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[22.05] A model for the chemical deposition of minerals in an evaporation basin on early Mars

David C. Catling (NASA Ames Research Center)

Geological features on the surface of Mars seem to indicate an earlier epoch of sustained hydrological activity during the Noachian period [1]. Given the lower luminosity of the Sun at this time, liquid water would require a much larger greenhouse effect than the present atmosphere provides, and it has been widely hypothesized that this was most probably caused by a higher partial pressure of carbon dioxide (pCO₂) in the early atmosphere [2,3,4,5]. Minerals forming in evaporite basins on Mars [6] may act as tracers of this early atmosphere [7]. Furthermore, it is possible that evaporitic minerals from this period and later epochs may be present in Martian meteorites [8,9].

Aqueous thermodynamic calculations are used to investigate the solute fractionation in a closed basin resulting from sequential evaporation. The model calculates the endogenic precipitates in a water column including carbonates and other minerals. The basin is assumed to be fed initially with terrigenous stream-water containing ions derived from the weathering of young ultramafic volcanic rocks in a thicker CO₂ atmosphere. Siderite (FeCO₃), the most insoluble of the major carbonates, is always the first carbonate to precipitate, provided the atmospheric pCO₂ level exceeds \approx 0.1~bar. In conditions of seasonal water supply and evaporation, siderite varves would be an important facies component in early Martian sediments. Generally, a carbonate sequence of siderite, calcite/dolomite and hydromagnesite interspersed with silica (chert) is predicted, followed by gypsum and highly soluble salts like halite. Higher pCO₂ causes gypsum precipitation earlier in the sequence at the expense of calcite. In ice-covered lakes, supersaturation of trapped CO₂ [10] may lead to little calcite and much gypsum. Further development of such models is important for interpreting future in situ mineralogy from landers and rovers, returned samples, and remote sensing results.

{\bf REFERENCES}: [1] M. H. Carr (1996) {\it Water on Mars}, Oxford University Press. [2] J. B. Pollack et al. (1987) {\it Icarus}, 71, 203-224. [3] J. F. Kasting (1991), {\it Icarus}, 94, 1-13. [4] F. Forget and R. T. Pierrehumbert (1997) {\it Science}, 278, 1273. [5] Y. L. Yung et al. (1997) {\it Icarus}, 136, 222-224. [6] R. D. Forsythe and J. R. Zimbelman (1995) {\it J. Geophys. Res.}, 100, 5553-5563. [7] D. C. Catling (1998), LPSC XXIX, 1568-1569. [8] J. C. Bridges and M. M. Grady (1998) LPSC XXIX, 1399-1400. [9] P. H. Warren (1998) {\it J. Geophys. Res.}, 103, 16759-16773. [10] D. Anderson et al. (1998) {\it Antarctic Science}, 10, 124-133.