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

[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...