Spectral reflectance properties of minerals exposed to martian surface conditions: Implications for spectroscopy-based mineral detection on Mars

Spectral reflectance properties of eight hydrated minerals including zeolites, sulfates, and silica-rich sinter were investigated under simulated Mars surface conditions. The minerals were selected based on their past detection on Mars by remote sensing data and certain minerals merited further investigations based on previous spectral studies. Long duration 763-day (similar to 26-month) exposure (atmosphere of 5 Torr CO2 with two periods of UV radiation) and 6 weeks exposure to ambient temperatures after similar to 26 months was conducted to determine mineral stability on the surface of Mars by assessing changes to absorption band depths, widths, shape, center, and spectral slope. Changes in all of these spectral metrics were observed for at least one of the samples. We found longer-term changes of some spectral properties past the termination of previous published studies for certain samples and some which may extend beyond the duration of the experimental run. Some of the spectral changes were reversible when the samples were re-exposed to the ambient terrestrial environment. These spectral changes could be related to various processes, such as rapid loss and reaccumulation of adsorbed water, preferential loss of bound water versus structural OH, changes in Fe3+ bridging cations and structure, and decomposition of some phases. The changes in a wide range of spectral properties suggests that analysis of observational data for Mars should be approached with an appreciation for how exposure to Mars surface conditions can affect spectroscopybased interpretations of mineralogy. The results are also relevant to understanding spectral changes seen in roverexposed subsurface regolith, and for selecting samples for caching and return to Earth: i.e., stability before, during, and after sampling, caching, return to Earth, and subsequent analysis in terrestrial laboratories.