On Oct 12, 6:59*am, Robert Clark wrote:
Large deposits of carbonates had been expected to be wide spread on
Mars because of the extensive water in the distant past and because of
the CO2 atmosphere. Previously however deposits of carbonates had not
been seen from orbit. What was seen from orbit was small amounts of
carbonate in the form of dust sprinkled over the planet at perhaps the
2% amount. *This was confirmed on the ground by the MER rovers which
also saw carbonate only in the form of dust in small amounts.
*However, the Phoenix lander has found carbonates in significant
amounts at its landing site, perhaps in the 6%-8% range. The presence
of the carbonate here might be due to the alkalinity of the soil at
the Phoenix site compared to acidic soil, as indicated by the presence
of sulfates, at the other lander sites.
*A new report however to be presented at the upcoming "Workshop on
Martian Phyllosilicates: Recorders of Aqueous Processes?" will argue
that carbonate best fits the spectra in a deposit in the Nili Fossae
region on Mars:

PHYLLOSILICATES, ZEOLITES, AND CARBONATE NEAR NILI FOSSAE, MARS:
EVIDENCE FOR DISTINCT ENVIRONMENTS OF AQUEOUS ALTERATION. B.L.
Ehlmann1, J.F. Mustard1, G.A. Swayze2, J.J. Wray3, O.S. Barnouin-Jha4,
J.L. Bishop5, D.J. Des Marais6, F. Poulet7, L.H. Roach1, R.E.
Milliken8, R.N. Clark2, S.L. Murchie4, and the MRO CRISM Team. 1Dept.
of Geological Sciences, Brown University, 2U.S. Geological Survey,
Denver, 3Cornell University, 4JHU-Applied Physics Laboratory 5SETI
Institute 6NASA Ames 7IAS, Université Paris-Sud, 8JPL-Caltech
)http://www.lpi.usra.edu/meetings/aqu...8/pdf/7019.pdf

This is to be published in an upcoming article in Science.

Other interesting reports from this conference:

Program
Workshop on Martian Phyllosilicates: Recorders of Aqueous Processes?
October 21–23, 2008http://www.lpi.usra.edu/meetings/aqueous2008/pdf/program.pdf

* * Bob Clark

Carbonates and clays (phyllosilicates, aluminum silicates) are
largest constituents of ocean sediments. Both clays and
olivine are made of hydrated silicon, SiO4 (silicate), which
requires ~200 atmospheres of hydrostatic pressure for its
formation.. Note H4SiO4 in the plume of deep-sea volcano.
http://www.pmel.noaa.gov/vents/chemi...ges/vents2.gif
Allowing for the difference in gravities, we can estimate the
minimal depth of ancient oceans on Mars to be ~6 km.
Caution to undergrads: the ideas are speculative,
not suited for the classroom. John Curtis