Are there carbonate deposits in the Valles Marineris, Mars?

The thick sequences of layered deposits found in the Valles Marineris have recently been determined by S.S. Nedell, S.W. Squyres, and D.W. Andersen (1987, Icarus, 70, 409-441) to contain a minimum of approximately 10(5) km3 of material. We suggest that this material could be derived from the precipitation of 30 mbar of atmospheric CO2 as carbonates in lakes. As surface temperatures fell on Mars, the presence of an insulating ice cover would have allowed liquid water to exist, fed by transitory surface melting. Atmospheric CO2 would have accumulated in such lakes enhancing carbonate precipitation. Support for this hypothesis comes from processes that occur in the perennially frozen lakes in the dry valleys of Antarctica where the mean temperature is 253 degrees K and the lake water is supersaturated with atmospheric gases. A search of the recently reissued Mariner 6/7 infrared spectrometer data in the wavelength region between 2 and 6 micrometers failed to confirm the presence of carbonates. Due to the fact that no spectral footprints appear to directly overlie the layered deposits, and that deposits may be blanketed by an eolian mantle, this negative result is inconclusive. We feel that the canyon deposits are still a prime site for future searches for carbonates on Mars.