Wow! I wasn't expecting my posting of Squyres's comments to open up new arguments on this! (Even from some who didn't participate much earlier! ) I have always felt the central tenet of Depositionism was that dark dust sand blew up out of the crater and deposited in the streaks. That has now been confirmed by Squyres. Further issues such as how much cleaning of light dust goes on (there will always be some), are secondary and there doesn't seem to be much prospect for resolving them now (unless perhaps we return to the streaks after some time, though that seems unlikely from Squyres' comments).

It still would be interesting to hear more from SS on this, perhaps that's a topic for another Q+A...

I congratulate the Sweepers on some very good arguments the past few weeks. We had some very stimulating exchanges and you sure got me thinking hard about this.

I'm happy that depostion is at least part of the answer (confirmed by SS).

Regardless, I'm sure some sweep is involved. I just never thought it was a clean sweep. How much is depostion and how much is erosion? "The answer my friend is blowin in the wind."

Well - there has to be as much material moved away from these areas as there is being deposited - or they would be visible ridges wouldn't they? I can't get that principle out of my head - am I missing something?

Doug

Errr - surely that depends on what the rate of deposition and the duration of deposition are. Deposition does not require a large amount be deposited. The sources of dark dust within the crater may shift with time. The winds may shift with time. A million years ago there may have been dark streaks out of different bays or no streaks at all.A point others have tried to make again and again, too!

After looking at the topo map it looks as if the streak bay exits are at least 5-10 (is that in meters? if the topo units legend is any guide) lower than the Duck or Bottomless Bay exits, so this could explain why there are not streaks at most of the other apparently downwind bays.

Although counter-intuitively, this somehow must be setting up what is still a net Clockwise air rotation inside the crater, since the interior dark deposits have collected on what appear to be downwind slopes of the mounds inside the crater. Considering that the streaks bend into what is to be inferred as the predominant wind direction, there has to be something that can explain both the exterior and interior asymmetries.

If we are to assume SS's contention that the streaks are composed of just the normal dark plains basalt material, blown in from abroad and somehow sorted by the wind dynamics into these features (if that is indeed the implication), and not sourced from new erosion from the walls of VC, then why no dark deposits at other areas of the crater, why are they only below the dark streaks, can sorting explain this? the article does say: "the streaks were sand blown out from the crater floor" that is, specifically not the Crater walls.

On the other hand, if the dark material is instead locally sourced, there seems to be a perfectly placed supply in the cliffs below to the streaks, which are undergoing enhanced erosion as evidenced by the boulders and steepness.

Surely you don't mean "dark dust". That would really muck up
the discourse! Can we stick with dark sand and light dust?

Pun intended?

"As this rover continued to the last planned stop on its clockwise tour around the
rim of Victoria... it dusted off any doubt as to the source of the streaks."
http://www.planetary.org/news/2007/0430_Ma...ate_Spirit.html
(from the second paragraph)

Oops! Yeah, I didn't mean to introduce something new. I should've said sand.

Still, this reminds me that the MIs showed the dark grains to be pretty small (much smaller than the grains at El Dorado). I haven't actually measured them and I don't know if there are official size ranges for "dust" versus "sand".

several billion years? Why do you think that this has been going on that long? Why not several thousand? Or how about just serveral years? We don't know how long this has been occuring.

I'm not trying to over-simplify or even to simplify at all. I have always been will to concede that erosion may be occuring. I just don't see any indication that there is any more erosion occuring in the streaks than off the streak. The dark material is falling out because of weaker winds. There is no reason that the wind would be any stronger on the streak than off.

This is my interpretation of the mounding on the streaks.

By the definitions I'm familiar with, it is dust. It is definitely larger than the "light toned dust" that is ubiquitous on Mars. It is close to the lower limit of sand defined by wikipedia.

http://www.engineeringtoolbox.com/particle-sizes-d_934.html
Here, the minimum size of beach sand is given as 100 microns.
The maximum size of concrete and coal dust is also 100 microns,
so 100 microns looks like a good dividing line.

http://astrogeology.usgs.gov/Projects/MER-...pic_imager.html
From this site: "The following images are simulated MI images (at 20-microns/pixel)..."
I don't know if this is representative of the reolution of the images from Mars,
but if it is it looks like 100 microns is at the very low end of what is visible as
individual grains in the MIs.

Oh great. "You snooze, you lose," as the saying goes. I came home from work tonight and was deluged with new information from Mars and elsewhere, not to mention several new pages in this thread. I think I'll wait for the paper, but what SS said seemed pretty clear.

If you thought the dark streaks at Victoria were strange, take a look at these on a new THEMIS image from Odyssey...

The streaks look cool.
The area looks very frosty so I'd guess winds are hitting prominent features in the landscape and causing turbulence downwind causing defrosting.
Just a guess as I said and they are nice paisley patterns.
Roy F

Clearly depositional.

Seriously, it's an incredible view. I take it this is polar frost?

Glad y'all like it, here's the link...

It looks like much of the darkening occurs near the ridgetops of dunes, as the frost sublimates. And furthermore, as that sublimation occurs, it seems that it destabilizes the crest, and dune grains (sand?) roll downhill. You can see the dark areas sloping away from the crest, with bits of debris at the bottom (downhill side) of the dark patches.

David

It appears to me that Oppy has recently been traveling over small ripples, similar to some seen early in the mission. She is also traversing an area where a fainter dark streak extends northwestward from Bottomless Bay. I'm not sure without a new map from Tesheiner, but it seems that the rover was in the same position on sol 1199 as it was on his 1198 route map.

It seems that this is a place where ripples have a lighter side and a darker side. The latest foreground pancams are interesting. Is this a light/dark boundary?
Click to view attachment

There is a curious gradation of grain size from the tops of the crests to the troughs.

Given the angle of illumination these look more like grooves than ripples. If that's an optical illusion created by albedo contrast it is a curious one indeed. Anyone got an anaglyph?

ngunn: How about two anaglyphs? On the left is the L7R1 pair, and on the right are L2R2.

Gray: I would have cut off an arm or a leg and burnt it as an offering at Mount Ithaca if I thought it would have gotten us an MI from sols 1198-1199. That little field of ripples that Opportunity crossed there is very visible on Tesheiner's HiRise route map.
Click to view attachment

Thanks, and very interesting they are too. I notice the 'impossible' shadow on the sunward side of the ripple only appears on one of the two. Can you explain what this means in beginner's language? At the risk of sounding like Oliver Twist - perhaps colour would help?

You must be looking at it differently than I am. To me, the shadow appears to be on the right side of the ripple crest, which is the side facing away from the sun. I don't really know why it appears more prominently in the L2R2 anaglyph, except to say that the two pairs were imaged at two very different wavelengths.

Well it looks to me as if individual 'berries' in that image are all illuminated from the right and slightly above, whereas the ripple 'looks' as if it is illuminated from the left. Where am I going wrong?

The sun is almost directly behind the camera. You can tell because the camera mast shadow is in the image. That means that the side of the ripple facing the camera is also the side facing the sun. The berry shadows do have a slight offset in the opposite direction. If the image was centered on the camera mast shadow, you would see all of the berries would have their shadows offset toward the center of the frame (towards the shadow of the camera).

What a difference the anaglyphs make. Now I can see that the larger grains are in the low areas and the finer ones are on the ripple crest - makes sense. Thanks for preparing it Cosmic.

I'm not sure about sacrificing an arm and a leg. If you did we might have to turn you into a MER-like robot.

Thanks for that E.S. After reading your post and a bit of head scratching I now see how this counterintuitive effect comes about. However even when I understand it it still looks peculiar, at least to me. You regular scrutinisers of Martian surface details probably think I'm barmy. A strategic retreat to the outer solar system is in order, I think . . .

NOW, you tell me. I won't. My brain has inverted the topography many times on orbiter images and MIs. I'd hate to tell you what I thought I had discovered on a couple of occasions.

Yeah, Tom, I hear you -- like the time I thought I had discovered shoulder-to-shoulder tiny craters in one of Spirit's first MIs of the soil at Gusev. I had to whack myself in the head (nearly literally) to see that they were actually tiny pebbles, not craters...

-the other Doug

Publications of Opportunity's study of Victoria's wind streaks:

From the abstract the full paper:
"Opportunity's observations suggest that the Victoria wind streaks are deposits of basaltic sand blown out of the crater from the dark dunes nestled below the crater rim, particularly at the base of the alcove leading up to the darkest streak.

Geissler, P.E.; Johnson, J.R.; Sullivan, R.; Herkenhoff, K.; Mittlefehldt, D; Fergason, R.; Ming, D.; Morris, R.; Squyres, S.; Soderblom, L.; Golombek, M. Journal of Geophysical Research, artilcle in press (2008). "First In-situ Investigation of a Dark Wind Streak on Mars." (pay-for article, abstract not yet available)

and the LPSC abstract:
Geissler, P.E.; Johnson, J.R.; Sullivan, R.; Herkenhoff, K.; Mittlefehldt, D; Fergason, R.; Ming, D.; Morris, R.; Squyres, S.; Soderblom, L.; Golombek, M. LPSC 39 (2008) Abstract 2286. "First In-situ Investigation of a Dark Wind Streak on Mars." (freely available here)

The full paper gives an overwhelming amount of evidence to support their conclusions, including:

• MI images of off-streak on on-streak terrain (including how the blueberries got pressed in by the Mossbauer)
• Albedo and photometry of off-streak and on-streak terrain
• Elemental abundance differences by APXS
• Thermal inertial measurements (higher thermal inertia in the streak, consistent with coarser grains)
• Wheel tracks in off-streak terrain and on-streak terrain (off-streak blueberries get pressed into the dust layer, on streak the blueberries don't get pressed into the dark sand layer)
• Rim ripples near small depressions in the streak area
• Observed erosion of lee deposit near rock (Alicante) during imaging campaign (observed in both Pancam images (this tread, post 307) and in MI images.) thought to be due to the presence of the rover.
• Mossbauer spectra (which indicates low dust abundance)

The basaltic sands coming from Victoria are thought to have been trapped there from the surrounding plains. It is the swirling of these sands that may cause the erosion that creates the scalloped rim of Victoria.

-Mike

I can't wait to read this one. Until I do, I'm still a "clean streaker".

I was a clean-streaker, too. The full paper sounds like a must-read to me, but it's one I'll need to go to the library to access. Thanks for pointing it out, Mike.

I read the LPSC abstract several times last March, and came away less than convinced each time. The swirling sand erosion hypothesis is interesting, if it can explain the scallops all around this type of crater.

Exactly, I was thinking the same thing.