Near- and mid-infrared reflectance spectra of hydrated oxychlorine salts with implications for Mars

by Hanley, J.; Chevrier, V. F.; Barrows, R. S.; Swaffer, C.; Altheide, T. S.

The presence and distribution of oxychlorine salts (e.g., chlorates and perchlorates) on Mars have implications for the stability of water, most notably, that they lower the freezing temperature. To date, elemental chlorine has been measured by all lander missions, with the perchlorate ion identified at both the Phoenix and Curiosity landing sites, but detection by near-infrared (NIR) and mid-infrared (MIR) remote sensing has been limited to deposits of anhydrous chlorides. Given that oxychlorine salts can form numerous hydrated phases, we have measured their NIR and MIR reflectance spectra from 1 to 25 mu m for comparison to data collected from orbiting spectrometers. Anhydrous oxychlorine salts show almost no features in the NIR, except for small bands of residual adsorbed water. However, hydrated oxychlorine salts show numerous features due to water in the NIR, specifically at similar to 1.4 and similar to 1.9 mu m. Increasing the hydration state increases the depth and width of the water bands. All oxychlorine salts exhibit an additional feature at similar to 2.2 mu m due to a Cl-O combination or overtone feature, although it is less prominent in the hydrated perchlorate salts, likely overwhelmed by the ClO4-H2O feature at 2.14 mu m. All oxychlorine salts show features in the MIR due to the fundamental vibrations of Cl-O longward of similar to 8 mu m. The NIR spectral features of hydrated oxychlorine salts are similar to other hydrated salts, especially hydrated sulfates; thus, identification from orbit may be ambiguous. However by utilizing the NIR and MIR laboratory data presented here for comparison, oxychlorine salts may be detectable by orbiting spectrometers.

Journal of Geophysical Research-Planets
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2169-9100; 2169-9097