The entire planet is coated with it. The frequent global dust storms are very efficient at circulating the stuff and depositing it everywhere.

That's my impression, too -- and a good part of my rationale to answer Fred's questions. Though the question of basalt isn't the right one, I think -- I believe that the ubiquitous red dust is actually oxidized basalt.

In regard to "clean sweeping," though, I'm thinking that you're right, the dark soil components on Mars are, in general, composed of larger grains than the brighter, redder dust. They move primarily by saltation and secondarily, for grains in the small end of the size range, by aeolian transport. Martian winds pick up the lighter, smaller-grained redder dust, and keep it in atmospheric suspension, more easily than they can pick up or keep suspended the larger, heavier, darker dust grains.

That has two different consequences:

First, a wind across the Martian surface will tend to sweep away the brighter, redder dust and deposit it thinly downwind. For a given set of wind conditions (i.e., wind speed and direction), this will set up a dynamic equilibrium after a certain time, during which areas that are more exposed to the winds are swept of bright dust to a greater degree than upwind sweeping re-deposits similar red dust. These areas slowly become darker, creating the wind-generated darkening events that have been visible even from Earthly telescopes for centuries. When wind patterns change (which they do, seasonally), portions of the terrain that are darkened may brighten, and other areas may darken. But the thing to remember is that, under each type of prevailing wind pattern that a given spot on Mars sees in a year, an equilibrium is reached and given areas are swept clean and darkened more than they are re-brightened by re-deposition. But, and this is the important point, *nowhere* on Mars is seeing *just* dust deflation or deposition. Both processes are always happening, everywhere -- it's all a question of which process, deflation or deposition, is dominant over a given landform at a given time.

Second, each type of dust, the bright red stuff and the darker gray stuff, exhibits a range of grain sizes. While bright red dust can be swept off from the darker gray dust, the smaller grains of the dark gray stuff will also be picked up by the winds. Because these darker grains are larger and heavier than the lighter red grains, they fall out of the air much faster and more easily. They don't stay suspended in the air for nearly as long, and aren't swept as high off the ground. So, in an area that is being swept, you'd see the smallest grains of the dark dust (which are still larger and heavier than the largest grains of bright red dust) piled up in aelian forms, in the lee of rocks and protrusions where turbulence degrades the wind's ability to keep them suspended. That's why it's important to see how well sorted the grains are in the dark aeolian forms -- if they're very well-sorted, it indicates that the dust that's forming these dark aeolian forms is dropping out from suspension because it all becomes too heavy to be supported when the wind hits the lee of the protrusion. In other words, it's not being deposited from very far away, this material is being picked up from the ground inches or feet away from the dark dune, moved the very short distance the wind can carry it, and being dropped as soon as the conditions keep it from being suspended in the air.

That's also probably why Victoria's dark streak seem to feather out from the center -- the bright dust that gets removed is deposited hundreds of meters, if not kilometers, away, while the small amount of dark dust that the same winds can transport only gets a few meters away from where it was picked up. So, along the edges, the darker dust actually does get deposited over the brighter dust at the margins, giving it that feathered look.

All of this explaining of what I think I'm seeing is moving me back towards the clean sweep side of the fence...

-the other Doug

Not at all. I and others here have pointed out that what's happening here is almost certainly a mix of deposition and sweeping, as I think we agree has been the case at El Dorado. The question is "is there a dominant process that causes the darkness in the streaks?"

What I meant was where does this light dust come from, in these wheel tracks. If the light dust has been removed from the dark streaks by wind sweeping, why is there light dust in the wheel tracks? If the light dust has been coated by dark deposits, I could see the wheels exhuming the light dust.

Dvandorn, your description of varying particle sizes fits beautifully with my view of deposition! We have some source inside the rim. The strongest winds are able to lift the smallest dark particles out of the crater through the bay wind funnels. As the wind spreads over the plain it slows, dropping the largest dark guys first, then smaller, etc. At any distance from the rim there is a certain size of dark particle just barely in suspension, so any obstacle like the rock will cause them to drop in its lee, leaving a well-sorted deposit.

This leads to a definite prediction: the dark particles in the streaks should be largest near the rim and smaller farther out. Perhaps we'll be able to test this as we move towards the rim.

Of course these winds can also clean light dust if it's there to clean. But the Depositionist claim is that the Darkness is primarily due to deposited dark stuff.

Thanks for the excellent photos of dust devils downwind from craters. When I first saw pictures like that I wondered whether in those cases vortices were picking up darker soil from the crater and depositing it downwind, depositing less dark soil -- and hence becoming fainter -- as they got further from the crater.

Your model implies vortices only form downwind of craters. Mine has dust devils more widely distributed, but those that pick up dark soil from craters leave darker tracks. In that view most vortices are sweepers, but some (after passing through craters with exposed dark soil) are depositional for a while.

I'm convinced by the dark soil in the NE corner of Victoria that the streaks downwind of that corner are depositional.

Steve

Fredk, do you think this prediction will also hold for the drift behind the rock?

Would you predict that the size of the particle grains behind the rock be smaller than those on the ground? The lower wind speed should help drop smaller particles than normally found in this location. (Can a quick MI at the drift boundary help solve the debate?)

Would a sweep scenario also predict smaller particles in the drift?

-Mike

(Oh. I see from the Hazcam images they already took the MI pictures. Remind me to check these things BEFORE I post.)

-Mike

I'm sorry folks, but I am growing tired of this debate. I really am trying to keep an open mind, but most of what I see seems to be cleaning. Obviously, the depositionists are also confident in their theory. All I can say is that deposition seems to require a more complicated set of conditions than sweeping does. According to the MIs, we are seeing down to where the coarser grains last moved, when the winds were strong enough to move them. That's clean enough for me.

Dust devils form on sun-facing slopes, then migrate with prevailing winds picking up and redistributing the lighter fines, revealing cleaned, darker soil underneath. It's well documented process - and one that we've seen upclose and personal within Gusev crater many times.

As for Victoria, darkness and the wheel tracks....

• Dark soil with Berrys on top
• Dust falls out the sky as it does everywhere, lands on top of the berries and the darker soil underneath.
• Wind comes along, blows dust off the top of the berries, but doesnt 'touch' the soil underneath because the berries cause a wind shadow.
• Thus you have darker berries visible from above with a lighter soil underneath
• Wheel comes along, pushes berries into the soil - and now you see the dark soil again but with a lighter dust coating on top, thus appearing a little lighter.

The tracks here are VERY reminiscent of Eagle craters airbag imprints.

Doug

Nice rocks there
(Sol 1045)

So you are officially a clean sweeper?

> Nice rocks there

Remember we were trying to pinpoint the location of those ones when they were barely visible on the horizon (on sol 1096 perhaps?)

BTW, this rock pile was named "Granada". OT: Those who have been there should know the Alhambra.

Regulars will know that I don't side on these (frankly a bit silly) 'With us or against us' arguments. I'm just commenting on what I see

And I've visited the Alhambra, it's utterly breathtaking.

Doug

You answered your own question. The darker matterial is larger grains. Harder to move than the light dust (but not impossiblie).

Tesh' : You mean this rocks (at the left center of the horizon)?

Clean sweepers are not addressing the deposits on the walls of the crater.

Please review and explain if you are in the clean sweep camp.

This dark matterial is clearly being deposited and not being uncovered.

There are convincing arguments on both sides - in theory. When the evidence is taken in its totality, deposition is the dominate process.

The one thing I would point out here is that the wind outside the crater is not moving substantially faster than on the plains. The reason for the dark desposits is that there is a fountain of dark particles coming out of the bays. The smaller particles are lofted high than the larger ones. Once they enter the prevailing winds, they are swept down wind and dropped. The smaller particles going farther. This is evident in that the dark streak does not protrud straight out of the bay. It is immediately swept down wind. Talk of vortexs or wirl winds are not supported. This has been stated by steve. If you don't remember this, I will look up the reference.

I appreciate you carrying the ball here, fredk...

Yes - but darker than what? Darker than very bright rock, sure - but the same 'darker' than the dark streaks against the surrounding terrain?

There are convincing arguments on both sides....full stop.

Doug

The orbital view clearly shows the dark matterial in the streaks to have the same albedo and color (at least within a limited part of the spectrum) as the deposits around the north side of the rim.

So to answer your question - Yes, the same dark as the streaks...

In case anyone missed it, that "rock pile" is a rather fresh-looking crater. It's not very apparent unless viewed in 3D.

I'll hazard a guess that it's another secondary (or even tertiary!) chunk of ejecta from Beagle impact. It's very reminiscent of the rubble pile we passed on the way south to Beagle.

I'm not sure exactly what silliness you're referring to here, but my interest is to try to understand the nature of these streaks given what (limited) information we have publicly available. Of course jpl has all the data and will no doubt come to a conclusion and (hopefully) we'll hear it (indeed the fact they've driven out here highlights the importance they place on this issue!). But what's extraordinary about this mission is that we all have the opportunity to take part in this process in real time! We can observe some feature, speculate about its origin, and then look at what further observations do to support or refute our ideas. I find this extremely fun, but then science is my thing.

This "taking sides" business is a normal part of real science. You provisionally take a position based on what you see, then debate with people taking different stances, and in the end (hopefully) come to a conclusion. Nothing silly in that.

Admittedly things can get out of hand in these public discussion groups. But you do a pretty darn good job of keeping things under control!

Indeed!
Click to view attachment

Agreed. If we all just sat here waiting for Steve S to tell us what every single feature was, well, the yawns would be deafening. Remember how much fun we all had discussing / debating / arguing about Beacon's location and nature? That was one of the highlights of the whole mission so far for me, even tho I turned out to be on the wrong side of the fence crater, simply because it drew everyone here together and set us a challenge, which I think we rose to. I still think the streaks are deposits, but if I'm wrong then I'm wrong, and at least I'll have had fun joining in the discussion and having the opportunity to talk with people who know a helluva lot more about this stuff than I do. ( Which is pretty much everyone here, I know, but that's ok too. )

I'm NOT convinced yet. I'm sure Oppy will be heading soon in Beacon's direction to double check this
Seriously, I hope they'll stop there this time.
BTW, I agree on the need to debate even if the subject could be futile. If we don't, the Rovers will last more than ourselfs

Hmmm.... fresh-looking crater...

Click to view attachment

And again, in 3D:
Click to view attachment

Debate over?
After reading all these great thought-provoking arguments in the last few pages, its apparent that the same points are being argued by both camps, mainly:
--> Of all the dust being deposited on the 'dark streaks', it is primarily dark dust.
--> Of all the dust being 'swept away' from the dark streaks, it is primarily light dust.

Simply because the light dust is too ephemeral and blows away based on the preponderance of wind speeds whereas the dark dust just tends to tumble up and along slowly, and both process occur in a somewhat simultaneous and overlapping-offset fashion based on seasonal wind differences, a tortoise and hare co-evolution.

- I'd characterize the streaks as "depositional" (i hope i dont sound like im massaging the semantics to fit) because the dark streaks contain extra dark particles over and above that of the rest of the apron and consist of material that was not part of the original apron material. Perhaps a centimeter or so in the thickest areas, with the blueberries setting some sort of upper limit due to their wind interaction and can only be partially buried (or maybe this is due to generally chaotic air movement exiting VC which doesnt allow blueburying drifts to build under such conditions). The real 'depositional' clincher here is if there exists a build-up of dark material on the bare bedrock at the mouth of the bay, which looks to be the case from the HiRise images, although even then it looks to be a thin veneer.

Sweepers and Depositors can live together in peace and harmony and indeed need each other to survive(!):
--> The dark streaks wouldn't be visible if it werent for the clean sweep activity going on to reveal them.
--> The clean sweeped areas wouldnt be apparent if there werent a preponderance of dark dust beneath them to give them contrast with the rest of the apron.

Late to the party, but here are some thoughts.

As I'm sure has been noted previously, the blueberries are obviously much less wind-transportable than dust. Given the observed density of them across Meridiani by Oppy, the key discriminator between deposition & erosion seems to be whether the berries are darker or lighter than the substrate (I personally can't tell from Oppy's raw B&W imagery what the reflectance/absorption characteristics of the berries are, much less how they would appear to MRO) . Therefore, a primary investigation strategy would be to determine if ground-level obs of the 'plumes' reveal more or less blueberries per m^2 of terrain sampled.

Probably just re-invented the wheel there, but made it square thiis time...

I'm not sure what Martian wind velocity would be required to move significant numbers of those blueberries.

I wonder if the wind was strong enough to roll a few of them off the northern apron of Victoria's annulus.

I know I said I was tired of this, but I might not be. I came across an image from sol 1109, when Opportunity was in an inter-streak area on the Cape of Good Hope that I must have missed earlier. In my mind,this pancam target pretty much condenses most of the differences between the light and dark surfaces into one pancam subframe for me.

The attached image is a montage showing the subframe image in context. From the rover's eyes it seems that this area is one that is generally lighter, but which contains some darker patches. The pancam overlapped the navcams displaying the darker patches in context, and provides a more detailed view. To me it appears that the light area in the bottom of that subframe is coated in light dust, and that the top half of the image is mostly swept clean of light dust.

The transitional boundary between the two displays partially buried berries. I swear, this is my last comment.

Thanks for the interesting montage, CosmicRocker.

[And yes, I'm going to try to tease more comments from you...]

That's an impressive pile o' blueberries in the upper part of the subframe in your montage. How do you think this compares with the images from the dark streak zones? Can you explain the differences in blueberry density?

I think that the the montage you made gives really good clues about deposition/ablation of material at the subframe location, but I'm not sure I understand if it formed exactly the same way as the dark streak zones or if it is yet another part of the story at Victoria.

What do you think was the sequence of events that explains the image from the subframe image location?

-Mike

I totally agree. But it all really boils down to focusing on the right question:

"What key observation/experiment would help decide between the different scenarios?"

-Mike



I still preserve my right to waffle as new evidence arrives.

Yup.



... named "Huesca" (spanish city near the Pirenees).

Somehow I can't imagine features getting names after British cities...just not quite romantic enough

Milton Keynes
Bradford
Sutton Colefield
Slough
or even Leicester

There's Robin Hoods Bay
Lulworth Cove.

I don't see why the same arguments about the dark deposits lower on Victoria's slopes
can't apply to deposits higher up on the wall. They are the same material that occurs
all around the rim, but swept clean of light dust, OR they are a different, dark material
that has been blown up from deposits of this dark, mystery material from below.

What a great demonstration of clean and dusty berries!

And going back to details of MIs from sol 1136 and 1139, the off streak
berries (top) look dusty while the on streak berries look clean.
Also, I think it would be hard to argue that there is more dark fine material
seen in the lower image than in the upper. The main difference is the
cleanliness of the berries. Perhaps this is the only difference between on
and off streak areas -- the cleanliness of the berries.
Click to view attachment
Note the mini Iapetuses (Iapeti?) in the bottom image .

Just for a bit of fun - I botched a little animation of the timeline for a cleaning hypothesis using grey ground, a grey-blue single berry, and salmoney dust.

Dust falls onto ground and onto berries. Makes everything look a little brighter. Then - along comes the wind, but the ground is sheltered by the berries - so only the top of the berries get cleared off - thus you see a whole lot more berry (dark) and less soil + berry ( light )

A similar sort of process also happened to an extent up at the year old tracks they observed on the way out of Endurance. For eons the berries sat on the surface, the wind blowing over the top...grains of soil underneath not really feeling the wind. Then, along comes a wheel - pushes the berries into the ground, and thus the soil within the track is re-exposed. the wind then works away at the soil and you end up with the pushed-in-berries being re-exposed at the bottom of the wheel tracks.

All just speculation of course..just a bit of fun

Do you see me saying that it isn't deposition or it has to be cleaning? Nope. I'm just trying to visualise what the MI's seem to be showing.

It's very hard to compare those two MIs since one is in shade and the other in complete sunlight. Much of what may appear to be dust on the top (shade) frame looks like it may be jpeg artifacts.

So am I (see avatar) ... but on the other side of the rim

I don't know. I would love to see homage paid to such memorable locations as Amlwch, Swanage, Corriemuchloch and of course, Clonmult, Sadberge and Buldoo.

Brian

I knew I'd be drawn back into this discussion, even though I swore I had made my last comment. I am not sure why the berries are sometimes aggregated into local surface concentrations. There are other forces acting on them that may or may not be relevant to the dark streak question. We have often seen the berries piled up into local concentrations on various scales, and in areas where the streaks were weak or absent. I am not yet convinced that the dark streaks mean simply that more berries are visible. I think the cleaning extends down to the sand-sized grains below them.

When I first proposed that these were streaks were clean, I really thought it was a slam-dunk situation. In some ways I still do. But when you get down down in the dirt with the imagery we have to work with, it seems to me that there is quite a lot going on. I might never have seriously considered the dark deposition idea were it not for the fact that there are accumulations of dark sediment inside the crater's north side, as Edward has reminded us. I explain that away by proposing that those internal dark deposits exist all around the crater, and that they have been selectively cleaned of light dust on the north side of the crater by the same winds that made the external streaks. Without other supporting information, that leaves me with a somewhat empty feeling, though.

Another interesting observation would be fredk's images of light dust blowing south from the wheel tracks. I'm still working on that. I think this is going to make one heck of an interesting paper when they eventually publish it.

Another little "Berry Bowl"...?

Click to view attachment

Bigfoot!!

Keep in mind that, per Doug's model, the amount of light
colored dust in the on and off streak areas is "the same",
only the distribution is different, with the dust in the streak
having been blown off the berries but not out of the streak.
This dusty layer, even in the streak area, becomes
prominent when the rover pushes the berries below.

Yes, the lighting and focus variables make it difficult. But won't
stop me from trying.

In The latest, sol 1148 on streak MIs (right), the berries have the same stoney (not shiny)
look as the off streak berries from sol 1135. (Both images are crops of the originals)
Maybe it's my bias, but the right image (on streak) has a cleaner, less dusty look.
There could be an argument made for more dark fines though, and what is the explaination
for the lack of intermediate fragments in the on streak image?

Click to view attachment

But the MIs on-streak appear to show clean, well-sorted dark sand/dust between the berries. It looks like either the light dust has completely blown away, or has been covered by the dark sand/dust. Or some combination of both!