Nature of Victoria's dark streaks, swept clean, deposited, or other? Options

[centsworth_II]

... dunes of the ordinary dark sand I assume is all over, but usually under the light dust.

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.

[CosmicRocker]

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.

[centsworth_II]

... 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?

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.

[Guest_Edward Schmitz_*]

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.

Attached thumbnail(s)

 

[Stu]

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.

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...



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...

[Stu]

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

[Stu]

And finally...



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!

[fredk]

Why is the bed rock darkening on the streak?... Is there not enough wind two or three feet farther down the slope? Isn't the wind highest in chute?

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

[centsworth_II]

Why is the bed rock darkening on the streak?

My bet is dust free berries.

[Guest_Edward Schmitz_*]

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

[Gray]

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?

[Guest_Edward Schmitz_*]

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.

[centsworth_II]

...[bedrock that is within the streak] 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.

[Gray]

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.

[fredk]

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.