Hydrous Environments on Mars from Visible-Infrared Orbital Data

Introduction: Data acquired over the past 5 years from the Mars Express OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) [1] and Mars Reconnaissance Orbiter CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) [2] show a wide diversity hydrated mineral phases distributed across Mars [1, 3, 4, 5, 6, 7]. The increased spatial resolution of CRISM (18 m/p) has lead to the identification of additional mineral phases as well as to refinements in the understanding of the geologic setting in which they exist. Numerous analyses of the specific geologic settings in which the aqueous minerals are identified have recently been summarized [8] where a systematic assessment identifies at least ten distinct hydrous environments: Deep phyllosilicates, layered phyl-losilicates, phyllosilicates in intra-crater fans, plains sediment, intra-crater clay-sulfate deposits, carbon-ate-bearing deposits, Meridiani-type layered deposits, Valles-type layered deposits, hydrated silica-bearing deposits, and surrounding the North Polar Cap gyp-sum plains. The listing of deposits follows the approximate age of the units that host the deposits, from oldest to youngest. We will discuss a few of these deposits classes here. The apparent distinction between Noachian-aged terrains enriched in phyllosilicate minerals and Hes-perian-aged terrains enriched in sulfate minerals was a fundamental result from the OMEGA experiment [1]. Bibring et al [1] proposed a model where the transition between these distinct mineralogic eras was a consequence of a fundamental reorganization of the martian global system. While the detailed obsera-vations of CRISM have supported the apparent distinction between older phyllosilicate terrains and younger sulfate terrains [5], it has also provided additional data on the specific geologic environments and identified several distinct envionrments not previously identified [8]. The largest class of aqueous deposit is the deep phyllosilicates. This is found thoroughout the southern highlands, deep exposures in Valles Marineris as well as large impact craters in the northern plains that penetrate beneath the cover of Hesperian-aged ridge plains and Vastitas Borealis formation. In the southern highlands the phyllosilicates are commonly observed associated with rims, ejecta, walls and central peaks of impact craters. The broad distribtion among all elements of impact craters implies that the target rocks were altered prior to crater formation. A survey