I wonder if there will be any MI, or other study done of the exposed
rock layers in off and on streak areas? It would be interesting to see
the darkness of fine grains caught in cracks and voids in the rock.

For example, what would MIs from the areas circled in red reveal?
(This is a crop from Tesheiner's Opportunity Route Map)

Click to view attachment

It sure is hard to make all the idea pieces fit!

Deposition, of course!

Recall our earlier posts on this.

In the "subframe image" CosmicRocker posted, it looks like the brecciated rock pavement may lie pretty close to the surface. I suspect because of the shallow depth of the sand layer, that sand/dust was removed, and the revealed berries were gently blown (or just rolled) into little local hollows in the rock topography. The berries are simply lying on the cracks and dips.

After this, there was a deposition event(s) that emplaced a cute little local sand drift right in front of the bowl o' berries (was this dune downwind or upwind? where would you expect a similar cute drift near the rock in the on-streak image). Then another layer of fine dust was put over everything, since the pavement bedrock doesn't seem directly visible in this part of the image.

So in this "subframe image" I am interpreting this as evidence for both ablation and deposition at work at this "subframe image" location. I don't know if the nearness of the bedrock (shallowness of the soil layer) makes this processes behave different compared to the dark streak area. (e.g. deposition might work better on soft sand than hard rock, less bounce).

-Mike

To my "... what is the explaination for the lack of intermediate fragments in the on streak image?"


But is it deposition of a dark material unique to the streaks, or of the same sand
found throughout Victoria simply rendered dust-free, and thus darker? In other
words, would the sand of the off streak areas look the same as that of the on
streak areas if the dust were blown from it, without the addition of some new,
dark material?

Glad you've been drawn back, Rocker!

That patch of berries from sol 1109 is clearly tied to a change in slope - look at this 3D image. But the berries don't appear to be in a bowl, instead on a slope. Probably wind is involved as well.
Click to view attachment

Former mini-dunes o' berries?

-Mike

Here's an explanation for the lack of the smaller, what people are calling intermediate grains, on-streak.

The intermediate grains (smaller than the concretions, larger than the soil grains) are likely *not* hematitic concretions. They may well be busted-up pieces of the evaporite.

Perhaps the higher wind conditions in the streak areas have eroded these small grains away to nothing, while the winds just aren't strong enough to erode them down further outside of the streak areas?

I think we need to look at compositional answers for why these smaller grains are missing on the streaks...

-the other Doug

The smaller grains are buried. You can see them in the study that CosmicRocker did...

Compare the last image with the others. There are very few partially covered pebbles or berries in the other frames. In the last one, all of the pebbles and berries are partially covered. By inferrence, the missing ones are buried.

To figure out what is behind all these little berry-drifts, and we see them everywhere, it might help to consider to what depth has the bedrock been ablated here, what, maybe a half meter or so with high variability? The initial apron was surely more of a chaotic jumble of evarporite chunks than it is now after having been sanded down over the eons to a very smooth level overall, and if we consider that the berries dont move unless they are on bedrock to roll away or the sand beneath them moves, then it would be possible that these little berry drifts are whats left behind after a chunk of bedrock gets ablated and all the berries drop out and roll down what was then the existing side of the rock and pile up at the base of their parent rock's eroding surface, which is now long gone, but the berries in most cases dont move, they exist as sort of chalkmarks outlining the deaths of old bedrock chunks, some are more pronounced than others due to the topography of the rocks they exited and further erosional factors.

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
Click to view attachment
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.

Click to view attachment

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.

The pattern is dictated by the wind. It may dictate that the cause was not
turbulent dust devils, but does not rule against a constant, stiff breeze.

Wind would be involved in dust sweeping or in deposition... or both at
the same time. How would the pattern be different for either sweeping
or deposition if the same wind is involved?


Maybe not, but we do need to see the deposited material. If you argue that MIs in the streak areas
show a deposition of the same sandy material that is found in off streak areas I could agree. But
if you say there is some different, dark material that has been deposited in the streaks, I would say
that the MIs do not show any such material to be present. What they do show is a surface
swept clean of dust.

There are varying amounts of streaks coming off of the bays. If you look at each bay there is a strong corrolation between the amount of dark matterial in and under the bay with the darkness of the streak.

If anyone is thinking - "that just means the wind is cleaning the area below the bay too." The whole clean sweep theory is based on how the wind exits the bay. There needs to be another mechanism to clean the area below the bay.

Why....precisely?

Wind patterns on the rim clear dust from certain spots. Wind patterns
within the crater clear dust from certain spots.

Another point: the dust removal theory for the streaks is
Occam's razor friendy. The deposition theory, not so friendly.

I know my suggestion waaaaaaaaay up the page didn't get much interest or response, but I still wonder if there's some kind of dark mass beneath the crater to the north and north east... something that is eaten into when erosion of the bays eats into it, briefly releasing fine dark material that then 1. spills down into the crater, and 2. puffs up and then away from the crater. Only a few bays show this dark material. Maybe the areas beneath the bays aren't cleaned at all but the dark material is covered by lighter dust blowing into the crater from the surrounding terrain, I dunno...

The invention of this mass to explain is a good example of why
the dark matter deposition theory is not so Occam's razor friendly.
The simplest explaination does not always end up being the
correct one, but it is the one that needs to be ruled out first.
Does anyone here think the most detailed evidence to date --
the MIs -- rule out the sweep theory?

I haven't 'invented' it; I'm basing my suggestion on actual observations... There are only rays coming out of a handful of bays, which are bays that cut quite deep into the surrounding terrain... there are large craters in Victoria's area that could have deposited material nearby when they were formed... there are layers visible in the cliffs on numerous pancam images... the rippled dust dunes on Victoria's floor are proof that winds move material around down there, at least suggesting winds blow on the features higher up the crater too...

The slash of Occam's Razor doesn't murder my suggestion, does it?

If the jet of air exiting the crater is required to clean the surface, how is the area below the cliff going to get cleaned.

Did you look at the pictures that I posted?

Did you see the difference between streaking bays and non-streaking bays?

Can we at least agree that there are dark deposits there?



Occam's razor is not proof for anything. It is only a reminder to put aside far-fetched explainations UNTIL you have eliminated the more obvious ones. besides - deposition is simpler in that there is obviously dark mobile matterial there just waiting to be blown up on to the apron.

Clean sweep requires an as yet undefined reason why some bays cause air blasts while others don't. How is that simple.

Stu,

I concure... There may well be that mass of matterial there. Chances are, we've gotten our closest look as those cliffs already. We may never know where it came from...

Sooner or later the depositionists are going to have to get into specifics about the "dark mass", namely "What is it made of? and "Where can we see the original mass?" and "Point to the dark material in an MI." and "Why haven't we seen this dark mass before, either from Oppy or from orbit?"
If those challenges cannot be dealt with, the hypothesis will remain entirely speculative.
Numerous workers in the field recognize the process of bright dust removal to produce darker streaks. The erosional hypothesis requires only the wind.

If the entire area were uniform in composition, why even traverse around the crater at all? Just sample one spot and you're done...

As Stu points out, it is a theory based on observation. The totality of observations will lead to the most complete explaination.

So, I'll ask again. Can we at least agree that there are dark deposits at the base of those cliffs?

Let's just say it's a close shave.

Ok, I'll take that mic...

What is it made of? Well, it might be a big, dirty, dusty chunk of Phobos that splatted down here and is now being eaten into... more likely, it could be material from farther below the surface than we've seen so far; it's not a huge leap to suggest that the impacts that blasted the larger, nearby craters out of Mars could have excavated deeper lying material and vomited it in VC's direction, laying it down as a localised deposit of dark material in the north/north-east area that was later covered by dust and VC's own ejecta. Just like grass and dirt covers a cow pat.

Where can we see the original mass? We can't, it's beneath the surface, beneath VC's apron.

Point to the dark material in an MI. Hands up, I can't. Yet.

Why haven't we seen this dark mass before...? Because it's beneath the surface, and the material from it is only exposed when the erosion of a bay "breaks into" it, exposing it to the air and releasing it. But we have seen the streaks, on MOC and MRO images, haven't we?






Like your style cents!

What is it made of? Irrelevant - it's there.
Where can we see the original mass? Irrelevant - it's there.
Point to the dark matterial in an MI? How - the MI's don't provide that kind of information.
Why haven't we seen this dark mass before, either from Oppy or from orbit? I cirlced it in my previous posts.

Entirely speculative... Are there or are there not, dune that appear to be the same color and darkness as the streaks right below the cliffs?

Are those dark deposits in the craters on the left?
I say no, that they are just the same as the craters
on the right but have had light colored dust swept
from them. Do you say that the craters on the left
have actual dark material present in them that is
not present in the craters on the right albeit beneath
a layer of dust?
Click to view attachment

Mr. Centworth,

Your interpretation of those images are the same as mine. Which don't look anything like what we see at VC. That is dust removal. What we see at VC is deposition.

Well. I can't argue with that.

Cents,

I actually think you've helped our case with your pics... the rays coming out of VC broaden, while the "rays" and marks coming away from the craters in your left pic taper to points, suggesting two very different processes.

Thanks!

The overall process: dust removal by wind, is the same.
What is different is the type of wind: dust devils vs. breezes or gusts.

And, the images from the south pole are definitly deposition and match the patterns seen at VC.

O.K. I get the growing feeling that I've fallen down a wormhole and been transported into a forum of another color. I've decided to leave the bickering and to wait until the MER team announces its findings.

Ok... so I'm standing on the edge of Victoria Crater, just above Soup Dragon, looking over the crater towards where Oppy is now, Occam's Razor clutched tightly in my hand. It's a gorgeous martian day - but nippy, maybe, but the sky is clear and pink and the Sun is shining brightly. I feel a wind on my back, a wind that wafts across the crater and passes over the other side heading north. As it passes over the far cliffs and bays, I'm puzzled... why does it only clear dust away from the ground behind two or three of the bays, and not every bay? Maybe because this particular wind was focussed quite narrowly, and only passed over a few bays? Ok, but there must be winds blowing across Victoria all the time, over every bay, so why aren't they all the sources of dark streaks..? Very curious now, I walk around the crater, towards the streaks... doesn't take long, not as long as those old rovers did anyway... and as I walk up one of the streaks, away from the crater, I wonder how a passing wind could clear away more dust farther away from the bay than it did closer to the bay... shouldn't the wind's strength have diminished? Or did the wind gather strength after passing over the crater somehow?

Standing there I feel a slight tremor run through the ground beneath my feet and I feel an urge to turn around, and do so just in time to see a puff of dark dust billowing up into the air above one of the nearby bays. Maybe, I wonder, there's been a landslide or a rockfall? But then a billow of darker, finer dust puffs up out of the bay, quite violently, and, caught by the wind, the material blows towards me and over me, the fine particles falling softly to the ground, and onto me.

What the frak was that?

A mystery indeed...

Oh Shaka, this isn't bickering, I think this is pretty good-natured debate

I agree things are getting a bit out of hand. Based on the evidence we have we clearly can't convince each other on the nature of the Darkness. Both sides feel strongly that their views are the simplest, most economical ideas consistent with what we see.

It would be great if we could find more ideas to rigourously discriminate the opposing models. A while ago I posted that Deposition predicts larger dark grain size the closer you get to the rim. Would sweepers be convinced by such an observation? What else might it take to convince both sides that Deposition is the better model? I could add one more discriminator: the observation with the other instruments of a clear compositional difference between off- and on-streak soil, ie a clear signal of a Darkness of different composition from the rest of the apron. I'd say this is sufficient, but not necessary for Deposition.

I don't recall hearing any such prediction for the Sweep model. Is there a clear prediction of Sweep that would distinguish it from Deposition? What would it take for someone from either camp to be convinced Sweep is essentially correct?

I don't see any evidence that things are "getting out of hand"... from where I'm sitting this is just a good ol' to-and-fro debate... no name-calling or gloves being slapped across faces... but oh well, I guess I'll just shut up and try and get this sleeping cat off my keyboard... not joking...

Hey Fredk,

I believe the grain size hypothosis would be a strong indicator of deposition. The MI's don't provide brightness or color. Composition via the other instruments is no help. That can occur with either model. I also don't see any mechanism for partial to full burial of the larger berrys and pebbles in the clean sweep model. If more evidence of burial were to surface (no pun intended), would that sway anyone?

I agree with Stu. I don't think this is out of hand. I do agree with Fredk that it seems rather futile without more information. I've made the best case I can given the data at hand.