Spectral characterization of acid weathering products on Martian basaltic glass

Abstract

AbstractFor the first time, direct infrared spectral analyses of glasses with Martian compositions, altered under controlled conditions, are presented in order to assess surface weathering and regolith development on Mars. Basaltic glasses of Irvine and Backstay composition were synthesized and altered using H2SO4‐HCl acid solutions (pH 0–4). Scanning electron microscopy/energy dispersive spectroscopy, X‐ray diffraction, Raman, and infrared spectral measurements were acquired for each reaction product. Infrared spectra were also acquired from previously synthesized and altered glasses with Pathfinder‐measured compositions. Acid alteration on particles in the most acidic solutions (pH ≤ 1) yielded sulfate‐dominated visible near infrared (VNIR) and thermal infrared (TIR) spectra with some silica influence. Spectral differences between alteration products from each starting material were present, reflecting strong sensitivity to changes in mineral assemblage. In the TIR, alteration features were preserved after reworking and consolidation. In the VNIR, hydrated sulfate features were present along with strong negative spectral slopes. Although such signatures are found in a few isolated locations on Mars with high‐resolution spectrometers, much of the Martian surface lacks these characteristics, suggesting the following: acid alteration occurred at pH ≥ 2; small amounts of sulfates were reworked with unaltered material; there is a prevalence of intermediate‐to‐high silica glass in Martian starting materials (more resistant to acid alteration); primary or added sulfur were lacking; alteration features are obscured by dust; and/or large‐scale, pervasive, acid sulfate weathering of the Martian surface did not occur. These results highlight the need to better understand the spectral properties of altered Martian surface material in order to enhance the interpretation of remote spectra for altered terrains.