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

[CosmicRocker]

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.

[ngunn]

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

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?

[CosmicRocker]

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.

[ngunn]

To me, the shadow appears to be on the right side of the ripple crest, which is the side facing away from the sun.

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?

[Guest_Edward Schmitz_*]

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).

[Gray]

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.

[ngunn]

The sun is almost directly behind the camera.

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 . . .

[CosmicRocker]

... 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.

NOW, you tell me.

... ...probably think I'm barmy. ...

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.

[dvandorn]

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

[Juramike]

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

[Shaka]

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

[CosmicRocker]

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.

[Doc]

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.