Wow, Stu, I felt like I was there! Beautiful description!

It reminds me that, according to the Depositionist model, I don't see why the Darkness needs to have a different compostion than a dark component of the soil we've seen all over the apron, ie the dark stuff that the Sweepers claim is revealed by the wind. That's why I wrote sufficient but not necessary above. Perhaps the Darkness is the same old stuff, but just available inside the NE rim in enough quantity (due to exposure after rockfalls, perhaps? We do see more boulders in the NE of Vicky) or in small enough grain size to be wafted up by wind into the streaks, as Stu described so well.

Would this ease the mind of a Sweeper at all? It certainly sounds economical. I had the impression some here thought that according to the Depositionists the Darkness had to be some unique, bizarre, previously unseen type of material. Shaka wrote above: "Point to the dark material in an MI." Well, in any MI on-streak, it's the stuff between the berries! We all know that. The question is, what is it, and was it deposited or revealed by sweeping.

One of the (many) reasons I love it here is because it gives me the opportunity to take part in these volleying debates, without fear of being insulted or slapped down. I've enjoyed tonight's discussion, thanks to everyone who stuck their head above the trench!

I certainly didn't mean to shut anyone up! I only meant we seemed to be going in circles as far as the arguments presented so far. That's why I'd like to hear some new ideas, in particular, what would it take to convince both sides that either model is correct?

Man, I wish I had a cat...

Here, have this one...

Click to view attachment

I expected the smoke monster from "LOST" to appear.

"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..." -- Stu
Click to view attachment
http://www.lostpedia.com/wiki/Image:23psalmmonster2.jpg

Maybe I'm not that far from (some) depositionists. My main problem
with deposition is when it invents some new, isolated, dark mass.

It is obvious that the sand consists of dark and light grains. I can believe
that some persistant wind pattern could selectively sort the grains resulting
in areas with an enhanced amount of dark grains. This, combined with the
removal of light dust by the same winds could create both the dark areas
below and on the rim of Victoria. [But] If there is an increase in the percentage
of dark sand in the streak, it is not enough to be obvious to me in the MIs.

Going back to Steve Squyres' description of El Dorado:
"...chemistry very similar to average Gusev soil though somewhat higher in olivine.
This is very clean stuff. Mössbauer mineralogy: lots of olivine, pyroxene, essentially
unaltered, low Fe3+, very clean." http://www.planetary.org/blog/article/00000497/

Note that even the obvious dark deposit of El Dorado had "...chemistry very similar to
average Gusev soil". No new mystery substance.

Another way to discern between deposition vs clean-sweep is to look at the mouth of the bays where the bedrock is exposed:

* If there is 'dark material' lying on the bedrock that is not on adjacent bedrock areas that arent dark, then we could bolster the case for depostion, since the bedrock couldn't be hiding lighter dust under a surface layer of dark grains.

* if the adjacent bedrock areas have more dust on them (presumably on top of the dark material), then you could make a better case that the darker areas are swept clean.

...and it certainly looks from the image that the mouths of the bays look just a little bit of 'darkened' so presumably they have a slight peppering of dark material that has accumulated on them. Yes, its hard to tell, and each camp could take it either way, but its the next waypoint as far as im aware, oppy's toe-dip isnt it? so we shoudl have our results soon.

Why yes, it would, as you can see from my post above.
I'm still not sure if there really is a significant increase in the percentage of dark grains in the streaks
or if the darkness just results from lack of dust. There does look to be and increase in sand -- both light
and dark -- in the streak, so I do see clean (dust free) sand being blown up and over the rim. The dust
having been blown away while the sand still sat below the rim.


Well, some depositionist comments did give this impression.

I think this is a good idea. A look at grains caught in the the nooks and cracks of the
bedrock at the rim edge, both in streak and non streak areas, could be instructive.

Here's my take on possible sample sites.
Click to view attachment
(This is a crop from Tesheiner's Opportunity Route Map)

Yeah, that would likely convince me (though, while I still lean towards clean-sweep, I'm not comfortable that it explains the apparent medium-grained cover over the intermediate-sized soil grains in the on-streak soils). If deposition is the correct theory, I would also expect to see a similar relationship in larger dark grain size from the sides of the streaks into mid-streak. Considering the feathered appearance of the boundaries, if the streak comes from the wind's dispersal of a fount of dark material coming out of the crater, then deposition ought to peter out at a predictable rate as you move to the edges in any direction.

That wouldn't convince me as much, since both deposition and clean-sweep could result in clear compositional differences -- especially at the resolution available. If you could prove to me that there are compositional *matches* between the streak soil and the occasional darker spots within the crater, that would convince me more -- but not entirely, since both could be areas where similar underlying material is being exposed by deflation.

Problem is, not even the APXS and Mossbauser are capable of resolving the composition of individual soil factors. And mini-TES is nearly useless for such a fine-scale resolution. We get good data, but there's a limit to its resolution. And composition of individual soil factors is beyond that limit.

I fear that, without being able to say "yes, the soil here has 12% more grains of this mineralogical characterization," or somesuch, the averaged compositional data we get for soil will only fuel speculation, not provide definitive answers.



-the other Doug

fredk: Your suggestion that we agree on certain observational tests of each hypothesis is a good idea. I think it is difficult to come up with a simple observation that would satisfy everyone. For example, I thought the simplest way to convince people was to show MIs along the trek where the difficult-to-move sand grains between the berries essentially remained the same, while the smaller dust fragments became sparser as the rover moved from light to dark areas. I thought that was a good test of clean sweep.

Your proposal that "Deposition predicts larger dark grain size the closer you get to the rim" would be very convincing to me, if that process was the only one to operate on the soil grains in the past. I don't think sand-sized grains are being moved much in the current wind regime. Sand-sized and even larger clasts obviously were moved by the wind at some time in the past, but that period in Martian history seems to predate these streaks. For the several years that this rover has been observing the surface, it has only seen evidence of relatively fine material moving with the wind. At some time in the past, however, I can imagine stronger gusts of turbulent wind blowing up the northern bays and moving the sand grains, resulting in a size sorting where a greater population of larger grains remains near the rim, and smaller grains are distributed further out.

If you folks could find evidence of very small, dark grains accumulating widely in the dark streaks, that would be convincing.

Well, some depositionist comments did give this impression.

That would be me. I'm not sure if the dark material is coming from a vein of familiar stuff, or from a vein of new stuff from elsewhere, very dark subsurface ejecta laid down after an impact yonks and yonks ago. Just thinking outside the crater box. Mind, I really don't think it's that crazy an idea, for reasons given in a previous post. Guess we'll find out soon when Oppy reaches the crater end of one of the streaks. But I do think it's significant that the streaks are all found jabbing away from bays in the area of the crater where (to my untrained eye) we see the most dynamic erosion features - the most eroded cliff walls, the most rockfalls, the most disturbed dust deposits on the slopes, etc...

Looking around, you see me standing on the lip of the crater, one bay along, in the act of launching a model glider into the wind. It's an odd-looking thing, with large but low aspect ratio wings - something to do with the Reynold's Number, you think.



I'll be tacking the plane in front of the slope - to the left of the diagram, where the thin air is forced upwards - in order to gain height.

For fun, and with enough altitude, I'll sometimes fly it away from the crater, turn back into the wind, and do a fast pass over our suited figures. But what I won't do, is fly low and into the rotor you get just behind the lip of the crater. This is an area of sink: it can be unpredictable and fatal.

You can tell where the sink is, as it's where the wind-borne dust gets preferentially dumped on the ground. Behind it, away from the lip, the airflow returns to something a little more laminar, where the dust can be cleared.

Andy

Now THIS is fun! Flying gliders on the edge of a crater on Mars!

Click to view attachment

I LOVE this place!

Seriously, that made a lot of sense and cleared some things up for me, so thanks for that Andy.

No sign of solar panel clean up yet ? This will be the definitive answer.

Gliding isn't easy on Mars ( as anyone who's flown X-Plane will tell you ) - but I have flown RC gliders off terrain not that different to the Bays and Capes of Victoria. You have to fly SO fast through the thin Martian atmosphere to get the lift to stay up - but your motor (gravity) is so weak. It's an interesting engineering challenge though. Planes on Mars might look a lot like the U2 Spy plane.

Doug

Could small vortex (not a dust devil) form in the bays and help lift dust?

This may explain the dark patch inside the NE bays. Maybe little vortexes form (not large or self-sustaining enough to be a dust devil) in these particular bays due to local topography.

This gives a localized cleaning event on the rocks of the bays, as the vortex lifts out it disperses into downwind gusts which gives a cleaning event on downwind of the bay mouths.

So the darkness in the bays may be due to cleaning events in the bay when then translate to downwind of the bays. The orientation/shape of the bays just happens to prefer vortex formation.

If the prevailing wind direction shifted, another set of bays somewhere else on the crater would get cleaning events.


So, what observation would eliminate this hypothesis?

-Mike

That's more or less the original Sweep idea, as I understood it. What got me thinking about this a long time ago was that it seemed very odd that you'd get that kind of vortex cleaning from the NNE bays but not from the NW bays, like Duck Bay and neighbours, which appear to be symmetrically placed with respect to the prevailing winds (recall my original post in this thread).

Of course we can't say there isn't something odd about the local topography in the NNE that results in far more/stronger vortices, but it has always seemed like a stretch to me.

Okay, here's our "playground"...

Click to view attachment

It's clear that there are winds wafting and wufting about in this NNE area because the dark material here is sculpted into dunes. I guess the question is, are the wind effects here strong enough to make the leap up out of the crater and clear the streaks we see?

And if the shape of the bays "channels" the wind, wouldn't the streaks be aligned with the bays, and not jutting away from them at the broken leg angles we see?

The angled plumes of the dark streaks are indicative of a "racetrack" wind flow within the crater that breaks out at the north and northeast bays.

Think of a bowl of soup. Half-fill a bowl that has several scallops in its rim and begin swirling the soup in the bowl -- in this case, counter-clockwise. When the soup rises high enough along the sides of the bowl, it will stream out of the lowest scallops in the rim and continue on in a line that starts out tangential to the rim, but that sweeps out from that tangent as the materials flows outward. Leaving exactly the kind of "broken leg" pattern we see at Victoria.

Martian craters seem to be very good at "spinning up" the winds within them into racetrack patterns. If the coriolis forces are strong enough, and if the surface heating conditions are right, this spin-up can cause organized vortices (i.e., dust devils) to form. Even when dust devils don't form, though, most craters still develop these racetrack-style wind patterns. It's quite common to see signs of such racetrack patterns as they blow out of low spots in crater rims.

-the other Doug

Thanks for explaining that t'Other Doug; makes more sense now.

Here's another little gedankenexperiment for those of you who are having a hard time understanding why there are only streaks from some of the bays, and not all of them...

Prop a bicycle up so that one of its tires is free to rotate, without touching the ground. To take gravity out of this little experiment, set the bike on its side so that the wheel's axle is perindicular to the ground. Now take a garden hose with a spray attachment and spray a high-pressure stream tangential to the tire, to spin it up.

You'll find that the water which stays on the wheel as it turns will be sprayed off primarily from a fairly small arc of the wheel, about 120 degrees in the direction of rotation from where the main stream is hitting the tire.

It has to do with momentum and how long the material can be accelerated before the force holding it to the wheel (adhesion, mostly) is overcome by inertia. And it has to do with the original vector and location of the water stream.

Similarly, wind spun up inside a shallow bowl isn'g going to stream out of every available bay equally -- it will spin out where its momentum tosses it out, and since the pressures within the crater aren't even, that will be somewhere between 120 and 270 degrees "down-rotation" from where a majority of the wind enters the crater.

So -- the answer as to why Vicky streams her streaks out where she does is because of the direction of the prevailing wind, where the wind primarily sinks into the crater, and where the pressure bulge of that wind's entry tends to push it back out.

-the other Doug

doug,

explain how the streaks in fit into your model...

Dvandorn, there's certainly no difficulty explaining the asymmetry of the dark streaks given an asymmetric wind flow within the crater! But the problem is that the prevailing wind defines a single direction, and does not by itself give preference to "to the left" or "to the right" of that direction. You need to break that symmetry somehow. Ie, why should the wind flow counterclockwise inside the crater rather than clockwise? It's not obvious how a more-or-less circular crater could break that symmetry in a significant way, without an odd dependence of wind on subtle details of topography.

Coriolis could do it in principle, but should be important only on very large scales, like 10s of km and up.

I don't think the bays give the asymmetry here -- I think it has to do with the uneven height of the rim. Wind will enter the depression along the lowest points of the rim, and that likely determines how and where it spins up its racetrack pattern.

In fact, there are some indications that, at some times of year, the racetrack wind pattern in Victoria changes from counter-clockwise to clockwise. Look at the muted streaks coming off the western side of the crater -- those seem to indicate a spin in the opposite direction from the streaks off the north-northeast side of the crater.

Hey, maybe that explains why Victoria has bays and other, similar-sized craters in this same area do not. Victoria is situated on a slope, and that slope allows its internal wind pattern to shift seasonally. So, instead of a billion years of counter-clockwise rotation, it sees seasonal switches in wind direction, which enhances aeolian erosion along the crater rim. So, the same winds that would otherwise have reinforced the circularity of the depression by eroding everything in the same direction all of the time instead do this seasonal push-pull routine, which encourages greater and greater erosion along the rim. Probably also generated the overhangs and undercutting we see in the cape cliffs.

-the other Doug

Thanks all, for an interesting discussion!

Victoria crater does seem to have an assymetry in the bowl shape. Looking from straight down, the S side seems to have a shallower slope then the north side of the bowl. (The sand dunes at the base also seem situated slightly to the North of center).

Could the assymetry in the bowl shape, coupled with a prevailing wind direction from a vector different to the N-S axis, be enough to cause the effects we are seeing?


-Mike

["Gedankenexperiment" must be German for "will be fun on a summer day!"]

Again, it certainly is possible that topography, including rim height, breaks the symmetry. But remember that Meridiani is extremely flat. From the newly released topo map the rim heights vary by only a few metres, compared to 70 metres or so total depth.

My appologies for sounding like a broken record one day after complaining we were going in circles with these arguments!

Here's a way to maybe break the deadlock. Perhaps someone could build a 3D model of Victoria, then perform some tests by letting wind blow over it and tracing the airflow with smoke. I don't know what would be involved in capturing the dynamics on such a different scale. Perhaps the test would need to be done underwater. Dip your model in a running stream and inject squirts of food colour or silt particles or something. Definitely a fun way to spend an afternoon!

I would almost be buying this. And I'd purchase the (wet?) wind tunnel tests too. Stu - time to dip the plasticine Victoria in the bath and pull the plug.

<rhetorical>How many forums could I meaningfully say that last line in???</rhetorical>

Doug - the RC issues. I personally think the pressure/gravity situation is a killer for realistic small flight on Mars. But there's always a good side: you'd be hard-pressed to find radio interference on your chosen channel.

Andy

Mars and UMSF: you've got to love this place!

How 'bout a bike trip down tp to the beach on a day with a constant breeze?

We can build a model of Victoria in wet sand in the right orientation, then dribble sugar sand near the rim of the model and see where it ends up.

(Then we do the Gedankenexperiment to wash the sand off our bikes!)

-Mike

dvandorn:
"Here's another little gedankenexperiment ... Prop a bicycle up so that one of its tires is free to rotate..."

Edward Schmitz:
"explain how the streaks in fit into your model... "

Uh... They're the spokes?

Phil

I just tried the mind-numbing exercise of going through the MI images and counting representative zones and putting grains in "bin-colors" in order to quantify if there was any difference between on-streak particle count and off-streak particle count.

I figured the ratio of brightest to darkest should be independant of lighting.

There may be a difference, but it might be really subtle. Again, I'll leave the particle color breakdown to image-meisters and their software toys.


I was almost able to convince myself that it "looks" like there is a concentration of dark particles in wind shadows of the beads. I was also able to convince myself that this has been seen in many other images, whether on-streak or off-streak. If this is real (I need a second opinion - I'm pretty much seeing inkblot images of my third-grade bully at this point), then perhaps there is a dark material being deposited all over in little pockets near berries. And perhaps this dark material is just a little bit more concentrated in the on-streak area than off, but just enough to make a difference. We're talking just a few % here.

Has anyone quantified the gross color difference between the two zones? Assuming that the color difference were due to jet black particles, how much % increase would be required?

Here's a picture that "seems" to indicate dark stuff in the windshadow of the berries or is it just shading.

Only the Shadow knows for sure....

-Mike

And while perusing MI's I also came across similarities in MI images between Victoria apron, and those near Vostok. The ("polished"?) aspect of the berries seems very similar.

-Mike

Andy, I might have to get The Model down off the wardrobe...

Click to view attachment

Ah, happy days, happy days! When we were all still open-mouthed with wonder at the first real sight of Victoria...

So, I pour flour in, point a hairdryer at it from one side, and see what happens...?

Thats food for thought, but i took a different track here (but are there any meteorologists in the house that can help?):

* The racetrack winds are actually spinning clockwise: Duck Bay and Bottomless Bay form the preferential exits for any relative pressure building up in VC due to the prevailing winds, simply because they are gradually sloped and seem to be the lowest points. Since the air pressure primarily exits there, the air speed becomes faster on the left side of the crater, this causes a net clockwise rotation inside the crater which will tend to push the dark grains over the edge of the little humps formed by the debris from the cliffs and pile up the the rotational windshadow on the cliff slopes. there seems to be actual evidence of this process in this photo

* the reason why the dark stream changes course and veers to the west is merely due to the prevailing winds which are flowing in the 'general' direction the the streaks end up pointing. The air after exiting the crater becomes part of the larger windstream and follows that course, forming the deflected streak. From looking at the overhead shot showing the apparently different directional streams, it seems apparent that there is no deviation from this process since any air should exit the crater according to the crater local topography (radial from the crater) and then merge with the prevailing wind direction. The apparent deviation to this flow in the lower right looks more like a contrast artifact from the lighter evaporite dust that has blown onto the SE.

* so why arent there dark grains on the left side of the crater? There might actually be darker material eroding from the steeper cliffs on the NE than there is anywhere else in the crater for this reason: Note that the steep areas, which are probably not coincidentally associated with the dark streaks, all are formed along what is arguably the most linear feature in the crater as seen in the overhead image as well. his suggests this might be a fault or other feature that might have some concentration of one type or another material or might be more easily eroded.

Ah, following the rigorous experimental example of our illustrious founder?

This new view of Alicante shows more berries in the lee of the rock compared with the earlier pancam view. They must have been revealed when the instruments touched the surface.

It's interesting, because that means that many of the berries in this MI were originally burried. This view is a closeup of the lee deposit. You can see this same region in this MI taken earlier with fewer berries.

It's remarkable - I never would have guessed that the berries in the first MI were burried before we exposed them.

There is absolutely no sign of APXS or Mossbauer contact plate impressions on the soil in the area in question, in your "after" shot. And besides, in every other experience we've had with Meridiani soils, when you press down on the soil, blueberries are shoved *under* the fines and disappear from view.

Oppy didn't exhume those blueberries. There is no conceivable mechanism by which they could. Especially not without any other signs of soil disturbance. (Maybe, just MAYBE, if you RATted the soil, you could exhume buried berries. That has very obviously not happened here.)

If some berries are visible in the "after" picture that weren't visible in the "before" picture (and I agree, there are), then the dust that had been covering them has to have been swept off.

By the wind.

Clean sweep party, anyone...?

-the other Doug

Well, Opportunity was doing something over there.
Click to view attachment

I'd agree that the topography itself is subtle enough not to be the cause of this - but there is another factor.

Just before dawn at Victoria, the crater has cooled all night, and is going to be more or less filled as a hollow with a pool of cold, (relatively) dense air. The prevailing wind will blow more-or-less unhindered over this.

Dawn.

Shortly after the sun rises it's going to be beating straight onto the western cliff faces - they'll never get more insolation than this - while the eastern ones are still in darkness. As the western rocks heat up, they'll warm the air next to them. Rising air on the west side of the crater will draw air down from the east side. Add in the prevailing wind (anything southerly) think about the vectors at the interface, and the air in Victoria will tend to rotate clockwise.

Throughout the morning, this clockwise flow will peak and then will lose power, and sometime after noon it will stop when the western cliffs lose the sun. In the evening the flow is reversed (warm east cliffs, cold west ones), and as dusk arrives, the flow will be at its strongest in an anticlockwise direction.

Question - given the potential diurnal movement of air within Victoria, what does this say about the deposition or cleaning events? My gut reaction is that the greatest mass loss (dust sourcing) from the walls of Victoria will be caused when those rocks change temperature the most. The western walls at dawn (clockwise flow), the eastern walls more subtly at dusk (anticlockwise flow).

Andy

The daily heating cycle is a good point, Andy. It's not obvious to me how rising air on one side leads to either clock- or counterclockwise flow, but in principle there could be an asymmetry due to what I assume would be more rapid heating on the west side in the morning than on the east in the evening. Another factor might be wind speed or direction changes during the day.

Actually there is an imprint visible on the soil. It is visible in L7 and R1, so it's not an artifact:
Click to view attachment
You're absolutely right that normally an imprint buries the berries. But recall an earlier post of mine where I pointed out that the wheeltracks looked very different on-streak compared with off-streak. The berries were still visible in the tracks on-streak. That is exactly the kind of anomalous behaviour that you've pointed out here, in the lee of Alicante, with berries becoming visible after contact is made.

What all of this points to is that the behaviour of the fine soil on streak is distinct from that off-streak, and unlike anywhere else to my recollection. That suggests something unique might be going on here, and perhaps deposition fits the bill.

Heating leads to dust devils in some smaller craters.
perhaps it leads to sub dust devil currents in larger craters.
Maybe not a circular current within the whole crater, but
a current within an individual bay.

In addition to my previous comment, I'll add that what you're proposing here is the removal of the relatively coarse dark grains by the wind, not just light dust (of which there's very little on-streak) as Sweepers normally invoke. In other words, you're invoking winds powerful enough to do what Depositionists require!

One more point. We know the arm touched the region where the new berries appeared. What are the odds that wind would happen to expose the berries at that very spot just after we examined it with the arm? Can you find any other areas where berries have been exposed? Occam argues that the simplest explanation is that the berries were exposed due to the action of the arm.

Come up with a believable mechanism by which contact with the arm could have generated the *opposite* result to what we've seen every other time it's been pressed into the soil, and I'll grant you the point.



-the other Doug

Well, we're in an interesting situation here, all right. We can see with our very eyes that berries have appeared after the arm did its thing. The only explanations we have yet are:

1. Bizarrely timed and unusually strong wind gust happens to expose the berries just at that spot just after we were there, or

2. Unusual property of soil/substrate allows buried berries to be exposed.

Given the unusual appearance of the rover tracks on-streak, we have corroboratory support for 2.

I'm no soil expert, but couldn't you imagine a hardish layer below the berries, so that when you press down on the surface the berries can't sink much, but you either compress the fine grains or expell them outwards and expose the berries?

Again, perhaps it's time to get out some flour or icing sugar or whatever and some ball bearings and see what you can come up with!

The soil here is highly mobile. Much more so than any place else we've seen. The exposed berries are not where the contact was made but just next to it. We've all played in the sand. There is a big difference in the way that loose sand acts as opposed to crusty sand.

There are not other spots that have any different in the two images. It's not credible to suggest that we just happened to pick the spot very spot that would show a difference.

As the instrument pushes down, a berm is pushed away from it. The pressure wave hits the near side of the berry and the sand next to it at the same time. Since the berry is much larger, the far side is pushed long berfore the sand grains on the far side see the pressure wave. They pop right out.

btw: Fredk, that was a great catch...

Here's something else. The lee of the stone should be a low wind area. if there was a gust that would uncover stuff in the lee area, shouldn't it have disturbed other areas as well?

From the UK IMAX discussion:

Hmmm...

Click to view attachment