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

[fredk]

I was looking for some HRSC imagery that might show some changes over time. The resolution of what I found was not impressive, and the changes might be more related to lighting, atmospheric conditions, or something else; but there are large differences between the two images in this animated gif... I'm not sure that these help us understand very much, though.

Thanks for this, Cosmic! I think I'm more optimistic than you, though. If you look at these two images, the two MOC images I posted earlier, and the Hirise images, I think there is a pattern:

My first and your second frames were both taken during southern summer, and the streaks in the NNE of Victoria are not very dark in these frames relative to other features. My second, your first, and the Hirise images were all taken during southern winter, and all show the NNE streaks as very dark.

My picture is that during southern autumn or winter, winds blow towards the NNW, and gather dark sand/dust from inside the NE rim and deposit it outside in the few darkest streaks. Then in southern spring/summer, winds blow towards the SE, and the darkest streaks are coated with the ubiquitous light dust.

If this is anything like correct, we're lucky we got here in time since we're now well into southern spring and the dark streaks may be getting covered in light dust. This may help explain why the first streak appeared less distinct from the ground than we may have expected, though viewing geometry was probably the biggest factor.

A windsock, a windsock, my kingdom for a windsock!

[CosmicRocker]

That's interesting, fredk. I meant to eventually compare them carefully, but I got caught up in this new project. It would be interesting to verify which direction the seasonal winds blow. I could swear I found that information once. I'll try to chase it down again. Unfortunately, it seems that those directions work just as well for the dust cleaning hypothesis.

Here's another montage using MIs from sols 924 to 1139. I tried to choose the best focused ones. They are reduced to 50% of their size to make this thing manageable, so you might want to look at the full-sized images for the best clarity.

I think most would agree that the MIs from sols 924 and 1069 are noticeably dusty. They are from areas well away from the main dark streaks. The images from sols 1103, 1135, and 1139 all appear to show significantly less dust. They are all from the general area of streaks, and even though two are from inter-streak locations, one might expect them to be cleaner than areas where no or faint streaks occur. That observation fits well with the clean-streak idea. I'm pretty convinced that the sol 1139 MIs are the least dusty of all. The very well sorted sand in those really clinched it for me.

1001 KB

[Shaka]

The next best thing to a windsock is our trusty RAT; the grindings are sometimes blown out in a dust trail. So I would propose a sampling program as follows: (Of course, our PIs may well dismiss the streak issue as trivial and non-controversial, and so be unwilling to spend the time.)
1. Make at least 2 more MIs at different spots here.
2. Make at least 2 overhead PANs for counting berries and fragments.
3. Then proceed to another location (or two) in mid-streak to repeat the MI and PAN routine,
4. Proceed to the area of exposed rock close to the rim and do a RAT, observing the dust plume.
5. Eventually repeat steps 1 and 2 at locations outside of the streak to the east.
That should provide statistically conclusive evidence to resolve the issue for even the most devout Depositionist.

[fredk]

I think most would agree that the MIs from sols 924 and 1069 are noticeably dusty... That observation fits well with the clean-streak idea. I'm pretty convinced that the sol 1139 MIs are the least dusty of all. The very well sorted sand in those really clinched it for me.

Thanks again, CR! I think though that it is tough to compare dust levels from these images, especially when some have bad focus or are taken under very different lighting conditions (crucially, sol 1139 is under full sunlight). I am much more confident that 1139 contains many fewer small berry pieces than 1135, and given those two sols' proximity, and surface homogeneity on metre scales based on hazcam and pancam navcam imagery, this observation carries much more weight.

I have to ask you though what exactly do you mean by "the very well sorted sand in those really clinched it for me" - how does the sorted character support sweep vs deposit? If the darkness is deposited, then as the air jet emerging from the bay moves northward out over the plain and slows down, it should drop the heaviest dark particles first, and then lighter and lighter. Couldn't that lead to the "well sorted sand"? How would sweep give you sorted sand?

[fredk]

(Of course, our PIs may well dismiss the streak issue as trivial and non-controversial, and so be unwilling to spend the time.)... That should provide statistically conclusive evidence to resolve the issue for even the most devout Depositionist.

Given the drive we've taken out here solely for the purpose of examining the Darkness, I think they can't dismiss this issue as trivial! I'd sure love to know their thoughts on this now, though. Remember too that they've got more to work with (minites etc).

I had thought the MIs might conclusively settle this, but it seems that both devout Depositionists and Sweepers are maintaining their positions! The fun continues...

[Floyd]

I agree with fredk--I think it is deposition and that the ratio of sand to berries is critical. But it is even more complex. Both light dust and dark material from Victoria are being deposited as well as removed (possibly at different times of year).
If
dark deposition rate = dd/dt
light deposition rate = ld/dt
dark sweeping rate = ds/dt
light sweeping rate = ls/dt
You get a dark band when dd/dt+ls/dt > ld/dt+ds/dt
but if dd/dt much bigger than all the others, then you get a buildup of dark sand covering the little light fragments and all the dust---just what we see in 1139

[CosmicRocker]

I have to ask you though what exactly do you mean by "the very well sorted sand in those really clinched it for me" - how does the sorted character support sweep vs deposit? If the darkness is deposited, then as the air jet emerging from the bay moves northward out over the plain and slows down, it should drop the heaviest dark particles first, and then lighter and lighter. Couldn't that lead to the "well sorted sand"? How would sweep give you sorted sand?

In all fairness, Shaka was the first to note the well sorted appearance of the surface grains. If this doesn't pan out in the end, I'll blame it on him. I might have difficulty expressing this efficiently, but here goes.

You could come up with a dark depositional scenario that employs well sorted grains, but we really haven't seen evidence of sand grains of the the size that are apparent in the sol 1139 MIs that have undergone much transport recently. That sand was blown to where it now is a long time ago, and it seems to not be affected much by modern winds. That sand, along with the berries and other larger fragments aren't going anywhere these days, and I see them as comprising the base soil or surface regolith here, and it has usually been been darker in appearance when we've seen it elsewhere.

I think I see tinier, fluffier dust grains in the MIs from earlier sols that are lying on top of and between the stuff that is too large to move in the current wind regime. They seem to be rarer in the most recent MIs. That is why I described the surface sand as very well sorted. I see fewer smaller grains.

[Shaka]

In all fairness, Freddo, it was you who first announced the cleanliness of the berries (post #67). I simply extended your observation to the sand grains. To me the trick is to look for the circular outline of the grains. The more clearly you can resolve them, the more dust-free (i.e. well-sorted) they are.

[Juramike]

"Dig we must."

[centsworth_II]

Basically, the clean, rounded sand grains at El Dorado were dark
from high viewing angles (and from orbit) for the same reasons.
Clean Martian sand dunes look dark, dusty ones look lighter.

I, like many, saw the dark deposits below the rim and the dark fans
extending from the rim above them and thought: Ah, yes, dark material
from the deposits is being blown out of the crater.

But if, like you say, the dark areas blow Victoria's rim are dark for
the same reason that El Dorado is dark (blown free of light dust). Then
the same wind that blows the patches below the rim clean also blows
the areas above the rim at those spots clean.

[centsworth_II]

I have to ask you though what exactly do you mean by "the very well sorted sand
in those really clinched it for me" .... How would sweep give you sorted sand?

Maybe he's referring to this sort of process described by Steve Squyres:
"El Dorado is a spectacular dune field.... We think that, in terms of how this thing got
here, because of the configuration of this terrain with respect to the prevailing winds,
it may be an aeolian cul-de-sac.... What you see in the Microscopic Imager is a sand
that is very well sorted, very well rounded.... This is very clean stuff."
http://www.planetary.org/blog/article/00000497/

Dark areas below Victoria's rim could present this same "cul-de-sac" situation.
Above the rim? Certainly not a cul-de-sac. But still blown clean of light-colored
dust (and fine sand?).

[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.

[Guest_Edward Schmitz_*]

In all four frames that are not on a streak, one can see the outline of the larger berries. One can also see their shadows. On the dark streak frame, no outline - no shadows.

The smaller grains are almost non-existant. One can only see the very top poking out of the soil.

The clear interpretation is that the fine grains are burying the larger ones. How can that be anything but deposition?

The dark streaks are dark grains being deposited on the lighter apron.

[Floyd]

Seems like the 1139 berries are more deeply buried. Most seem to be more than half way, which is more rare on other images.

[Juramike]

Assuming that the dark fine grains are burying the berries (and presumably overlying any lighter-colored sands) as the initial obervations suggest:

Are the fine particles heavier (denser) or less dense than the lighter-colored "normal" sands?

What does this say about the local wind speed? Is it higher or lower?

How did the dark streak form? What is the sequence of events for the deposition?

-Mike