness in the bays may be due to cleaning events in the bay when then translate to downwind of the bays. The orientation/shape of the bays just happens to prefer vortex formation.

If the prevailing wind direction shifted, another set of bays somewhere else on the crater would get cleaning events.


So, what observation would eliminate this hypothesis?

-Mike

Posted by: fredk Apr 20 2007, 04:05 PM

That's more or less the original Sweep idea, as I understood it. What got me thinking about this a long time ago was that it seemed very odd that you'd get that kind of vortex cleaning from the NNE bays but not from the NW bays, like Duck Bay and neighbours, which appear to be symmetrically placed with respect to the prevailing winds (recall my http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=87495).

Of course we can't say there isn't something odd about the local topography in the NNE that results in far more/stronger vortices, but it has always seemed like a stretch to me.

Posted by: Stu Apr 20 2007, 04:11 PM

Okay, here's our "playground"...

It's clear that there are winds wafting and wufting about in this NNE area because the dark material here is sculpted into dunes. I guess the question is, are the wind effects here strong enough to make the leap up out of the crater and clear the streaks we see?

And if the shape of the bays "channels" the wind, wouldn't the streaks be aligned with the bays, and not jutting away from them at the broken leg angles we see?

Posted by: dvandorn Apr 20 2007, 04:20 PM

The angled plumes of the dark streaks are indicative of a "racetrack" wind flow within the crater that breaks out at the north and northeast bays.

Think of a bowl of soup. Half-fill a bowl that has several scallops in its rim and begin swirling the soup in the bowl -- in this case, counter-clockwise. When the soup rises high enough along the sides of the bowl, it will stream out of the lowest scallops in the rim and continue on in a line that starts out tangential to the rim, but that sweeps out from that tangent as the materials flows outward. Leaving exactly the kind of "broken leg" pattern we see at Victoria.

Martian craters seem to be very good at "spinning up" the winds within them into racetrack patterns. If the coriolis forces are strong enough, and if the surface heating conditions are right, this spin-up can cause organized vortices (i.e., dust devils) to form. Even when dust devils don't form, though, most craters still develop these racetrack-style wind patterns. It's quite common to see signs of such racetrack patterns as they blow out of low spots in crater rims.

-the other Doug

Posted by: Stu Apr 20 2007, 04:22 PM

Thanks for explaining that t'Other Doug; makes more sense now.

Posted by: dvandorn Apr 20 2007, 04:31 PM

Here's another little gedankenexperiment for those of you who are having a hard time understanding why there are only streaks from some of the bays, and not all of them...

Prop a bicycle up so that one of its tires is free to rotate, without touching the ground. To take gravity out of this little experiment, set the bike on its side so that the wheel's axle is perindicular to the ground. Now take a garden hose with a spray attachment and spray a high-pressure stream tangential to the tire, to spin it up.

You'll find that the water which stays on the wheel as it turns will be sprayed off primarily from a fairly small arc of the wheel, about 120 degrees in the direction of rotation from where the main stream is hitting the tire.

It has to do with momentum and how long the material can be accelerated before the force holding it to the wheel (adhesion, mostly) is overcome by inertia. And it has to do with the original vector and location of the water stream.

Similarly, wind spun up inside a shallow bowl isn'g going to stream out of every available bay equally -- it will spin out where its momentum tosses it out, and since the pressures within the crater aren't even, that will be somewhere between 120 and 270 degrees "down-rotation" from where a majority of the wind enters the crater.

So -- the answer as to why Vicky streams her streaks out where she does is because of the direction of the prevailing wind, where the wind primarily sinks into the crater, and where the pressure bulge of that wind's entry tends to push it back out.

-the other Doug

Posted by: Edward Schmitz Apr 20 2007, 04:55 PM

doug,

explain how the streaks in fit into your model...

Posted by: fredk Apr 20 2007, 05:01 PM

Dvandorn, there's certainly no difficulty explaining the asymmetry of the dark streaks given an asymmetric wind flow within the crater! But the problem is that the prevailing wind defines a single direction, and does not by itself give preference to "to the left" or "to the right" of that direction. You need to break that symmetry somehow. Ie, why should the wind flow counterclockwise inside the crater rather than clockwise? It's not obvious how a more-or-less circular crater could break that symmetry in a significant way, without an odd dependence of wind on subtle details of topography.

Coriolis could do it in principle, but should be important only on very large scales, like 10s of km and up.

Posted by: dvandorn Apr 20 2007, 05:18 PM

I don't think the bays give the asymmetry here -- I think it has to do with the uneven height of the rim. Wind will enter the depression along the lowest points of the rim, and that likely determines how and where it spins up its racetrack pattern.

In fact, there are some indications that, at some times of year, the racetrack wind pattern in Victoria changes from counter-clockwise to clockwise. Look at the muted streaks coming off the western side of the crater -- those seem to indicate a spin in the opposite direction from the streaks off the north-northeast side of the crater.

Hey, maybe that explains why Victoria has bays and other, similar-sized craters in this same area do not. Victoria is situated on a slope, and that slope allows its internal wind pattern to shift seasonally. So, instead of a billion years of counter-clockwise rotation, it sees seasonal switches in wind direction, which enhances aeolian erosion along the crater rim. So, the same winds that would otherwise have reinforced the circularity of the depression by eroding everything in the same direction all of the time instead do this seasonal push-pull routine, which encourages greater and greater erosion along the rim. Probably also generated the overhangs and undercutting we see in the cape cliffs.

-the other Doug

Posted by: Juramike Apr 20 2007, 05:48 PM

Thanks all, for an interesting discussion!

Victoria crater does seem to have an assymetry in the bowl shape. Looking from straight down, the S side seems to have a shallower slope then the north side of the bowl. (The sand dunes at the base also seem situated slightly to the North of center).

Could the assymetry in the bowl shape, coupled with a prevailing wind direction from a vector different to the N-S axis, be enough to cause the effects we are seeing?


-Mike

["Gedankenexperiment" must be German for "will be fun on a summer day!"]

Posted by: fredk Apr 20 2007, 06:05 PM

Again, it certainly is possible that topography, including rim height, breaks the symmetry. But remember that Meridiani is extremely flat. From the newly released topo map the rim heights vary by only a few metres, compared to 70 metres or so total depth.

My appologies for sounding like a broken record one day after complaining we were going in circles with these arguments!

Here's a way to maybe break the deadlock. Perhaps someone could build a 3D model of Victoria, then perform some tests by letting wind blow over it and tracing the airflow with smoke. I don't know what would be involved in capturing the dynamics on such a different scale. Perhaps the test would need to be done underwater. Dip your model in a running stream and inject squirts of food colour or silt particles or something. Definitely a fun way to spend an afternoon!

Posted by: AndyG Apr 20 2007, 06:56 PM

I would almost be buying this. And I'd purchase the (wet?) wind tunnel tests too. Stu - time to dip the plasticine Victoria in the bath and pull the plug.

<rhetorical>How many forums could I meaningfully say that last line in???</rhetorical>

Doug - the RC issues. I personally think the pressure/gravity situation is a killer for realistic small flight on Mars. But there's always a good side: you'd be hard-pressed to find radio interference on your chosen channel.

Andy

Mars and UMSF: you've got to love this place!

Posted by: Juramike Apr 20 2007, 07:40 PM

How 'bout a bike trip down tp to the beach on a day with a constant breeze?

We can build a model of Victoria in wet sand in the right orientation, then dribble sugar sand near the rim of the model and see where it ends up.

(Then we do the Gedankenexperiment to wash the sand off our bikes!)

-Mike

Posted by: Phil Stooke Apr 20 2007, 09:53 PM

dvandorn:
"Here's another little gedankenexperiment ... Prop a bicycle up so that one of its tires is free to rotate..."

Edward Schmitz:
"explain how the streaks in fit into your model... "

Uh... They're the spokes?

Phil

Posted by: Juramike Apr 20 2007, 10:24 PM

I just tried the mind-numbing exercise of going through the MI images and counting representative zones and putting grains in "bin-colors" in order to quantify if there was any difference between on-streak particle count and off-streak particle count.

I figured the ratio of brightest to darkest should be independant of lighting.

There may be a difference, but it might be really subtle. Again, I'll leave the particle color breakdown to image-meisters and their software toys.


I was almost able to convince myself that it "looks" like there is a concentration of dark particles in wind shadows of the beads. I was also able to convince myself that this has been seen in many other images, whether on-streak or off-streak. If this is real (I need a second opinion - I'm pretty much seeing inkblot images of my third-grade bully at this point), then perhaps there is a dark material being deposited all over in little pockets near berries. And perhaps this dark material is just a little bit more concentrated in the on-streak area than off, but just enough to make a difference. We're talking just a few % here.

Has anyone quantified the gross color difference between the two zones? Assuming that the color difference were due to jet black particles, how much % increase would be required?

Here's a picture that "seems" to indicate dark stuff in the windshadow of the berries or is it just shading.

Only the Shadow knows for sure....

-Mike

Posted by: Juramike Apr 20 2007, 10:30 PM

And while perusing MI's I also came across similarities in MI images between Victoria apron, and those near Vostok. The ("polished"?) aspect of the berries seems very similar.

-Mike

Posted by: Stu Apr 20 2007, 10:34 PM

Andy, I might have to get The Model down off the wardrobe...

Ah, happy days, happy days! When we were all still open-mouthed with wonder at the first real sight of Victoria...

So, I pour flour in, point a hairdryer at it from one side, and see what happens...?

Posted by: atomoid Apr 20 2007, 11:08 PM

Thats food for thought, but i took a different track here (but are there any meteorologists in the house that can help?):

* The racetrack winds are actually spinning clockwise: Duck Bay and Bottomless Bay form the preferential exits for any relative pressure building up in VC due to the prevailing winds, simply because they are gradually sloped and seem to be the lowest points. Since the air pressure primarily exits there, the air speed becomes faster on the left side of the crater, this causes a net clockwise rotation inside the crater which will tend to push the dark grains over the edge of the little humps formed by the debris from the cliffs and pile up the the rotational windshadow on the cliff slopes. http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=10181

* the reason why the dark stream changes course and veers to the west is merely due to the prevailing winds which are flowing in the 'general' direction the the streaks end up pointing. The air after exiting the crater becomes part of the larger windstream and follows that course, forming the deflected streak. From looking at http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=10182, it seems apparent that there is no deviation from this process since any air should exit the crater according to the crater local topography (radial from the crater) and then merge with the prevailing wind direction. The apparent deviation to this flow in the lower right looks more like a contrast artifact from the lighter evaporite dust that has blown onto the SE.

* so why arent there dark grains on the left side of the crater? There might actually be darker material eroding from the steeper cliffs on the NE than there is anywhere else in the crater for this reason: Note that the steep areas, which are probably not coincidentally associated with the dark streaks, all are formed along what is arguably the most linear feature in the crater http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=10181. his suggests this might be a fault or other feature that might have some concentration of one type or another material or might be more easily eroded.

Posted by: lyford Apr 21 2007, 12:15 AM

Ah, following the rigorous http://www.unmannedspaceflight.com/index.php?showtopic=514&st=0&p=2494&#entry2494 of our illustrious founder?

Posted by: fredk Apr 21 2007, 04:54 AM

http://qt.exploratorium.edu/mars/opportunity/pancam/2007-04-20/1P230275243ESF81OBP2536R1M1.JPG of Alicante shows more berries in the lee of the rock compared with http://marsrovers.jpl.nasa.gov/gallery/all/1/p/1143/1P229654197ESF81NEP2599L7M1.JPG They must have been revealed when the instruments touched the surface.

It's interesting, because that means that many of the berries in http://marsrovers.jpl.nasa.gov/gallery/all/1/m/1148/1M230096787EFF81O7P2956M2M1.JPG were originally burried. This view is a closeup of the lee deposit. You can see this same region in http://marsrovers.jpl.nasa.gov/gallery/all/1/m/1145/1M229830466EFF81O7P2956M2M1.JPG taken earlier with fewer berries.

It's remarkable - I never would have guessed that the berries in the first MI were burried before we exposed them.

Posted by: dvandorn Apr 21 2007, 05:56 AM

There is absolutely no sign of APXS or Mossbauer contact plate impressions on the soil in the area in question, in your "after" shot. And besides, in every other experience we've had with Meridiani soils, when you press down on the soil, blueberries are shoved *under* the fines and disappear from view.

Oppy didn't exhume those blueberries. There is no conceivable mechanism by which they could. Especially not without any other signs of soil disturbance. (Maybe, just MAYBE, if you RATted the soil, you could exhume buried berries. That has very obviously not happened here.)

If some berries are visible in the "after" picture that weren't visible in the "before" picture (and I agree, there are), then the dust that had been covering them has to have been swept off.

By the wind.

Clean sweep party, anyone...?

-the other Doug

Posted by: centsworth_II Apr 21 2007, 06:13 AM

Well, Opportunity was doing something over there.

Posted by: AndyG Apr 21 2007, 08:04 AM

I'd agree that the topography itself is subtle enough not to be the cause of this - but there is another factor.

Just before dawn at Victoria, the crater has cooled all night, and is going to be more or less filled as a hollow with a pool of cold, (relatively) dense air. The prevailing wind will blow more-or-less unhindered over this.

Dawn.

Shortly after the sun rises it's going to be beating straight onto the western cliff faces - they'll never get more insolation than this - while the eastern ones are still in darkness. As the western rocks heat up, they'll warm the air next to them. Rising air on the west side of the crater will draw air down from the east side. Add in the prevailing wind (anything southerly) think about the vectors at the interface, and the air in Victoria will tend to rotate clockwise.

Throughout the morning, this clockwise flow will peak and then will lose power, and sometime after noon it will stop when the western cliffs lose the sun. In the evening the flow is reversed (warm east cliffs, cold west ones), and as dusk arrives, the flow will be at its strongest in an anticlockwise direction.

Question - given the potential diurnal movement of air within Victoria, what does this say about the deposition or cleaning events? My gut reaction is that the greatest mass loss (dust sourcing) from the walls of Victoria will be caused when those rocks change temperature the most. The western walls at dawn (clockwise flow), the eastern walls more subtly at dusk (anticlockwise flow).

Andy

Posted by: fredk Apr 21 2007, 04:04 PM

The daily heating cycle is a good point, Andy. It's not obvious to me how rising air on one side leads to either clock- or counterclockwise flow, but in principle there could be an asymmetry due to what I assume would be more rapid heating on the west side in the morning than on the east in the evening. Another factor might be wind speed or direction changes during the day.

Posted by: fredk Apr 21 2007, 04:33 PM

Actually there is an imprint visible on the soil. It is visible in L7 and R1, so it's not an artifact:

You're absolutely right that normally an imprint buries the berries. But recall http://www.unmannedspaceflight.com/index.php?s=&showtopic=4081&view=findpost&p=88292 where I pointed out that the wheeltracks looked very different on-streak compared with off-streak. The berries were still visible in the tracks on-streak. That is exactly the kind of anomalous behaviour that you've pointed out here, in the lee of Alicante, with berries becoming visible after contact is made.

What all of this points to is that the behaviour of the fine soil on streak is distinct from that off-streak, and unlike anywhere else to my recollection. That suggests something unique might be going on here, and perhaps deposition fits the bill.

Posted by: centsworth_II Apr 21 2007, 05:03 PM

Heating leads to dust devils in some smaller craters.
perhaps it leads to sub dust devil currents in larger craters.
Maybe not a circular current within the whole crater, but
a current within an individual bay.

Posted by: fredk Apr 21 2007, 05:48 PM

In addition to my previous comment, I'll add that what you're proposing here is the removal of the relatively coarse dark grains by the wind, not just light dust (of which there's very little on-streak) as Sweepers normally invoke. In other words, you're invoking winds powerful enough to do what Depositionists require!

One more point. We know the arm touched the region where the new berries appeared. What are the odds that wind would happen to expose the berries at that very spot just after we examined it with the arm? Can you find any other areas where berries have been exposed? Occam argues that the simplest explanation is that the berries were exposed due to the action of the arm.

Posted by: dvandorn Apr 21 2007, 06:11 PM

Come up with a believable mechanism by which contact with the arm could have generated the *opposite* result to what we've seen every other time it's been pressed into the soil, and I'll grant you the point.



-the other Doug

Posted by: fredk Apr 21 2007, 06:49 PM

Well, we're in an interesting situation here, all right. We can see with our very eyes that berries have appeared after the arm did its thing. The only explanations we have yet are:

1. Bizarrely timed and unusually strong wind gust happens to expose the berries just at that spot just after we were there, or

2. Unusual property of soil/substrate allows buried berries to be exposed.

Given the unusual appearance of the rover tracks on-streak, we have corroboratory support for 2.

I'm no soil expert, but couldn't you imagine a hardish layer below the berries, so that when you press down on the surface the berries can't sink much, but you either compress the fine grains or expell them outwards and expose the berries?

Again, perhaps it's time to get out some flour or icing sugar or whatever and some ball bearings and see what you can come up with!

Posted by: Edward Schmitz Apr 21 2007, 07:00 PM

The soil here is highly mobile. Much more so than any place else we've seen. The exposed berries are not where the contact was made but just next to it. We've all played in the sand. There is a big difference in the way that loose sand acts as opposed to crusty sand.

There are not other spots that have any different in the two images. It's not credible to suggest that we just happened to pick the spot very spot that would show a difference.

As the instrument pushes down, a berm is pushed away from it. The pressure wave hits the near side of the berry and the sand next to it at the same time. Since the berry is much larger, the far side is pushed long berfore the sand grains on the far side see the pressure wave. They pop right out.

Posted by: Edward Schmitz Apr 21 2007, 07:02 PM

btw: Fredk, that was a great catch...

Posted by: Edward Schmitz Apr 21 2007, 07:06 PM

Here's something else. The lee of the stone should be a low wind area. if there was a gust that would uncover stuff in the lee area, shouldn't it have disturbed other areas as well?

Posted by: ElkGroveDan Apr 21 2007, 08:44 PM

From the UK IMAX discussion:

Posted by: Stu Apr 21 2007, 09:29 PM

Hmmm...