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Posted by: nprev Mar 14 2008, 03:37 PM

I remain mystified by the layers of bright material(s) that Spirit discovered just under the surface via the dragging wheel (which may go down in history as one of the most serendipitous breaks in the history of science!) The mystery is why do these layers exist so discretely, just barely below the surface?

The lunar regolith seems well mixed from continuous impacts of all sizes. On Mars, winds & dust-devils in addition to relatively frequent mid-sized meteor impacts presumably provide even more transport, at least within the upper few cm of the soil. So why are the layers there at all?

One reason I'm interested in this is that more than thirty years ago I built a Mars jar for a science fair project. My soil mix consisted of various iron oxides and sulfides/sulfates (magnesium sulfate ws one of them), all in fine powder form. The sides of the tank were transparent, and I noticed after a few weeks that the white MgSO4 formed discrete layers under the surface. Thought this was some sort of screw-up in the mixing on my part at the time, so it was odd indeed to see Spirit find something so similar.

Anyhow. Any thoughts on how these layers in Gusev came to be, and how they apparently manange to persist?

Posted by: ElkGroveDan Mar 14 2008, 04:29 PM

It's probably more precise to talk of the subsurface layers of the 'Columbia Hills' rather than Gusev in general in this context, since 90% of Gusev's subsurface layers are likely basalt. But then again maybe I'm just splitting hairs.

Posted by: nprev Mar 14 2008, 07:13 PM

I'll buy off on that, Dan; the Columbia Hills do seem quite different mineralogically than the Gusev plain areas.

Posted by: JRehling Mar 14 2008, 09:07 PM

I'm puzzled by the hills in the first place. My mental model is of Gusev as an impact basin that formed in a time/place where the planet was thermally dead. Which then had floods, possibly catastrophic. And which only after that received a cumulatively large deposit of other stuff, either from impacts or wind. I would expect that stuff to be no more active in any sense than a flowerpot on your porch. The big question is where the hills came from in the first place, much less how there was any activity of any kind IN them.

A summary of the local epochs:

1) Crust forms.
2) Gusev forms.
3) Floods from Ma'adim.
4) Stuff fills Gusev.
5) Something bursts up.
5a) To form the hills.
5b) Within the hills.

Gusev made the list of MER sites because of (3). Anything of interest that Spirit found is owing to (5b). Very strange.

Posted by: ngunn Mar 14 2008, 10:30 PM

A nice summary of the situation JR.

Suppose Gusev got flooded by basalt lava that covered earlier hydrated rock layers. Where would the water cooked up at depth break through to the surface? Maybe at the thinnest point of the flood basalt - directly above the original crater's central peak?

Posted by: ElkGroveDan Mar 14 2008, 10:44 PM

I've always imagined the Columbia Hills as an "Islands in the Stream" situation, though I promise not to break out in song.

If there were violent inflows of water into a crater with a central peak, the peak could grow by way of an accumulation of flowing material and debris, and it could even migrate over an extended flow period, meaning it needn't even be situated in the geometric center when it was all over.

Posted by: ngunn Mar 15 2008, 12:09 AM

Central peaks tend to be rather chaotic anyway. The highest point could easily differ from the geometric centre.

Posted by: EckJerome Mar 15 2008, 12:52 AM

I have long been under the impression that the Columbia Hills are composed mostly of ejecta from a later impact within Gusev Crater...and that's why they're such a jumble of different rock types. It's likely that the hills contain material that used to be below the basalt which now covers most of the floor of Gusev.

Home plate itself obviously formed in place, though I don't know if that was before or after the formation of the Columbia Hills.

Posted by: dburt Mar 15 2008, 02:05 AM

nprev - Getting back to your original question, the bright "wheel-drag" material just below the surface tested out as largely unspecified ferric sulfates, if I recall correctly, and its distribution was rather spotty, not a continuous layer. Nevertheless, it is probably an efflorescence (or "flowering") of salts, analogous to the MgSO4 layer produced in your Mars analog experiment of many years ago, and also analogous to the bright sulfate-rich layer found just below the surface of Meridiani Planum, and to colorful ferric sulfate "efflorescences" seen on desert mine dumps above where sulfide minerals are weathering. The cause in all cases is believed to be capillarity or "wicking up" and evaporation of salts dissolved in moisture, with the occurrence below the surface, rather than at the surface, perhaps controlled by the much lower atmospheric pressure of Mars (or possibly by frost leaching from the very surface). The subsurface bright calcium carbonate layer (caliche, or calcrete if there is enough of it) in mature desert soils of the Southwest is believed to form analogously, although evaporation of far more water is required, owing to the lower solubility of carbonates (calcite). The late Roger Burns (and my 2006 Eos article "Mars and mine dumps") would have the Gusev ferric sulfates form as on a desert mine dump, by the moist weathering and oxidation of ejected iron sulfides. Another possibility is owing to condensation of Fe- and S-rich impact vapors, or, to explain the local occurrences, of fumarolic vapors released by buried impact melts interacting with moisture or melting ice. The MER team likes condensation of fumarolic volcanic vapors to explain the local occurrence. Take your pick. Only the mine dump mechanism has a well-understood terrestrial analog.

The persistence of those efflorescent ferric sulfates to the present from whenever they formed presumably indicates that Mars has been either too dry or too cold (or both) for those sulfates to interact significantly with liquid water, which would eventually dissolve them incongruently, releasing sulfuric acid and leaving goethite or hematite behind.

-- HDP Don

Posted by: Bill Harris Mar 15 2008, 08:24 AM

Don and I are on the same page on this item. I look at the shallow subsurface geochemistry of the Columbia Hills area as shallow groundwater wicking sulfate salts into the aeolian or volcanic regolith. The source of the sulfates is likely the weathering of iron sulfides emplaced by hydrothermal or volcanic activity in the Hills area.

--Bill