from Part II - Elemental Composition: Orbital and in situ Surface Measurements
Published online by Cambridge University Press: 10 December 2009
ABSTRACT
The Gamma Ray Spectrometer (GRS) onboard the Odyssey spacecraft has made the first global measurements of the elemental composition of the Martian surface using gamma rays measured from polar orbit. We report results for Si, Fe, K, Th, Cl, and H. The nominal spatial resolution is 450 km in diameter. Gamma Ray Spectrometer 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 in situ. K/Th is variable, but 95% of the surface has a weight ratio between 4000 and 7000. The mean (5300) is double that in terrestrial crustal rocks and in the inferred bulk silicate Earth. Cl varies substantially, with the highest values in the region west of the Tharsis Montes. Surface Types 1 and 2 (ST1 and ST2), identified from the Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS), are indistinguishable except in the amount of K and Th they contain: ST2 is enriched in both elements by about 30% relative to ST1, while both types have similar K/Th ratios. The H2O mass fraction (stoichiometrically derived from the H content) in equatorial regions ranges from about 1.5%–7%, indicative of the presence of hydrous minerals.
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