Can Chabazite Account for the Water Observed on the Equator of Mars?

Up to 13 weight% of water-equivalent hydrogen has been reported for large near-equatorial areas of Mars based on neutron data from the Mars Odyssey orbiter (Feldman et al., 2003). Water ice is unlikely to explain this observation because it is not stable near the martian equator. Magnesium sulfate hydrate, clays, and zeolites have been suggested as possible water-bearing mineral constituents on the surface of Mars, perhaps partially accounting for this enigmatic water. Whether these minerals can retain H₂O under extreme Martian surface conditions is, however, not well constrained. The present study focuses on the natural zeolite chabazite (ideally Ca₂Al₄Si₈O₂₄.12H₂O) because chabazite is a common alteration product of basaltic rocks similar to those that are likely to be common on Mars. It also forms in the soils of the Dry Valleys region of Antarctica, cited by several studies as analogs of martian near-surface alteration. The objectives of this study were to determine experimentally the hydration/dehydration behavior and energetics of this zeolite over a wide range of temperature (T) and water-vapor pressure (P(H₂O)) and predict its hydration state under low-T and low-P(H₂O) conditions such as those existing on the surface of Mars. Chabazite-H₂O equilibria were investigated by isothermal thermogravimetry using a system equipped with automated relative humidity (RH) control. Isothermal data were collected over a range of temperature from 25 to 315° C and pressure from 0.3 to 26 mbar. The relationship of the equilibrium constant (K) to the water content (X) was used to develop a thermodynamic model for hydration (X= gram of H₂O/gram of dry chabazite; K=(X)/(P(H_2O))). Fits of ln(K) vs. X isotherms clearly indicate the existence of three distinct hydration sites in chabazite with different energies. Extrapolation of the corresponding thermodynamic data to low-temperature and low-P(H₂O) indicate that if chabazite formed in the past on the surface of Mars it would still retain some of its water today (from 11 to 22 weight%). In order to explain the rather large amount of H₂O observed by the Mars Odyssey observer, chabazite or similar zeolites and clays would have to present at abundances >59 %. This is unlikely and other sources of water may be required to explain the H₂O present at equatorial regions on the surface of Mars. Nevertheless, hydrous minerals such as zeolites and clays, and hydrated salts, could account for a measurable portion of the water observed in martian regolith by the Odyssey spacecraft. Reference: Feldman, W.C., Prettyman, T.H., Boynton, W.V., Squyres, S.W., Bish, D.L., Elphic, R.C., Funsten, H.O., Lawrence, D.J., Maurice, S., Moore, K., Tokar, R.L., Vaniman, D.T. (2003) The global distribution of near-surface hydrogen on Mars. Sixth International Conference on Mars, July 20-25, 2003, Caltech, Pasadena, California, Abstract 3218. This research was supported by Los Alamos National Laboratory-Directed Research and Development funding.