HR: 08:00h
AN: P21C-01    [Abstracts]
TI: Recent Results from the Spirit Rover at Home Plate and "Silica Valley"
AU: * Squyres, S W
EM: squyres@astro.cornell.edu
AF: Cornell University, Space Sciences Building, Ithaca, NY 14853, United States
AU: Team, A S
AB: The Mars Exploration Rover Spirit has spent more than 500 sols exploring Home Plate in the inner basin of the Columbia Hills, and adjacent materials. Home Plate is a plateau of layered rocks 80-90 meters in diameter and ~2 meters high. The rocks are clastic and of moderately altered alkali basalt composition, enriched in some volatile elements. A coarse-grained lower unit is overlain by a finer-grained upper unit. Textural observations, including a prominent bomb sag, indicate that the lower strata were emplaced in an explosive event. Geochemical similarities to nearby volcanic rocks and the enrichment in volatile elements favor an explosive volcanic origin. Along the northern portion of Home Plate, the upper unit is very well sorted and composed of well rounded sand sized grains, pointing to textural maturity and suggesting an eolian origin. Along the southeastern portion, however, the upper unit contains some coarser granules too large to be transported by saltation. While their size is comparable to other clasts on Mars that have moved by saltation-induced creep, the observed textures clearly are consistent with emplacement as a pyroclastic surge. The upper and lower units are effectively identical in composition, so the upper unit probably represents a finer-grained fraction of pyroclastic materials that may have undergone some local reworking by wind. Rocks along the margins of Home Plate show a consistent dip toward the center of the plateau. We interpret Home Plate to be the eroded remnants of a formerly more extensive sheet of pyroclastic materials, perhaps produced in a phreatomagmatic eruption. The inward dips may have arisen when pyroclastic materials overrode and partially buried a pre-existing bowl-shaped depression such as an impact crater, draping the topography. Immediately to the east of Home Plate is a narrow valley bounded on one side by Home Plate and on the other by Mitcheltree Ridge. While operating within this valley, Spirit's inoperative right front wheel excavated a small patch of high albedo soil. Mini-TES spectra of this soil were well fit by amorphous silica, and subsequent investigation with the APXS showed a composition that was more than 90% SiO₂. The deposits are also enriched in Ti. Mini-TES spectra of nearby rocks also show a strong signature of amorphous silica, and APXS spectra of these rocks also confirm a high silica content. We consider two hypotheses for the formation of these silica-rich deposits. One is that they developed via precipitation from hydrothermal fluids. Siliceous sinter deposits are common in terrestrial hydrothermal environments where fluids dissolve Si from host rocks at high temperatures and then reprecipitate silica at lower temperatures. Alternatively, the Si-rich materials may represent the remnants of formerly basaltic materials that have been extensively leached in a fumarolic environment under acid sulfate conditions. In either case, the proximity to Home Plate is consistent with formation via the interaction of basaltic volcanism with groundwater. The astrobiological implications of these Si-rich deposits may be significant. Both hydrothermal systems and fumaroles are capable of supporting microbial ecosystems on Earth, and precipitated silica deposits in both environments can preserve strong textural evidence of microbial life.
DE: 5419 Hydrology and fluvial processes
DE: 5480 Volcanism (6063, 8148, 8450)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting