Marcel:

Got to agree. I reckon these are really *old* dunes!

Which doesn't deny the possibility of salty sludge moving about here and there - we've seen surface frosts, and micro-climates may allow water flow in locations with appropriate solar heating etc (like, perhaps, some crater sides).

Bob Shaw

During post-Viking geologic analysis, there was a fair bit of "arm-waving" discussion of salt transport by "monolayers" of water molecules on soil grains. Gas or liquid molecules can be adsorbed <not absorbed> on a solid surface, held there by relatively weak electrostatic bonds to the atoms and molecules in the surface.

One of the major objectives of Mars Polar Lander was to bake fine soils and measure the evolved gas composition with temperature to quantify gas and water adsorbtion on martian "fines" A significant fraction of the martian atmosphere could be adsorbed on soil grains, to be released at higher temperatures and humidities. Some suggested several times as much gas in the atmosphere could be globally adsorbed in the soil.

Water, with its polar charge distribution and hydrogen bonding between water molecules, likes to form adsorbed monolayers. As humidity goes up, they become multiple molecules thick, but are still not liquid layers in a conventional sense. Such monolayers, it was suggested could transport salts over long time intervals in apparently dry soils, and produce duricrusts like the Viking Landers orbserved.

Interesting interview with Squyres on SPACE.com.
http://space.com/missionlaunches/050705_rovers_update.html
In particular, this statement: “We also have a hunch that the dunes that run from northwest to southeast, like Purgatory does, may be a little younger and a little softer than the ones that run north-south”

And clays are "polarized" by the nature of their crystal structure. I'm not sure what the mineralogy of the very fine, pervasive dust is, but I wouoldn't be surprised if it were a clay. Given the small size of these clay particles and the resultant surface:volume ratio I'd suspect that a large amount of water could be involved.

--Bill

Hi Bill,
There's a huge difference between fine dust of mineral origin and clay. There's no evidence that there's clays at Mars at all. Maybe there actually is clay, but we simply did not have the instrument capabilities up there to proove it microscope needed). Clay is clay if it consists of a claymineral (with cation exchange capacity: C.E.C.) AND if it is built up in the form of micron/nano scale, uniformly oriented plates. Only then the material has the chemical and physical properties of a clay; the possibility of long term bonding of water and elements via chemically induced adsorption of positive ions on a TREAMENDOUS surface (Na, Ca, K, Mg, Li, etc.).

Besides clay, I cannot think of another soil that actually can hold water in the Martian environment for long periods. If it is just grains (round, angular, blocky, whatever), eventually everything will dry. Except for chemically bond water like in Gypsum, when the watermolecules are built in nanoscale crystals and can't get out.

However: your theory is interesting. I might be wrong. Maybe the extremely fine grains do (on a modest scale) hold water in their matrix. But i don't think the fines can hold "large amounts" of water.....

Would this mechanism account for the flows around the exposed platy surfaces in endurance, or on the steeply inclined surfaces at Methusalh? Or is it just a 'baking out' and mineralising effect in general?

Good discussion, Marcel.

I was actually thinking of clay as _one_ component of the mars dust. We don't know if clays exist on Mars, but we haven't been able to look at the minerology closely enough to say yes or no. I recall that clays are alteration or weathering byproducts of silicates but I'm not sure if the conditions on Mars were ever conducive to clay formation.

But clays are found, that could be another data point for a wet Mars.

--Bill

Bill:

Clays are also prime suspects in pre-biotic chemistry, possibly providing a matrix allowing not-quite-alive 'cells' to form.

Bob Shaw

It appears to me that Purgatory, and the tops of most dunes in general, have a pavement of somewhat smaller grains, spaced further apart, than the pavement between the dunes.

I *THINK* the TES team on Global Surveyor or some group working in the middle infrared (1 to 5 micromters) has reported seeing minor <few percent> clay mineral spectral signatures in global dust spectra. And not terrestrially familiar clay minerals like montmorillonite and others, but uncommon ones on Earth. I have not read the actual published papers on these results, so I'm mentioning things I've watched float by on the info-tide.

I've wondered about that, too. I suppose that, logically, smaller lumps of material should get the chance to float higher than bigger, heavier ones...

Have a look at: http://www.spacedaily.com/news/mars-future-05p.html

And what does granite weather to?

Granite? Weather?

*Everything* weathers!

On a galactic timescale, I suppose so.