Regional and global crustal context of soil and rock chemistry from ChemCam and APXS at Gale crater
Friday, 19 December 2014
The chemistry of rocks and soils analyzed by Curiosity represent a diverse population including mafic and felsic compositions. The data from Gale Crater can be compared with the accumulated data for martian materials from other landing sites, the Gamma Ray Spectrometer (GRS) experiment on the Mars Odyssey Spacecraft, and the data for martian meteorites. Variations in the CaO/Al2O3 ratio in primitive igneous rocks can provide a fundamental signature of crustal formation on Mars. Abundances of other elements like Fe in the surface rocks can reflect later differentiation effects. Comparing the chemistry of Gale samples with other martian data must take into account the different geochemical components in the samples. The most important distinction is between the volatile elements including H, C, Cl, S, and the lithophile elements including Al, Si, Fe, Mn, Ca, Na, Mg, etc. The large enrichments of the volatile elements SO3, Cl, and H2O in the soils may represent contributions from volcanic aerosols or other local sources of volatiles. Alteration and transport of fluid mobile major elements by aqueous or hydrothermal processes could complicate the estimation of crustal abundances of elements such as Ca but early results suggest little or no chemical fractionation attributable to alteration. Other clues to the role of fluids can come from the ChemCam data for the highly fluid mobile elements lithium and manganese. Regional comparisons of chemistry only make sense when considering the absolute abundances and elemental ratios within the different component classes. The use of elemental ratios avoids the problem of the correction required to get volatile-free abundance data for comparison of GRS data with meteorites and landing site rocks measured by the ChemCam Laser Induced Breakdown Spectroscopy (LIBS) experiment and Alpha Particle X-ray Spectrometer (APXS). The huge size of the GRS footprint makes it especially difficult to make the required corrections. Eventually data from the whole suite of experiments on Curiosity will allow us to improve volatile corrections for GRS observations of Gale Crater and the rest of Mars.