ACID ALTERATION OF GLASS-BEARING MATERIALS: FORMATION OF A MAJOR MARTIAN SURFACE TYPE

Low-albedo sand seas and sand sheets encircle the north polar cap of Mars and extend into the deepest part of the northern basin, Acidalia Planitia. These low albedo deposits are the type locality for TES Surface Type 2 (ST2), a silica-enriched, globally distributed, compositional unit identified from deconvolution of Mars Global Surveyor TES thermal infrared spectra. The nature of ST2 is not well constrained, as the high-silica component is spectrally consistent with both primary lithologies and alteration phases. We present new evidence from Mars Express OMEGA near infrared spectra of the north polar region and Acidalia Planitia that the primary high-silica component of ST2 is an amorphous silica coating. We interpret this coating as being associated with high concentrations of iron bearing glass that have not previously been detected in these regions. These high silica, glass-rich deposits are spectrally consistent with thin silica leaching rinds formed on Earth by acidic alteration of basaltic glass.

We identify ST2 concentrations with amorphous silica coating spectral signatures in a variety of geomorphic settings, including within craters and volcanic complexes, possibly indicating both impact melts and volcanic materials as alteration precursor materials. Furthermore, the lack of identified volcanic edifices or extensive impact melt sheets in the north polar region or in Acidalia Planitia suggests that the widespread silica coated sands in these regions may be the end product of alteration (for example, by olivine dissolution/glass leaching) of the basaltic sand seas that have occupied these regions throughout the Amazonian. These observations suggest that acidic alteration of glass-bearing deposits has been a widespread process on Mars, especially at high latitudes. Considered along with the recent hypothesis that non-ST2 dark regions on Mars may be soils produced by alteration of bedrock, these results indicate that chemical weathering may play a much more important role in the formation and evolution of martian soils than has been previously thought.