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Posted by: SigurRosFan Feb 2 2006, 07:01 PM

- http://www.msss.com/mars_images/moc/2006/02/02/ - Broken Plain

--- This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows polygonally patterned ground on the floor of a trough in the southern hemisphere of Mars. The polygons could be an indicator that ground ice is or was present at this location. The dark streaks were formed by passing dust devils. ---

Posted by: stevo Feb 2 2006, 08:04 PM

I am intrigued by the previous days image:
http://www.msss.com/mars_images/moc/2006/02/01/

If you look closely on the left side there's a layer of what appear to be boulders partway down the slope. Is this common on Mars ?
My ancient fragmented memories of terrestrial geology associate this with glaciation. Am I way off base ?

Steve

Posted by: Richard Trigaux Feb 2 2006, 08:10 PM

If we removed all the sand, perhaps Meridiani planum would look like this (with troughs like Anatolia).

Posted by: Richard Trigaux Feb 2 2006, 08:14 PM

If you speak of the boulders visible half-way of the gullies, I think that simply the gullies (and all the slope) were cut through several layers, one of these would give boulders and the others not. This, I think, has nothing to do with glaciations. (but the presence of gullies could).

Posted by: stevo Feb 2 2006, 08:23 PM

Sorry Richard, my question could have been better phrased:

What processes on Mars would lead to a layer of boulders like this?

Steve

Posted by: Richard Trigaux Feb 2 2006, 08:42 PM

Many.

It looks like there are at least three different horizontal parallel layers in this ground. A harder one gave boulders, the others not, because they were more fragile, or basically of a sandy or powdery structure.

The two possibilities to have multiple parallel horizontal layers are :

-sedimentary deposits (in a large body of water, but wind can sometimes do the job)
-multiple lava layers, eventualy hard ones (plain flows) or more fragile ones (ash deposits).

The second possibility is much more common on Mars, but we cannot exclude the first. And on this photo we cannot known. Perhaps some Mars specialists better know the context of this image and may have more accurate answers.

Posted by: SigurRosFan Feb 2 2006, 09:37 PM

And if we removed all Meridiani sands and the polygonal features, perhaps it would look like a big subsurface ice layer.

Posted by: Bob Shaw Feb 3 2006, 12:44 PM

Putting on my crater-counting hat, I see none. Zero. Nada. Zilch. Nil points.

So, this looks like a surface which is *younger* than other surfaces, such as Meridiani and Gusev (unless they just happened to be unlucky and got whacked by late rogues) and yet which has the presence of ice at least *implicated* by the troughs. Hmmmm...

Bob Shaw

Posted by: Bob Shaw Feb 3 2006, 12:57 PM

Steve and Richard:

I see more than four layers, and evidence of repeated (though brief) 'flood' events ('flood' because it doesn't *have* to be water, though we *are* looking at fluidised material as the cause of the gullying). Towards the lower right of the image are to be seen alluvial fans, small islands, and typical wadi structures - this has all been going on for a long time. At the top of the image, below the gullies, are also the remains of previous flows, including a bluff which has resisted the flow to some extent.

Again, not a crater - though I'd expect that there's lots of dusty sand, which would eat craters over time.

Bob Shaw

Posted by: AndyG Feb 3 2006, 03:29 PM

It reminds me of one of artist Andy Goldsworthy's http://www.artincontext.org/LISTINGS/IMAGES/FULL/S/AH2N9OES.htm.

Posted by: SigurRosFan Feb 3 2006, 06:22 PM

Exactly. One of my Goldsworthy favorites is 'Storm King Wall'.

Posted by: slinted Feb 4 2006, 01:04 AM

I couldn't agree more and I love his work. I just saw a recently finished piece he made showing http://www.meridiangallery.org/images/Goldsworthy.jpg (single linear crack, earthquake inspired, as opposed to polygonal, like his clay work). Brilliant stuff!

Posted by: RNeuhaus Feb 4 2006, 01:18 AM

The gullies might be the product of the acumulation of humidity on sands caused by the transit evaporation of water from poles. Then, when the weigth of sand (gravity force) becomes greater than the friction and slope force, then the sand start to slid on some surface that has no humidity (the line between humidity and dry sand) and the other possiblity, as Richard, as said is about the layers of sedimentary of land. The harder ones is the last surface to be slide.

Rodolfo

Posted by: abalone Feb 4 2006, 02:33 PM

Except there is not a single crater visible. what sort of age does that put on it, must be very recent.

Posted by: Bob Shaw Feb 4 2006, 05:21 PM

Yes, at least in Martian terms. There aren't any eroded remnants at all that I can see, nor any dust to mantle craters. Nor are there any Europan-like palimpsests which might betray the presence of old impacts into icy material which have subsequently relaxed back to the datum level.

Bob Shaw