Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p22a..01t&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P22A-01
Physics
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5410 Composition (1060, 3672), 5455 Origin And Evolution, 5470 Surface Materials And Properties, 6225 Mars
Scientific paper
The Mars Odyssey spacecraft carries a gamma-ray spectrometer (GRS) that allows measurement of several elements (K, Th, Fe, Si, Al, Ca, Cl, and H) on the surface of Mars. Although its spatial resolution is 500 km, it measures the composition in the upper few tens of centimeters (compared to microns to a few hundred microns for reflectance or emission spectral techniques). GRS provides the first global chemical perspective on the composition of the Martian surface, allowing us to assess the composition of the crust and, with judicious assumptions, the bulk silicate composition of the planet. The data also allow us to identify major geochemical provinces to understand the evolution of the crust and search for the effects of aqueous alteration. GRS data show that the Martian surface is chemically heterogeneous. Elemental concentrations vary across the surface, including variations within high-albedo areas that are presumably covered with dust. Fe concentrations are uniformly high, in accord with the compositions of Martian meteorites and most rock samples analyzed by landed instruments. K/Th is variable, but 95% of the surface has a weight ratio between 3000 and 7000. The mean (5300) is double that in terrestrial crustal rocks and in the bulk silicate Earth. These data indicate that bulk silicate Mars is enriched in moderately volatile elements and in FeO compared to Earth, suggesting that there was not extensive mixing throughout the inner solar system during planetary accretion. Cl varies substantially, with the highest values (0.8 wt%) in the region west of the Tharsis Montes. Surface Types (ST) 1 and 2 (identified from TES spectra) are indistinguishable in Si and Fe concentrations, but ST2 is enriched in K and Th by about 30% relative to ST1 (the K/Th ratio is similar in both regions). The combination of TES and GRS data suggests that ST2 is composed of a different suite of (probably) basaltic rocks than is ST1 and has been weathered. The H2O mass fraction (derived from the H content) in equatorial regions ranges from about 1.5 to 7 %, suggesting the presence of hydrous minerals. Variation in H2O and highly soluble Cl, coupled with some areas with anomalous K/Th, provide geochemical evidence for the role of aqueous alteration on Mars, although the generally uniform K/Th might indicate that wet periods were short.
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