[X] My Assistant

[Fran Ontanaya]

From Wikipedia:

"Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves)."

Also:

http://en.wikipedia.org/wiki/Ooid
http://en.wikipedia.org/wiki/Image:OoidSurface01.jpg (deja vu?)

This may be related too:

http://en.wikipedia.org/wiki/Calcite_seas

[ngunn]

Thanks, I'll start with those. The question is whether the dissolved calcium carbonate is in fact re-dissolved from another source of the solid material (a pre-existing biogenic limestone deposit) or created spontaneously by purely chemical reactions between dissolved atmospheric CO2 and calcium ions in the water. Hopefully the answer is in there somewhere.

EDIT: In this post I was referring to the articles about terrestrial oolites, NOT proposing Martian coral reefs - thought I should make that clear to passers by.

[Fran Ontanaya]

http://www.springerlink.com/content/e4n0vul0gcpxq6nt/

"Ikaite crystals (CaCO3×6H2O) have been found at 232- to 238-cm sediment depth in R/V Polarstern core PS2460-4 from the Laptev Sea continental margin in a water depth of 204 m. δ13C values of this phase average −36.3±0.4‰ PDB (N=2), which is significantly outside the range of normal marine carbonates. The CO2 involved in the precipitation of the ikaite is most probably derived from methane, which has extremely depleted 13C isotope values."

Edit: Check this abstract: http://cat.inist.fr/?aModele=afficheN&cpsidt=17478139

"Our results show that the thermal degradation of abiotic calcite starts at a temperature at least 40°C higher than the degradation temperature of any biotic calcite investigated. Consequently, in the case of a Martian in-situ study or in a sample return mission, the analysis of Martian minerals by DTA-TG represents a promising approach to detect evidence of past biological activity on Mars."

Also: http://space.newscientist.com/channel/astr...obiology/dn8534

[ngunn]

Thanks for catching me up on all that Fran.

So - abiotic deposition of carbonates on the bed of any putative martian ocean would be not only possible but expected.

(Apologies to everyone who already knew that!)

[marsbug]

The ocean is still a speculative idea. I would have thought that if heimdall or any other impact had drilled into a preserved limestone layer (as opposed to a one-off deposit) it would be a detectable band in the crater?

As the OP notes carbonates on mars seem to be found as tiny inclusions and tiny regolith grains. This sounds to me more like water condensing in pore spaces in the rock during damp periods, giving rise to carbonate grains which are then eroded out by wind action and mixed in with the soil. I don't even know if that can happen but I've never heard that carbonates need large amounts of water to form only that they need water.

For these to make up six percent of the soil it must have happened many times, (perhaps too many times to be plausable?) but if we are going to have a major sea or lake bed as our source we need a 'smoking gun' with some signs of large scale carbonate deposits.

Now for some unfounded speculation:
Could the carbonate grains even be more contempory? If water vapour can build up enough in pore spaces today to make small amounts of liquid (perhaps if the rock had the right salts included in its structure as impurities?) could carbonate grains be slowly forming today? If it occured in soil pores might that explain the TECP results; thin films of water don't form because the water is being drawn into the few soil spaces with conditions right for liquid H2O? Can osmotic pressure (dimly remebered from school) work that way?

Anyway I'd like to see a crater with a ring of carbonate or a half destroyed carbonate deposite before we decide the grains started out as one big piece. My babbling for today.....

[Vultur]

Assuming a lack of shellfish or coral ... does this tell us anything about the potential for more complex organic chemistry, maybe even simple life?