Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p23a1097r&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P23A-1097
Other
3616 Hydrothermal Systems (0450, 1034, 3017, 4832, 8135, 8424), 5410 Composition (1060, 3672), 5464 Remote Sensing, 6225 Mars
Scientific paper
Instruments from the Athena payload on the Mars Exploration Rover Spirit in Gusev Crater have identified materials remarkably enriched in silica. Spectra from the Miniature Thermal Emission Spectrometer (Mini-TES; ~5-25 microns) and Alpha Particle X-ray Spectrometer have revealed a high-silica phase in light-toned soil and heavily eroded outcrops extending as much as 50m from the low-lying, layered plateau known as Home Plate. The various forms of silica (SiO2) are readily distinguishable in the thermal infrared (TIR) wavelengths as is their purity. The Mini-TES spectra clearly indicate an amorphous phase with silica content >80% that is most similar to opal-A, the least crystalline form. The presence of amorphous silica is consistent with a variety of primary and secondary origins including obsidian, high-silica rhyolite/tuff, acidic alteration/leaching of silicate rocks, and precipitation from silica-saturated fluids. The Mini-TES spectra contain a prominent absorption near 8 microns and an emissivity maximum near 12 microns that depart from typical opal-A. Existing literature and new laboratory measurements demonstrate that these features can arise in certain forms of amorphous silica as a function of both geometric and scattering effects. The feature near 8 microns that typically appears as a shoulder in opal-A becomes a prominent absorption minimum with increasing emission angle as well as with the presence of porosity at the scale of a few 10s of microns. An emission peak near 12 microns is accentuated as a result of scattering due either to porosity or particle-size effects. Both of these features are common to siliceous sinter deposits that form as sedimentary precipitates around hydrothermal springs on earth. The Mini-TES spectra display these features and are best fit by spectra of natural sinter samples that contain microporosity and are measured at the high emission angles (>45 degrees) typical of Mini-TES observations. Primary igneous forms of amorphous silica such as obsidian and high-silica glass found in tuffs do not provide a good fit to the Mini-TES spectra. We have not explored fully the spectral characteristics of amorphous silica produced from acid alteration environments such as fumaroles, so this origin for the high-silica materials remains viable. However, their proximity to the Home Plate structure, which appears to be a depression buried by pyroclastic deposits, leads to the hypothesis that Home Plate represents a buried hot spring with an associated apron of siliceous sinter. Continued exploration of the site by the Spirit rover is intended to further investigate this hypothesis as well as the alternative hypothesis of a fumarolic origin for the high-silica materials.
Arvidson Ray E.
Calvin Wendy M.
Christensen Per Rex
Farmer Doyne J.
Johnson Jay Robert
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