Basaltic andesite, altered basalt, and a TES-based search for smectite clay minerals on Mars

The global-scale surface type 2 unit first identified with the Mars Global Surveyor Thermal Emission Spectrometer (TES) remains enigmatic. Competing hypotheses suggest that it represents either basaltic andesite or some form of altered basalt rich in dioctahedral smectite clay minerals and/or spectrally similar amorphous silica phases. Common dioctahedral smectites contain a diagnostic feature in a portion of TES spectra that has been excluded in previous studies because of atmospheric CO₂ absorption. We exploit the relative transparency of this spectral region to develop two indices that in tandem, distinguish between certain smectites and amorphous phases. Smectites are not detected in northern lowlands type 2 settings, consistent with other datasets, but the case for primary or secondary amorphous silica phases is strengthened. Separately, one index reveals an abundance of isolated type 2 occurrences in the southern high-latitudes (>50°) associated with dunes, the other shows olivine concentrations between -40° and +30°.