Gamma Ray Spectrometer - 2001 Mars Odyssey Lunar and Planetary Lab The University of Arizona

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Science Objectives

Data from GRS will be used to study a variety of scientific questions. They address unknowns in the area of distribution of volatiles (including water), past climate, and the origin and geochemical evolution of Mars.

GRS can determine H2O content by two independent techniques. Hydrogen emits gamma rays directly, which are detectable from the upper few tens of centimeters in the surface, while the neutron systems sample hydrogen in the upper meter. By comparing the results of the two independent data sets, it is possible to determine if the water is homogeneously distributed with depth or is concentrated in subsurface material. Thus the GRS provides a means of mapping the water content over the entire surface of the planet with some information about depth distribution in the upper meter. It will also permit a determination of the thickness of the seasonal polar ice caps as they wax and wane through the annual cycle.

The ratio of certain key elements, for example K (Potassium) and Th (Thorium), will constrain models of planet formation. These two elements are incompatible—they are not easily fractionated from each other, i.e. the ratio of their concentrations does not change. But K is much more volatile than Th, and the K/Th ratio, when compared to the solar ratio, from which our solar system formed, will significantly constrain models of the total volatile inventory in the planet. In addition, the composition of erupted magmas, which were generated in equilibrium with the subsurface minerals, allows one to constrain the composition of the Martian interior.

Mars Pathfinder results suggest that the Mars rocks may be silica-rich. The Martian highlands may be the source of the silica-rich rocks, and the GRS will be capable of answering this question. Finally, because the GRS is so sensitive to chlorine concentrations, it will be very easy to see if suggested lake beds or ancient ocean bottoms collected significant salt content.

In summary, GRS should add significantly to our growing understanding of the origin and evolution of Mars and of geologic processes acting on it today and in the past.

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