Geochemical prospecting on Mars

Since its atmosphere is so thin, there are things you can achieve from orbit around Mars that would be unthinkable for the Earth. One is imagery free of atmospheric shimmer or scattering, another is analysing gamma rays emitted by Martian rocks using a gamma-ray spectrometer (GRS), as carried by Mars Odyssey. Two processes produce the gamma rays: the decay of long-lived naturally-occurring radiogenic isotopes of potassium, uranium and thorium with their daughter isotopes, and by the interactions of high-energy cosmic-ray particles with other elements in surface materials. Again, with little atmosphere the Martian surface is heavily bombarded by cosmic rays. Using far larger gamma-ray detecting crystals carried on low-flying aircraft it is possible to remotely sense K, U and Th concentrations at the Earth’s surface. To get data on other terrestrial elements from far off would involve unsociable irradiation of the surface by artificial means.

Results from the Mars Odyssey GRS are somewhat blurred as the analysed radiation comes from 0.5º x 0.5º sampling ‘bins’ and is then filtered to a level of 5º x 5º (~ 25 x 25 km) (Taylor G.J. et al. 2010. Mapping Mars geochemically. Geology, v. 38, p. 183-186). So, the approach cannot match geological maps made by interpretation of high resolution images of reflected or thermal radiation. However, as well as K, U and Th estimates, the data cover Fe, Si, Ca, Cl and H2O: sufficient to crudely distinguish mafic and felsic igneous rocks and to detect any regional hydrothermal or groundwater alteration. The authors claim that the GRS separates  much of the Equatorial region of Mars into six kinds of geochemical province, all of roughly basaltic composition. With an estimated SiO2 range from 46.7 to 49.8% that doesn’t promise much by way of fractionation on the scale of terrestrial magmagenesis; i.e. there are no significant intermediate or felsic igneous rocks. A CaO range of 7.5 to 11.4 does indicate varying plagioclase feldspar content, but no anorthosites, unlike the Moon. The greatest variation is in K and Th content, but that does not match the much larger ranges in terrestrial basalts. The geochemical provinces do not match even a simplified photogeological map of the planet, and it seems quite likely that such variation as there is could have resulted from slight weathering and movement of dust and sand. Will a single returned sample of Mars basalt be all that is needed to characterise the Red Planet? More to the point, how does the estimated chemistry match that of purported Martian meteorites, or for that matter the analyses performed on the surface by the Martian rovers Spirit and Opportunity and by the earlier Mars Pathfinder? There is no comment…but Mars Pathfinder surface analyses revealed andesitic rocks at its landing site with up to 55% SiO2.

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