Constraining Paleoenvironments on Mars from Infrared Spectral Observations of Amorphous and Poorly Crystalline Alteration Products

Abstract:

The past 20 years of orbiter and rover-based infrared (IR) spectroscopy has shown that amorphous and poorly crystalline materials are important components of the martian surface at global and local scales. Due to limited spectral libraries, initial spectral models alleged that these materials were volcanic glass, but subsequent studies represented here have shown that they are often more consistent with amorphous and poorly crystalline aqueous alteration products. Major outstanding questions include: (1) What are the IR spectral signatures of the range of amorphous and poorly crystalline materials possible for Mars? (2) Can their spectra be used to unambiguously differentiate between paleoconditions on Mars? These questions represent a knowledge gap that inhibits our understanding of the geologic history and habitability of large regions of the martian surface.

To address these questions, we studied the IR spectral properties of a range of amorphous and poorly crystalline alteration products of mafic rocks formed under various conditions including: circumneutral pedogenesis, low pH open-system, alteration under small alpine glaciers and large persistent ice sheets, and laboratory freeze-thaw experiments. We have examined how micro- and nanoscale physical and chemical features affect the resultant spectra, and compared our laboratory spectra to the martian surface. A major finding is that natural secondary amorphous and poorly crystalline materials have heterogeneous compositions and crystallinities down to the nanoscale, suggesting that IR spectral studies of individual materials may not be representative of the true range of materials present on Mars. Additionally, we find that amorphous and poorly crystalline silicates are easier to identify than iron oxides because of the strong Si-O bands at thermal IR wavelengths. The products of alteration under most of these conditions provide comparable fits to orbital thermal IR spectra observed in the northern lowlands of Mars, one of the largest regions with signatures of silica-rich and likely amorphous materials. Ongoing work tests if thermal IR spectral models can differentiate between high-silica volcanic glass and amorphous and poorly crystalline silicate alteration products, and between alteration products formed under different conditions.