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Nature of Victoria's dark streaks, swept clean, deposited, or other?
dvandorn
post Apr 3 2007, 05:12 PM
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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


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BrianL
post Apr 3 2007, 06:20 PM
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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. biggrin.gif

Brian
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Shaka
post Apr 3 2007, 07:23 PM
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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?


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imipak
post Apr 3 2007, 08:02 PM
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QUOTE (BrianL @ Apr 3 2007, 07:20 PM) *
Ooh, I love adding my completely geologically untrained, marginally informed opinion to any question that divides us into camps.


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!" smile.gif


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ElkGroveDan
post Apr 3 2007, 08:07 PM
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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).


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Juramike
post Apr 3 2007, 08:35 PM
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QUOTE (dvandorn @ Apr 3 2007, 01:12 PM) *
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.



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


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helvick
post Apr 3 2007, 09:03 PM
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QUOTE
What do you think is particularly special about the dark streak areas?

They extend from bays that act as turbulence causing funnels given the predominantly ~SSE to ~NNW prevailing wind direction.
QUOTE
or do you think the local windspeed is faster in the dark streak area?(again due to turbulence effects?).

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.
QUOTE
Why are the dark streaks on the North side of the crater, but not on the South side?

Prevailing wind direction.
QUOTE
Where would you expect to find other localized dark streaks in Victoria crater?

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

Yes
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Juramike
post Apr 3 2007, 09:18 PM
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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


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fredk
post Apr 3 2007, 09:30 PM
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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. blink.gif 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:
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.
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fredk
post Apr 3 2007, 09:47 PM
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QUOTE (helvick @ Apr 3 2007, 09:03 PM) *
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.
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?
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helvick
post Apr 3 2007, 09:56 PM
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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.
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fredk
post Apr 3 2007, 10:05 PM
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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.
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MarsIsImportant
post Apr 3 2007, 10:08 PM
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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.
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Juramike
post Apr 3 2007, 10:14 PM
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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
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Guest_Edward Schmitz_*
post Apr 3 2007, 10:27 PM
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- 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.
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