HR: 11:50h
AN: P32A-07    [Abstracts]
TI: Salty Soils at Gusev Crater investigated by Mars Exploration Rover Spirit
AU: * Wang, A
EM: alianw@levee.wustl.edu
AF: Washington University, One Brookings Dr, St. Louis, MO 63130, United States
AU: Bell, J F
EM: jfb8@cornell.edu
AF: Cornell University, Space Science Bldg, Ithaca, NY 14853, United States
AU: Li, R
EM: li.282@osu.edu
AF: Ohio State University, 2070 Neil Ave, Columbus, OH 43210, United States
AU: Johnson, J R
EM: jrjohnson@usgs.gov
AF: US Geologicaal Survey, 2255 Germini Dr, Flagstaff, AZ 86001, United States
AU: Arvidson, R E
EM: arvidson@wunder.wustl.edu
AF: Washington University, One Brookings Dr, St. Louis, MO 63130, United States
AU: Crumpler, L
EM: Larry Crumpler@mac.com
AF: New Maxico Museum of Natural History & Science, 1801 Mountain Rd, Albuquerque, NM 87104, United States
AB: Mg-sulfate enriched salty soils were suggested by compositional correlation based on APXS data in two trenches made by the Mars Exploration Rover Spirit on the basaltic plains at Gusev Crater. During Spirit's traverse in the Columbia Hills region, light-toned salty soils were exposed again at eight loca-tions, with Paso Robles, Arad, and Tyrone having the largest exposures among them. The sites of Arad and Tyrone occur in local topographic minima, almost totally enclosed by surrounding ridges, while other exposures occur at the bases of hills, ridges, and outcrops. Where measured, the chemistry of the salts typically reflects the character of nearby rock, suggesting that the salts were formed locally rather than transported from a distant source. Candidate formation mechanisms include fumarolic processes and precipitation from salt-rich fluids. The salty soils at all eight locations were imaged with the Pancam 13 "geology" filter set that covers the VIS-NIR spectral region from 434 to 1009 nm. Spirit dragged its right-front wheel through the Tyrone area, that caused exposure of deeper light-toned soils ("yellowish" soils in Pancam false-color images) with 830-860 nm spectral absorption bands. In addition, these soils have a steeper spectral slope from 432 nm to 670 nm than the shallower soils ("whiteish" soils) exposed by the other five wheels. Similarly, more than one type of Pancam spectra, suggesting ferric sulfates, were observed at the Paso Robles and Arad areas. Mini-TES spectra from Paso Robles, Arad, and Tyrone all exhibit a 6 micrometer spectral feature suggesting hydrous materials. Three sets of in situ investigations were made at Paso Robles, Arad, and Tyrone, which indicate that ferric sulfates would be the important constituent of these light-toned salty soils, and possibly co-existing Mg- and Ca- sulfates. Further considering that Mg-sulfates are the major salts found in the two trenches on the basaltic plain, and that Ca-sulfates are the major sulfates within West Spur rocks, the heterogene-ous chemical nature of these sulfates found at Gusev Crater appears to demonstrate a correlation with the degree of alteration of local rocks/outcrops. After over 200 sols' exposure to current martian surface conditions (diurnal and seasonal cycles of tem-perature, dust, relative humidity, and low water vapor pressure), a relative change in the Pancam spectra of "yellowish" soils was detected from the seven repeated sets of Pancam 13-filter observations on Tyrone. The R(753 nm) over R(432 nm) slope in the spectra of "yellowish" soils decreased whereas that of "whiteish" soils did not change. This observation supports our model of the layered structure of salty soils at Tyrone. Indeed, we hypothesize that the "yellowish" soils were originally not in equilibrium with surface conditions because they were buried deeper. After exposure, the "yellowish" soils began to equilibrate with the current surface conditions, whereas the "whiteish" soils were more or less in equilibrium with surface conditions because of their originally much shallower burial depth. Laboratory experiments on the spectral features of some ferric sulfates suggests that dehydration can introduce the reduc-tion of spectral slope similar to what we see in the Pancam data of Tyrone. We are continuing to explore other possible origins for these changes.
DE: 3665 Mineral occurrences and deposits
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 5410 Composition (1060, 3672)
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly