HR: 10:35h
AN: P12A-02    [Abstracts]
TI: Spectral Evidence for Aqueous Alteration of the Plains Surrounding Valles Marineris, Mars
AU: * Milliken, R E
EM: ralph.milliken@jpl.nasa.gov
AF: JPL, 4800 Oak Grove Dr., Pasadena, CA 91001,
AU: Swayze, G
EM: gswayze@usgs.gov
AF: USGS, Kipling and 6th, Denver, CO 80225,
AU: Murchie, S
EM: scott.murchie@jhuapl.edu
AF: APL/JHU, 11100 Jonhs Hopkins, Laurel, MD 20723,
AU: Bishop, J
EM: jbishop@arc.nasa.gov
AF: NASA Ames, 515 N. Whisman Rd., Mountain View, CA 94043,
AU: Clark, R
EM: rclark@usgs.gov
AF: USGS, Kipling and 6th, Denver, CO 80225,
AU: Ehlmann, B
EM: Bethany_Ehlmann@brown.edu
AF: Brown Univ., 324 Brook St., Providence, RI 02912,
AU: Grtozinger, J
EM: grotz@gps.caltech.edu
AF: Caltech, 1200 E. California Blvd., Pasadena, CA 91125,
AU: Mustard, J
EM: John_Mustard@brown.edu
AF: Brown Univ., 324 Brook St., Providence, RI 02912,
AU: Weitz, C
EM: weitz@psi.edu
AF: PSI, 1700 E. Ft. Lowell, Tucson, AZ 85719,
AB: Data from the high-resolution CRISM visible-near infrared spectrometer onboard the MRO spacecraft reveal the presence of hydrous phases in the plains surrounding the Valles Marineris canyon system. These phases can be classified as either, phyllosilicates or, more generally, hydrated silicates. CRISM spectra of the former are consistent with Al and Fe/Mg-bearing smectites, whereas those of the latter may represent the presence of amorphous silica, hydrated glass (impact or volcanic), chalcedony, opal, or other hydrated Si-rich phases. The phyllosilicates occur in rough textured, light-toned, Noachian age outcrops exposed on the plains along the western portions of the canyon system, including the regions south of Eos and Coprates Chasma and in the walls of Ganges and Coprates Chasma. The presence of layering or total thickness of these deposits is currently unclear, but they are commonly covered by a thin, darker unit and it is likely that their lateral extent is greater than the size of the individual exposures. The spectra for these exposures exhibit H2O features near 1.4 and 1.9 um and Al-OH or Fe/Mg-OH combination bands near 2.2 or 2.3 um, respectively. The spectra are most consistent with smectites such as montmorillonite, nontronite, and/or saponite. In contrast, the hydrated silicates are found primarily in layered materials that exhibit parallel beds that vary in thickness and albedo in the Hesperian aged plains south of Melas and Ius Chasma and west of Juventae Chasma. The spectra and the smooth morphology and light-toned color of these units as observed in accompanying HiRISE and CTX images are inconsistent with lava flows and imply a pyroclastic, aeolian, or fluvial/lacustrine origin instead. Cross-bedding or other sedimentary structures indicative of aeolian or fluvial processes have not been observed to date, but this may be limited by the spatial resolution of the images or inadequate sampling of these units. Spectra of these materials exhibit H2O absorptions at 1.4, 1.9, and 3 um and an OH feature near 2.21 um, though sometimes this feature appears as a weaker band near 2.23 um. The OH feature is broader and sometimes more rounded than similarly positioned bands in Al-bearing phyllosilicates such as montmorillonite. The occurrences of these phases in the plains around V. Marineris do not appear to be associated with or directly adjacent to exposures of phyllosilicates or sulfates. The presence of phyllosilicates in the Noachian crust along V. Marineris, combined with observations of similar materials exposed throughout the ancient cratered highlands, suggests widespread aqueous alteration of the primary crust during the Noachian and is consistent with the model of Bibring et al. (2006). In contrast, the hydrated silicates in the Hesperian plains do not follow the convention of this model. Their composition may represent variations in parent lithology (e.g. volcanic ash instead of basalt), fluid chemistry, climatic changes, or a combination of these factors. Understanding the origin, age, relationship to other hydrous phases, and widespread of these phases may provide a better understanding of the transition from a phyllosilicate to sulfate dominated era on Mars.
DE: 5410 Composition (1060, 3672)
DE: 5415 Erosion and weathering
DE: 5419 Hydrology and fluvial processes
DE: 5464 Remote sensing
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting