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

[fredk]

Does anyone know if El Dorado was thought to be deposited dark sand or dark sand swept clean of light dust? "Aeolian cul-de-sac" sounds like a place where winds stagnate and drop deposits, rather than where winds are fast and lift off light dust. If El Dorado is depositional, then that supports the streaks being so too, since the grain size in the streaks is smaller than at El Dorado.

[Shaka]

Does anyone know if El Dorado was thought to be deposited dark sand or dark sand swept clean of light dust?

Both. It is depositional for large basaltic sand grains, and erosional for smaller, lighter grains of dust. In a past period of strong prevailing winds from a southerly direction, sediment of all sizes (including particles eroded from the vesicular basalts) were blown up against Husband Hill. The wind was strong enough to blow the fines up and over the hill (some got dropped up behind the summit in dunes), but the heavier basalt grains a significant fraction of a millimeter in diameter didn't make it. They blew partly up the hill in gusts and then rolled back, up and back, over and over until they got beautifully round and clean and sculpted into a dune field that could not climb over Husband Hill, and was such an effective light trap that it fooled a guy from Portugal into thinking it was an abyss.

Sure, you can say the Victoria streaks are both depositional and erosional in that sense, as well. Obviously there is sand and concretions in the streak and they had to get there some way. In the past they must have been blown there by some powerful winds - and deposited - but we have been debating why the streaks look darker than other parts of the annulus now. Every Martian year or three dust storms spread light pink dust over everything. The VC streaks are obliterated. They only reappear gradually over subsequent weeks as the prevailing southerly winds (winds less powerful than the storm winds) blow through the crater, circulate in some complex pattern molded by the crater topography, and funnel or rotate up and out through The Valley without Peril and adjacent bays, lifting and carrying the pink dust with them. Thus the streaks reappear through an erosional process, a process that cleans the dust from the underlying berries and sand. This dusty wind polishes the berries to a high sheen.

Why don't the winds funnel through and clean Bottomless Bay and Bay of Toil as well? Well, they do to a minor degree - you can see faint streaks if you look hard - but the winds probably aren't as strong through the western bays because the crater is higher here (remember Beacon?) or because of some other topographic asymmetry.

Parsimony favors the Clean Streakers because they don't have to postulate a discrete source of dark material being eroded out of the NE cliffs. Where is the source? What is it? I can't see it. I'm still waiting to see a Pancam shot with an arrow pointing to an outcrop of basalt or coal or god-knows-what black stuff. I'm not holding my breath.