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

[dvandorn]

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

[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.