Nature of Victoria's dark streaks, swept clean, deposited, or other? Options

[Shaka]

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.

[atomoid]

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:
[attachment=9947:attachment]
(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 heres a link to his post)

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.

[Shaka]

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,

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? )

so im officially camping in the dirt.

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

[fredk]

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.

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...

[MarsIsImportant]

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.

[CosmicRocker]

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.

Ooh, I love adding my completely geologically untrained, marginally informed opinion to any question that divides us into camps. ...

You really should consider a career in politics, if you haven't already.

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...90P2395L2M1.JPG
(see bottom of frame.) And there's the dark dune material inside the NE rim.

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.

... That would suggest that the whole area where the streaks are is saturated with blueberries beyond normal concentration for Meridiani. ...

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.

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

There's no doubt in my mind about that.

[MarsIsImportant]

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.

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.

[antoniseb]

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.

[MarsIsImportant]

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!

[CosmicRocker]

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.

[Juramike]

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

[MarsIsImportant]

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.

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.

[fredk]

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...00P2588L7M1.JPG
with this pancam from sol 995, near the Beacon:
http://marsrovers.jpl.nasa.gov/gallery/all...WUP2461L7M1.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 this sol 1135 MI 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!

[fredk]

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. Perhaps the splitting of the blueberries on impact created reflective surfaces, and those are the sparkles we see in that image!

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...LOP2587L7M1.JPG

[MarsIsImportant]

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.