Minimum times to Form Clay in Martian Surface and Near-surface

Introduction: Clay minerals are the most common alteration products of water-rock-gas interactions on Earth. Because these hydrous alteration minerals are thermodynamically stable over a wide range of P-T-x conditions [1], significant amounts of clay may have also formed in Martian environments where liquid water or water vapor was present. It has been speculated that liquid water or vapor may have persisted for weeks to thousands of years in diverse surface and near-surface Martian environments, ranging from an ocean and channelized surface flows, to the potentially higher temperature environments associated with volcanic or impact-induced hydrothermal systems [2]. If true, then a significant fraction of the surface of early Mars experienced water-rock-gas interactions that could have resulted in the formation of extensive clay deposits. Clay deposits are inferred to exist on Mars, based on orbital measurements of spectra at or near the detection limits of the instrument [3]. The duration of aqueous alteration on Mars can be estimated by evaluating the time required to form secondary alteration products. In this work, we estimate the minimum time required to produce detectable amounts of clay by aqueous alteration of volcanic glasses under different sets of idealized Martian surface and near-surface conditions. Model: Although several types of solid phases observed in martian meteorites or inferred to exist on the surface of Mars may react with water to form clays, our model assumes that hypothetical Martian clay deposits formed exclusively after the dissolution of volcanic glasses, and that the rate of clay formation was equivalent to the rate of glass dissolution. This approach is conservative, because mineralic phases generally dissolve about an order of magnitude slower than amorphous phases (i.e. glasses) of the same composition , and explicit reactions describing the formation of clay after glass dissolution may require the contribution of additional dissolved species, such as Al(OH) 4-, not considered in our model. To further minimize the time required to produce detectable amounts of clay, our calculations assume a spectral detection limit of 5 volume percent clay, although the actual limit may be significantly higher with current orbital data [3]. We used a standard rate law to describe glass dissolution rates on Mars, R(g/day) = k*As* a(H +)