THE CHEMICALLY ALTERED BASALTIC NORTHERN PLAINS OF MARS: TES AND GRS INTEGRATED DATASETS

TES and GRS provide unique and complementary insights into martian surface compositions. TES measures the composition of the upper hundred microns of the surface while GRS measures the upper few tens of centimeters. This study examines TES major oxide abundances of low-albedo surfaces and compares distributions to GRS element abundances to constrain the relative roles of igneous and alteration processes on Mars. TES oxide abundances are calculated by combining compositions of spectral endmembers in proportion to their relative modeled abundances. The most significant TES chemical trends are higher abundances of FeO for Surface Type (ST1) and higher abundances of SiO2 for Surface Type 2 (ST2). GRS reports Si, K, Fe, Th, and K/Th for 'regions' dominated by TES ST1 and ST2. GRS ST2 chemistries have higher abundances of Fe, K, and Th compared to ST1 while abundances of Si and K/Th ratio do not show spatial variations. One can account for the TES and GRS chemical trends, and discrepancies in silicon and iron, by considering the sampling depth differences between instruments. The constant GRS K/Th ratio across ST1 and ST2 is not consistent with subaqueous weathering of basalt as K would fractionate from Th. Fractional crystallization and subduction zone magmatism could enrich K and Th, however GRS does not detect an enrichment of Si as would be expected. The lack of any enrichment in Si between GRS ST1 and ST2 indicates that andesites are not present in high-abundances within the upper few tens of centimeters at global scales. The favored GRS model is thus initial bulk differentiation processes on Mars producing compositionally distinct magma source regions in the mantle. However, the differences in SiO2 between TES ST1 and ST2 must be taken into consideration. Thin coatings or rinds of secondary high-silica phases (tens of microns) significantly affect thermal emission spectra of basalt and may form on Mars from materials interacting with near-surface volatiles (ice). Limited degrees of alteration in only the upper few tens of microns of the surface could affect TES derived chemistries and be undetectable to GRS due to a deep sampling depth. GRS and TES chemistries thus support: 1) Distinct magma source regions and basaltic compositions for ST1 and ST2 and 2) Thin secondary coatings or rinds of amorphous high-silica phases on ST2 basalt.