EVAPORITE MINERALS IN MARTIAN SAMPLES: CLUES TO THE NEAR-SURFACE HISTORY OF MARS

Remote sensing data clearly show that the surface of Mars contains large- and medium-scale erosional and depositional features. It is commonly believed that many of the (non-aeolian) surface features are the result of aqueous activity, although other possible mechanisms such as erosion and deposition by liquid carbon dioxide, clathrate, and/or flowing lava, have also been proposed. Martian rock samples (meteorites from Mars), however, confirm that water has been actively present near the Martian surface. The Martian meteorites are igneous rocks (diabasic, basaltic, ultramafic), but traces of aqueously-precipitated secondary minerals, such as salts and phyllosilicates, are found along fractures and in voids in the rocks. The salts commonly occur as isolated microscopic grains or small clusters of grains. Some of the salts and other alteration products are demonstrably Martian. The distinction between possible Martian phases and terrestrial contaminants is not always clear, however. Martian salts seem to be very similar to those found in marine-type evaporites on Earth, and include carbonates, Ca-sulfate, Mg-sulfate, and halite. Earth-like layered sequences have not yet been observed in Martian occurrences. Ages of the Martian salts range from 3.9 Ga to less than 165 Ma, indicating that aqueous processes have been active on Mars for a very long time. The salts themselves commonly show evidence of weathering, suggesting that Martian aqueous activity may have been episodic. It is important to understand the Martian salts in order to decipher the near-surface history of Mars. Furthermore, the presence of evaporites on Mars has strong implications for the search for life there. The evaporites, along with other products of aqueous alteration, indicate that many of the components necessary for life on Earth, such as circulating water, potential nutrients dissolved in the water, and temperatures overlapping with the range for known organisms, have been available on Mars, possibly for a very large part of geologic history.