If so, it looks like they are buried, in the dark streak
by the same sand material that lies around them in
the off streak areas.

Looking at the four off streak and one on streak
image, I am not convinced that there is additional
dark material in the on streak sand. The only
obvious difference is that the on streak sand appears
clean and dust free compared to the sand in the
other images.

So if deposition is going on, it is of the same sand
that occurs off streak and not of some different,
dark material. The streak areas look dark because
they are free of light dust (on the surface). IMHO

I hope the Mössbauer and APXS results clear this up!

I really like that model, atomoid. We've seen quite a few areas were berries have been concentrated along the edges of eroded blocks. Even before Opportunity climbed onto Victoria's apron we saw similar features near some bedrock fractures all along the journey from Eagle crater. I'll be keeping that process in mind. That's what I call thinking outside of the box.

As for the "smaller grains/fragments," I'm not sure how much weight to give them. They are difficult to impossible to resolve in the pancams, so we can't say much about their appearance over larger areas of the surface. We can clearly see them in the MIs, but since the few MIs that have been captured cover such a tiny total area, it is hard to imagine that they provide us with a representative sampling of the surface.

Even if we assume that the relative distributions of the larger berries and the smaller fragments are homogeneous over the entire area and that both are being buried by finer, dark grains, I could simply postulate that the winds here have been high enough in the recent past to not only blow away the lightest weight and lightest colored dust, but also high enough to remobilize some of the sand-sized grains that seem to have been mostly dormant most everywhere else Opportunity has looked. Unfortunately, that brings us back to square one. I am still strongly swayed by the apparent cleanliness of the sand within the streak.

I didn't notice anyone mention it, but I thought it was interesting that the IDD reached out to that little dark ripple to the north of that likely meteorite chunk in front of the rover. Is that rock Alicante?

Thanks, Atomoid!

So given the average blueberry density in native bedrock, and the average amount of blueberries we see on the surface, how many meters of rock has been ablated? (Does 0.5 m still work out?)

Given the blueberry pile seen in the "subframe image", and the average blueberry density seen in the surrounding of the blueberry pile, does this still work out to the same value?

How many m3 of rock had to be concentrated to make the blueberry pile in the subframe image?

All this should give the minimum total ablation amount.

(I'm still assuming that the average blueberry count is the same on the on-streak and off-streak areas and that the ablation rates on both are similar.)

-Mike

[This is probably obvious to all y’all, but I’m still trying to crunch through this and get constraints on the various scenarios.]

I will assume that berries were emplaced where they eroded and were not significantly blown around. Berries are only being exhumed and not freshly blown in on the apron around Victoria.

The key observation is that the berry count is roughly the same both on-streak and off-streak. This implies that after the big erosion (of 0.5 m? of rock) that no major net overall deposition has occurred after the maximum exumation level was reached. The berries aren’t buried.

I struggle to imagine how after the removal of several cm of rock volume has occurred, that a “magic puff” occurred at the end of the ablation that would be able to selectively enrich native sand to only have dark material all around the apron. (After emplacement of a lighter layer all around the apron and perfect removal of the light sand layer would reveal the original lower layer in the streaks – this scenario doesn’t seem likely).
[Predicted w/this scenario (Scenario 0): on-streak: v. thin “native-enriched” darker layer, at surface, then “native apron” deep layers; off-streak: light deposited layer at surface, then v. thin “native-enriched” darker layer, then “native apron” deep layers).

More likely is that if after original emplacement level was reached, a new layer of lighter sand was added and this could be enriched slowly towards darker sands in the on-streak area. Because the berries aren’t buried, the net addition of light material off-streak cannot be more than a berry width deep. [Predicted w/this scenario (Scenario 1): on-streak: v. thin “enriched-deposited” darker layer at surface, then “native apron” deep layers; off streak: light deposited layer at surface, then “native apron” deep layers.]


Here is a possible sequence of events for the apron of Victoria:

Scenario 1:
1. Rubble breccia pile emplaced on impact.
2. Net Ablation of rock (? m). Blueberries emplaced where they drop.
3. Net deposition of sand (w light/dark particles) < berry widths ( we still see berries at surface).
4. On streak: Removal-of lighter sand deposit to give “enriched-deposited” darker layer at surface.


Here is another possible sequence of events for the apron of Victoria:

Scenario 2:
1. Rubble breccia pile emplaced on impact.
2. Net Ablation of rock (? m). Blueberries emplaced where they drop.
3. Net deposition of sand (w light/dark particles) < berry width ( we still see berries at surface).
4. Net deposition on-streak of v. thin layer of dark material over the sand deposition layer.


Another scenario that also should be considered (and eliminated though observation) is differential depostion of both materials:

Scenario 3:
1. Rubble breccia pile emplaced on impact.
2. Net Ablation of rock (? m). Blueberries emplaced where they drop.
3. (off-streak only) Net deposition of sand (w light/dark particles) < berry widths ( we still see berries at surface) off-streak only.
3. (on streak only) Net deposition on-streak of v. thin layer of dark material over the “native layer”.


For Scenario 1, the dark material comes from the deposition of wind-deposited sands w/light/dark particles. We should be able to get a better constraint on this hypothesis by measuring the amount of dark grains in the wind deposited sands (# particles/cm3), then observe the amount of enrichment in the dark region (# particles/cm2).

That should tell us the minimum amount of original sand wind-deposit needed to be present prior to enrichment.

Any imagemeisters up to the challenge?

-Mike

Why is the word "dust" conspicuously absent from all of your senerios?
Dust is famous Mars-wide as a primary affector of surface color/brightness
and is a primary element in many of the theories presented in this thread.
While the relative abundances of sand, fragments, and berries in MIs is
being considered, the relative abundance of dust cannot be ignored.

I'm assuming dust is "in the mix" as well. I was using the all-encompassing term sand to include anything that is rolled in, blown in, wind dropped, or settled out from the atmosphere that then ends up on Victoria's apron. Martian dust fits all these categores and is included in all the deposition scenarios.

My only exclusion is that blueberries are NOT in the deposition mix.

-Mike

A reality check for those of us that might be inclined
to explain all dark streaks on Mars in terms of dust
removal. Although the situations at the south pole and
on the rim of Victoria Crater are certainly very different.

From Emily Lakdawalla's Planetary Society Weblog:
Varying dust fans on Mars' defrosting south polar cap


http://www.planetary.org/blog/article/00000949/

I think this is a dangerous generalization. I suspect that size wise
there is as big a difference between dust and sand as between sand
and berries, with similar differences in their mobilities. If sand is
moved within an area while dust is removed from the area, these
are two different effects based on two different catagories of material.
I don't think a theory that lumps dust and sand together can explain
the dark streaks at Victoria.

You are right: I may have made a fatal generalization. I was trying to come up with a simple (AKA "niaive") explanation for possible scenarios.

In order to keep a selective-removal hypothesis possible (Scenario 1), I needed a mechanism to deposit a mix of different-colored particles, then selectively remove the lighter-colored components. I was assuming that the darker components would be heavier and thus left behind. I was trying to keep to a 3 component problem (light-white particles/heavy-dark particles, mix of the two) rather than a multi-body problem (light sand/dark sand/light dust/dark dust, and all the various combinations).

But the reality is probably more complex....

-Mike

While berries are on topic, I wanted to ask if we're able, given a surface berry density count, to make an estimate on how many "ghost" sulfate layers were deflated to concentrate the berries?

Does this assume that berry distribution is the same for all sulfate layers?

Related question: does the diameter of a berry give any clues where it formed in the layers? i.e. berries at the top of a sequence would be smaller than those at the bottom? (I realize on a debris apron of a crater, that the material was exhumed from the crater and probably jumbled, but thought we might see evidence of this in the crater wall when we finally get a closer look? Was this seen at Endurance?)

Bonus question: why is hematite the only form of iron we see at Meridiani? I'd think with iron and sulfates there'd be gobs of pyrite too, or does this hint at the acidity of the solution the hematite formed in?

Apologies in advance if I'm re-hashing questions from previous discussions.

I saw that too and was tempted to bring it in. There is a remarkable resemblamce to the dark streaks at VC. There are certainly diffenences in the process. The similarity that I would put out there is the fountain of particles and how they deposit down wind. Other places where there is turbulance induced cleaning has a very different appearance. This should be familiar to all of us. That is turbulance.

Compare the two images. Especially the bay at the 10 o'clock position. These are both examples of the particle fountain where the wind has shifted.

Comparisons between Meridiani and South Pole patterns are of course interesting and, perhaps, informative, but we need to keep in mind the apparent albedo contrasts in images, as distinct from absolute albedo levels of the materials present. The light background in the Polar image is either water ice or solid CO2, and therefore brilliantly white compared to the light dust covering ice-free areas of Mars, including VC. The dust fans at the Pole which look black in contrast to the ice may, in fact, match the albedo of the light, dusty apron around most of VC.

There is no doubt that wind streaks can be depositional or erosional in origin. They can be relatively light or dark in contrast to the background. The question we are dealing with here is: What is the immediate cause of the dark streaks on the Victoria apron? We should not have to invoke the entire history of Meridiani Planum to answer this. The "dirty" deposition hypothesis requires a discrete source of mineral in the northeast rim of VC that is dark in contrast to the common evaporite and concretion material composing the rest of the region. That deposit of dark mineral must be identified if the deposition hypothesis is to prevail.

two things...

1) The pattern of the streaks are not erosional. Those are the patterns of a substance being tossed in the wind stream and falling out down wind.

2) We don't need to find the ultimate source of the matterial for the deposition threory to prevail. There is mobile matterial in the north side of the crater that matches the streaks in both brightness and color. That's a fact. How it got there may never be known.

Neither of these "two things" are facts, except in your own mind.

In two crops (below) from the image you link, it's interesting to note that only craters
that have dust devil trails leading from them have dark "deposits" within them. Of
course these are not deposits but areas swept clean of dust by the same dust devils
that went on to leave a dust free trail leading from the crater in which they were born.



Concerning "...examples of the particle fountain where the wind has shifted.",
streaks caused by dust removal can also be laid down in different directions as
the wind forming them shifts.