Effects of glass content and oxidation on the spectra of SNC‐like basalts: Applications to Mars remote sensing

Abstract

We have investigated the effects of crystallization and oxidation on the visible and near‐infrared (VISNIR) and mid‐infrared (mid‐IR) spectra of materials with a Shergotty‐Nakhla‐Chassigny (SNC) basaltic composition in order to investigate potential links between Martian VISNIR spectra, Martian mid‐IR spectra, and SNC meteorite data. SNC basalt samples of varying crystallinities were synthesized and subsequently oxidized in air at 700°C for 1–7 days, and reflectance spectra of both fine (≤75 μm) and coarse (75–500 μm) basalt samples were obtained before and after oxidation. The effects of crystallization on the spectral properties are readily observed in the unoxidized products. In the VISNIR, increasing crystallization is accompanied by an increasing influence of pyroxene absorptions and decreasing influence of glass absorptions. In the mid‐IR, changes in spectral properties with crystallization are due to the variations in relative amounts of glass, pyroxene, and plagioclase. Oxidation alters the glass and pyroxenes within the samples, leading to changes in VISNIR and mid‐IR spectral shapes, although distinct signatures of oxidation products are not always observed. Both spectral and electron microscopy data indicate that hematite is the dominant oxidation product in the pigeonite and glass of the samples. In pigeonite, hematite is present in nanophase form and leads to the diminishment of the diagnostic 1‐ and 2‐μm pyroxene absorptions in the VISNIR as well as pyroxene absorptions expressed between 800 and 1100 cm−1 in the mid‐IR. In glass, hematite forms a near‐surface layer that is detected with varying efficiencies in the VISNIR and mid‐IR depending on the crystallinity and particle size of the analyzed sample. Oxidized SNC basalt samples of various crystallinities and fine grain sizes provide the best analogs to Mars remote‐sensing data in the VISNIR and mid‐IR. In the VISNIR, partially crystalline, oxidized SNC basalts can reproduce Martian dark region characteristics. In the mid‐IR, similarities exist between the spectra of a partially crystalline, oxidized SNC basalt and the basalt lithology detected by the Thermal Emission Spectrometer (TES). The andesite lithology detected by TES is best matched by the spectrum of a fully crystalline, oxidized SNC basalt. These results imply that plagioclase‐rich lithologies are not required by TES data and that observed variations in spectral character of dark regions across the Martian surface can be explained by SNC basalts influenced by oxidation coupled with variations in degree of crystallization.