NEW GEOLOGIC DISCOVERIES OF THE SPIRIT MARS ROVER MISSION

The Spirit rover celebrated its second successful year of operations on Mars in January 2006, considerably longer than its designed 3 month lifetime. While traversing across the plains of Gusev Crater, Spirit analyzed primitive, olivine-rich basalts that have flooded the crater. No underlying fluvial or lacustrine deposits (the reason that this landing site was selected) were found in excavated impact craters on the plains. During the subsequent climb (90 m vertical ascent) onto the uplifted Columbia Hills, Spirit encountered and analyzed a variety of altered rocks thought to be older than those on the plains. Most of these rocks are mixtures of two endmembers, one probably pyroclastic in origin and the other a basaltic sandstone cemented with sulfate salts. Mixing likely resulted from impacts. Small scale bedding is apparent in some rocks, but the gross stratigraphy is uncertain. Pervasive, non-isochemical alteration and oxidation of these rocks, as well as cavernous weathering in some cases, were promoted by exposure to liquid water. Unaltered float samples of trachybasalt found near the crest appear to be fractionation products of alkaline basaltic magma like that on the plains. During the descent down the other side of Husband Hill, Spirit encountered a thick sequence of ultramafic rocks, possibly representing a basement lithology beneath Gusev. These rocks contain large amounts of olivine, which are unlikely to have survived aqueous alteration, so only the cover rocks were in contact with water. Having completed the descent into a basin, Spirit is now exploring the geology of a new terrain within Gusev Crater. Its next challenge will be to survive a second Martian winter.