Gusev-style Alteration: Unique or Ubiquitous on Mars?

Introduction: Evidence for hydrous alteration on Mars has in recent years become unequivocal. The identification by a combination of orbital and landed instruments of water-derived hematite, sulfates, phyl-losilicates, Fe-oxides, and hydrated silica is robust. Even carbonates are now recognized [1, 2]. In some cases, it is possible to ascertain an environmental context for the alteration. Gusev Crater, which is still being explored by the Spirit rover more than five years after its arrival, displays a range of altered materials and secondary phases. Sulfates, Fe-oxides, phosphates, and hydrated silica have been identified using a combination of measurements from the Alpha Particle X-ray Spectrometer (APXS), Mössbauer spectrometer (MB), and Miniature Thermal Emission Spectrometer (Mini-TES). No phyllosilicates have been identified definitively. Most enigmatic is a phase or phases that resemble an amorphous silicate of basaltic composition [3]. No terrestrial analog has been identified for this material. Despite a powerful payload and a capable rover platform, the environmental conditions that have produced the range of alteration present in Gusev Crater are not well understood. From this lack of understanding comes an opportunity for modeling. Investigating the conditions that could produce the style of alteration observed in Gusev will lead to a better understanding of the range of hydrous environments across Mars. Alteration on the Plains: The rocks encountered by Spirit on the traverse across the plains to the Co-lumbia Hills are only lightly altered in most cases. The Adirondack-class basalts are the dominant rock type. Their primary mineralogy is that of a picritic basalt, with olivine (Fo50-60) clearly evident in the spectra from Mini-TES and MB [e.g., 4, 5]. Sulfur enrichment identified by the APXS that decreases with depth in the rocks [e.g., 6] recently has been characterized using Mini-TES spectra as a sulfate phase that most resembles kieserite [7]. This sulfate phase is a persistent spectral component among all Adirondack-class basalts observed with Mini-TES across the plains. Twenty-seven examples of these rocks have been identified as " exotics " in the Colum-bia Hills, likely lofted there by meteorite impacts onto the plains. The exotics also display the same sulfate phase. Combined with the observation that many Adi-rondack-class rocks have a ventifact morphology indicative of aeolian abrasion, the presence of sulfate at