So after 20 pages of discussion, thank god we appear to have an official answer.

That's his current take - not the official definative answer.

Doug

Opportunity was futzing around in that spot for several sols with its IDD. The grains there are not as
cohesive, perhaps due to a lack of dust, so impressions don't take. All the pressing and brushing of
the IDD caused a brazil nut effect, bringing additional berries to the surface. Lets also assume an
optimum depth of loose material over bedrock for this effect, whatever depth that is...

Or.... the finger of berryless sand that Opportunity was investigating
was not a flat surface feature, but a tiny ridge of windblown material
extending out from the tip of the rock. Opportunity's investigations
flattened out this ridge exposing berries that were within or below
it. Again, due to its lack of dust and fine particles, the sand flows
as a liquid: marks and impressions are not easily retained.

I'm happy to hear that Jim's preliminary assessment is favorable to cleaning, but the real excitement tosol seems to be the remarkable change between sol 1145 and sol 1148 MIs. That is an unexpected change. That little ripple that was on the lee side of the rock is now mostly gone. There is a dent in the area from an instrument that might have disturbed the soil, but I also see dust being removed from other areas well away from that disturbance. Could the proximity of the instrument package, IDD, and rover have altered the local wind vectors here, causing that fine stuff to blow away?

This was bewildering me tonight until a friend sent me a link to http://www.flickr.com/photo_zoom.gne?id=466694284&size=o

That's quite a clincher. It seems pretty clear that it is windy here. I think Oppy should configure it's movable parts to create the maximum wind turbulence toward its solar panels.

That's a great show of the appearance of previously hidden berries, but
I don't think it shows any results of short term wind action. I think the
"dust" you see in the color image is really the result of lower contrast
and lack of focus. In the black and white image the contrast and focus
jump and make it look like dust has cleared. I look around but do not
see the appearence of any previously unseen berries anywhere but at
the sandy spot the rover was looking at next to the rock.

I think the IDD was placed there specifically to get a look at built up
windblown material with no berries and the activity of the IDD caused
the sand to slide or shimmy away revealing the berries.

Before and after...

Click to view attachment

Pile o'rocks...

Click to view attachment

...meteorite, you reckon? If so I'd love to get stuck into it... what a great strewnfield to study and gather specimens from!

sol 1144-1149 L247:



Maybe Oppy should stick around here for a while.

And here is the L234567 color version of my animation CosmicRocker's friend sent him.

Horton, that flicker gif of Alicante is exactly what I wished I could do! Thanks for that!

Rocker, what about the flicker gif convinces you it's windy? We do know it's windy, recall the light dust blowing out of the rover tracks. But it seems quite a stretch to say wind exposed these berries.

OK, now I see berries moving in the wind.
I see a group of small berries just off the 10 o'cock position of the sundial ball.
I see several large berries along the very top, right of center, in the image.
I see others moving here and there, all to the right, and down a little.
I can't think of anything but wind moving them.
But why no "cleaning event"?

GREAT WORK HORTON!

Holy smokes...so what's going on with the power output?

Jim Bell's comments yesterday were accompanied by colour pancam's and an MI that seemed to me to indicate that the dark streaks were areas where cleaning effects predominate resulting in a surface layer that is more densely populated than average with blueberries that are themselves very clean. He was very specific about the terms "swept clean" and "polished surface's" when describing the areas but as Doug says these were just comments and not definitive rulings. That said his comments seemed to me to be a strong endorsement of the hypothesis that the dark streaks at Victoria's northern edge are caused by cleaning not deposition. He didn't comment on other areas that seem to be composed of dark materials that have been mentioned in this thread by the way.

I think the nature of the annulus (a very flat smooth surface with a relatively durable surface layer with lots of blueberries) is a major factor here. Whatever long term process that has created it must be compatible with the short term effects - if deposition of darker dust\sand scale material is the dominant cause then it should dominate the entire area eventually as the crater has changed shape and eroded back. What we actually see on the ground is the effect of MY\GY of very gradual erosion of both the crater and the original ejecta debris field that has lead to this very flat blueberry dominated crust where finer materials (dust\sand scale) are preferentially removed by the wind.

The animation of the wind action between 1144 & 1149 is dramatic. Not to disregard Jim Bell's remarks... If the wind is this strong here, what's it like at the mouth of the bay? It appears to be capable of moving berries. Shouldn't it be moving the dark matterial in the bays and below the cliffs? And if that's the case, is there any way it would not end up in the streak area?

It has crossed my mind that the polishing of the berries might be a contributor to the darkening...

Yes it should, if there is any "dark material" there to begin with. Otherwise
it will move all the sand -- light and dark -- and completely clear away the dust.
Still, I don't see any unique "dark material" in any of the latest images.

I think we *have* seen a great example of saltation in these latest images, though. The movement of the berries is a perfect example of the wind rolling larger masses along the ground.

I do wonder whether or not Oppy's presence might not be causing greater wind turbulence right here. After all, the Martian atmosphere does get denser right at the ground -- it's possible that outcrops and other irregularities create a greater degree of turbulence than we'd think, using our terrestrial experience as a guide...

-the other Doug

There does seem to be the phenomenon of brighter soil under darker soil on the streaks. We see it again here. This is unusual for meridiani and it has been persistant on the dark streaks.

Is is difficult to know what we're seeing when the camera is pointed at the ground. Relative brightness to other areas (off streak) will require calibrated images.

Yes, of course, if there is any dark matterial there to start with. The thing I can't figure out is why there is any question of the dark stuff's existance.

I am having trouble figuring out which direction the wind is blowing (1144 - 1149 flicker gif). It looks to me that it is right to left. I don't think the berries are actually blowing along. I think the soil is being undermined and they are rolling down hill.

An other impressions?

On the topic of diffinitive evidence...

If we saw the same level of wind activity at the 1150 location, would that nail deposition for anyone?

Ed: I agree. In Horton's calibration target animation, the fine material has been removed, undermining a few of the berries, and they rolled down slope. I don't think wind itself is moving them. I only see such movement in the wheel tracks, which disturbed the soil and created the local slopes. His Alicante animation seems to show only fines being removed.

I am trying to figure out which way the wind is blowing, too. I'd expect it to be southerly, as fredk demonstrated with the earlier wheel track image, but it would be nice to find more evidence.

Sorry, am I looking at completely different images to everyone else, or does the MRO pic of Victoria - a crop of which I posted a page or so ago - clearly show unusual dark material at the foot of the bays in question, blown into El Dorado-like dunes and crests, thus proving wind action there?

Great animation Hort, thanks for that. I was pretty pleased with my "before and after" pic (tho clearly no-one else was! ) but your animation is spot on and very intriguing, so much so that I wrote this for ya...

Strange, don't you think, that
out of all the sweeping landscapes,
all the epic features on this epoch-
rusted world a wind-curled creep
of dust - no larger than my thumb -
should be The One, the focus
of such merciless, screen-staring
scrutiny? See it? On Horton's gif?
There, beside that rugged rock
a ragged rascal of a dune
has died and tumbleweed berries
that were hidden from our sight
are now bathed in frigid sol-light.
How? well, that's the thing.
Each mouse click thickens
the plot; but as suspects form a line
not a single soul Knows for sure
what alien wind blows here.
Forget Cydonia's Face and pyramids,
the prickly polar trees; this is a real
Mystery of Mars, a Victorian
melodrama more gripping than Dickens',
a whatdunnit for a Photoshop
Elementary Holmes to solve.

Thanks Horton and Stu--You both contribute great things at just the right time!

Floyd

Stu,

There are definitely dark areas in the images but you cannot assume from the MRO picture that there is fine grained (dust\sand scale) dark material there. We know that one form of dark material erodes out of the surface layers - the blueberries - but we haven't seen any fine grained darker material sources in the area, yet.

The reason I specify fine grained is that the deposition theory needs this - coarse grained sand and blueberries require extremely high martian wind velocities (> 70 m/sec, 250kmh ) to get them moving on anything other than a smooth flat surface. Winds of that scale are not common - they would seriously rock the rovers and possibly could roll them over.

The blueberry and dust movement highlighted by Horton is significant enough to be seen over a very short period of time because the duricrust is broken and there is very fine grained subsurface material exposed in the wheel tracks. We've seen this often in the past (e.g. around Purgatory where the exposed subsurface material formed noticable wind tails within a few days) although the effect here is quite dramatic since it leads to movement of the berries as the finer material gets removed.

Stu said "Sorry, am I looking at completely different images to everyone else, or does the MRO pic of Victoria - a crop of which I posted a page or so ago - clearly show unusual dark material at the foot of the bays in question, blown into El Dorado-like dunes and crests, thus proving wind action there? "

But is it dark or just freshly disturbed?

Phil

Nice Stu. You do have a way with words...

Well, to my admittedly untrained eye, the fact that 1) it seems to lie on top of brighter material, and 2) we can see ridges and humps and crests in those areas...

Click to view attachment

... suggest that it is genuinely dark material, and not just light material that has been 'darkened' be being freshly disturbed.

Either that or there are some young Arrakian sandworms inside Victoria being drawn to the thumping of Oppy's wheels as she rolls along...

When I look at a the the full original MRO colour image and look at Ant103's excellent Valley Without Peril Panorama I'm afraid I don't see any obvious indications that the darker areas around the edge of Victoria are anything other than blueberries that have eroded out of the wall and rolled down the slopes over the mega-years. That definitely happens and we have imagery at all scales that shows it, so why do we need a different explanation for it? Individual berries are only resolvable by the Pancam up to a range of around 10-15mm, beyond that and a pile of blueberries will look just like a pile of dark sand. It could be sand and it could be some other fine dark\black dusty material I suppose but we already know there are piles of blueberries there so they would be the best explanation at this point in time. If further poking about in the cliffs exposes a coal seam or if some spectrometry reveals that the dark areas are not hematite then we need to find different explanations but blueberries fit the bill better than anything else right now.

You points are valid - this is definitely dark material overlaid on lighter material. Some of the lighter material is the lighter coloured bedrock but some certainly is the lighter coloured material that makes up the internal sand dunes (and is almost certainly the universal martian red oxidised basaltic dust). The darker stuff exists but the most likely explanation for it by far is that it is just a pile of blueberries.

Thanks for the very considered response Helvick, much appreciated. here's a thought I just had (cue groans, I know...) If you look at that last pic I posted, you can see not just ripples and crests (the "sandworm tracks") but also quite a few boulders and loose, large stones. Forgetting the largest boulders, how big are the smaller ones - four, five inches high? And looking at the sandworm tracks, some of them appear to be, what, half as tall as those smaller boulders and stones? Yet, looking at images taken around the edge of Victoria during all these past months, we have never - to my knowledge - seen more than a couple of blueberries stacked on top of each other... being round, it seems to me that they'll have a very hard time grouping together into anything taller than 2 or 3 berries high, for much the same reason that marbles are pretty tough to balance on top of each other... they seem to flatten out. Have we seen any berry "structures" anywhere?

...so, down there, at the foot of the bays, there are either 1) dunes and crests of darker, fine dusty material, or 2) finely balanced structures of unusually cohesive berries that are somehow surviving the winds we've heard about whipping around inside the crater... and we've just seen how even a light wind - i.e. not light enough to clean Oppy's calibration target - was enough to roll some berries about...

..?

Now I'm confused. I've always thought that the Sweepers had in mind that the light dust is swept away to reveal the darker, courser grains in between the blueberries, as well as cleaning off the berries themselves. In other words, I've assumed that we believe that there are dark (ie darker than the light dust) grains everywhere on the apron, under the dust.

That's why I've always thought that the idea of Deposition was not controversial in that it required no mysterious source of a strange new dark substance. As I've written before, it could be just the same old dark stuff being blown out of the crater into the streaks. But we still hear mention of "coal seams" etc.

Perhaps it's worthwhile to remind people of Squyres's words, from here: "It could be dark sand that has been blown out of the crater," Squyres suggested. "Alternatively, it could be places where there has been selective stripping of fine-grained dust by locally enhanced winds," he pointed out.

Do the Sweepers believe that the Darkness arises just from cleaning off the berries? Or have they always thought there are darker fine grains that can be exposed too? It seems hard to argue against this, since we've seen piles of dark sand, for example in the lee of Alicante.

Similarly, like Stu I find it a stretch to imagine that those are dunes of berries in the craters, as opposed to dunes of the ordinary dark sand I assume is all over, but usually under the light dust.

I have to correct you. The wind direction was actually northerly, ie from the north, blowing towards the rim, based on the light dust blowing from the wheel tracks. See my original post.

As far as Horton's calibration target gif, the pancam was looking towards the rover tracks roughly towards the SW in those images. So a northerly wind would be roughly towards the left in those images. As others have mentioned, it's likely the berries just rolled down little slopes.

Still, if you look at all the pancam caltarget images from that location, you can see that the entire change visible in that gif occured between sols 1145 and 1147. There were no visible changes before or after that. So it appears we had relatively short term wind/gusts rather than steady winds.

Spoken like a true sweeper.

I like to think the sweep view is a traditional view of Mars, where, on the whole,
the surface is continually coated with a layer of light-colored dust. When the dust
is blown away a generally darker surface is exposed.

One thing bothers me though. The light outcrop rock at the edge of the crater is
darkened where the dark streak begins. Is this due to dark sand, or an accumulation
of berries? It would be nice to find out.

It is difficult to imagine windblown, blueberry ripples, but we are seeing what I interpret as relict ripples on Tierra del Fuego that are covered with abundant berries. It's really not that different from ripples we've seen from the beginning. Many, if not most, have been covered by a residual layer of berries and coarse fragments. They armor the ripples and retard further deflation of fines. I don't know if that is what has happened to those dark ripples inside the north rim. I was thinking they were dark sand being moved around and winnowed by winds that were not quite up to the task of driving them all the way up the slope.

fredk: We agree in spite of the words used. I see that southerly can mean "toward the south" or "from the south." I always understood that your observation indicated winds that were toward the rim.

I also agree that we do not need an exotic source for dark grains. There are plenty of them around, both inside and outside of the crater.

Yes, the dark ripples you speak of are really there. And I doubt that anyone denies there is wind at work.
But how "unusual" is the dark material? It is probably the same sand found all around in Victoria, but just
selected (sorted) for larger, darker grains by the particular wind currents in those bays. I think the less
imaginative sweepers and depositionists will come to agree at this point:

The sand in the dark bays has been sorted for larger(?), heavier(?), darker grains. These may also
be blown up over the rim by extra stong gusts. All the while, below and above the rim, wind gusts are
keeping the area clear of dust, maintaining the dark contrast with the surrounding area.

Unfortunately, I doubt Opportunity will get a close look at the dark ripples to see how much the
material resembles that of El Dorado on a MI scale.. I wonder how much the mini TES can tell.

This will be my contribution to the pandemonium for the day...

Why is the bed rock darkening on the streak? The bedrock is light colored matterial. In a clean sweep, it should at best not change. Let's just assume that by the clean sweep theory that there is a darkening of the bed rock, by whatever mechanism. Why does it only start over the rim where it's relatively flat? Is there not enough wind two or three feet farther down the slope? Isn't the wind highest in the chute? Isn't the rest of the bed rock around the rim light because of clean sweeping there?

It's easy to explain the darkening of the bed rock in the streak if it is dark sand or dust that is causing the streak. The slope and texture of the rock at the top of the chute wont hold the particles there. Right over the top, where the rock becomes horizontal, the particles have no problem resting there.

Hang on, that's having it BOTH ways. You're saying there is dark dust blowing up out of the crater but also saying that wind gusts keep the area clear of dust...?

Here's a colour comp from today's pancams...

Click to view attachment

Now, looking at that purely unscientifically, forgetting all I know (or think I know!) about Mars, that little craterlet at the bottom there looks like it's been covered and partially filled by something... dusty. It looks smooth, rounded, a bit out of focus if you like. Not sharp-edged at all. If this area was being swept clean of dust, wouldn't that crater look a bit sharper? Or would erosion over time have left it looking so smooth? Or is it just full of bluberries?? Just wondering, not putting it forwars as pro-deposition evidence m'lud...

Oppy! Get your **** over to that crater edge and take a good look at what's there, because this is starting to give me a headache...

Actually she seems to be pretty close to the edge now...

Click to view attachment

And finally...

Click to view attachment

Right, talk among yourselves, I'm off to work. I expect this matter to be settled once and for all when I get back.

Not!

This is a great point, Ed. I hope we pause there on our drive back!

My bet is dust free berries.

the berries already look pretty dark to me. How much darker do you expect them to get?

Edward,
I'm not trying to argue one way or the other, I'm just not sure I understand what you mean when you say that the bedrock darkens in the streaks. As far as I recall, all of the bedrock surfaces I've seen have been a pale tone, even in the streaks. But, I haven't studied the images intently. What am I missing?

Gray,

I'm not sure... Did you open the image from that post? I have circled in red the bedrock that is within the streak. It appears to be darker than the rest of the exposed rock along the rim.

I believe there are only two possibilities: The darkening of the bedrock is caused by dark
granular material covering the bedrock, or berries, or a combination. My bet is on berries.

Edward,
Thanks I did miss the earlier post. I think I'd tend to agree with CentsII. My guess would be that the darker tone is due to something covering the bedrock. But if the bedrock itself is darker - wow, that would be very cool.

Nice new images showing the appearance of light dust streaming from the rover tracks at the 1137 location, corresponding to winds blowing towards the south (these are always called northerly winds, according to standard, if sometimes baffling, English usage!):
Compare sol 1138: http://marsrovers.jpl.nasa.gov/gallery/all...IBP2678L2M1.JPG
with sol 1140: http://marsrovers.jpl.nasa.gov/gallery/all...IBP2680L2M1.JPG
The images were taken about an hour apart, so the lighting is a bit different, but the change is obvious.

Opportunity did spend some time in an area of bedrock at the start of
the second largest dark streak. See the yellow circle I added to Edward's image.
Click to view attachment

In a shot from the rim and a crop of the same shot (below), a lot of berries can
be seen on the bedrock. There is also some granular material. Unfortunately
no MIs were taken while Opportunity was in that area.
Click to view attachment

Click to view attachment

If you want to look around at raw images from the sol 1120 to sol 1132 period that
Opportunity spent in that area, Check here: http://marsrovers.nasa.gov/gallery/all/opportunity.html

Okay, now I'm having one heck of a time trying to square the shape of the dark streaks with a surface-level wind pointing in, towards the rim. That's about as counter-intuitive to me as it's possible to get.

-the other Doug

Dvandorn, I believe we have seasonal wind variations here. I've posted evidence of this before, as has Rocker.

In fact, based on the orbital imagery, I speculated that the streaks tend to get covered by light dust brought in by northerlies in spring/summer. That's now supported by these wheel track images. (I got my windsock!)

For a long time I've taken the complex pattern of features outside the rim to emphasize the obvious, which is that likely there are multiple processes operating here, ie deposition and sweeping. The question is, is there a primary cause of the darkest streaks?

I was looking at your post, fredk. Something caught my eye and it triggered a memory. It looks to me as though the dominate wind direction is toward the SE (coming from NW). The SE rim of the crater has much more bare rock indicating to me that the stronger wind blows in that direction. It's pushing the loose matterial back.

The triggered memory: Hasn't the talk always been of the prevailing wind blowing toward the SE (prior to streak discussions)?

If that's true (big if), then the streaks aren't even forming during the highest wind periods. I guess it is still possible the streaks are forming during the highest speed winds but the most wind energy (slower but sustained) is flowing in the other direction.

(my contribution to the pandemonium)

Looking carefully at the picture of Alicante that Stu posted a while back: you can see that some berries have been uncovered in the lee of the rock.

But it also looks like some of the darker material was deposited in the front of Alicante and that it covered a few berries. Look closely at the blue circled area (especially near the 3 o'clock position) in the two views.


So the dark material is shifting around. Could we be seeing the dark zones slowly being erased?

Are the dark zones themselves seasonal?

-Mike