Presence of clay minerals can obscure spectral evidence of Mg sulfates: implications for orbital observations of Mars

Abstract

The martian crust is often viewed through the lens of its dominant secondary minerals, Noachian phyllosilicates and Hesperian sulfates, based on orbital spectral observations. However, the effects of surface exposure on the spectra of these hydrous minerals are not fully understood. We use an environmental chamber to measure changes in near-infrared (NIR) spectral absorptions related to H2O in smectite (montmorillonite) and Mg-sulfate under different temperature, pressure, and relative humidity conditions with relevance to the surface of Mars. Observed spectral differences are attributed to changes in water content (hydration state), mineral phase, and degree of crystallinity. It is observed that even minor changes in hydration state and phase (for Mg sulfate) cause perceptible changes in NIR H2O absorption features when measured in a controlled laboratory setting under dry Mars-like conditions. Based on these results and the known ability of smectite to rehydrate under increased RH, smectites exposed at the surface of Mars are expected to exchange water with the martian atmosphere under specific conditions, making them active participants in the present-day hydrological cycle of Mars, and in theory these hydration-dehydration processes should be detectable using NIR reflectance spectroscopy. However, some of the spectral changes associated with these hydration changes are subtle and may not be detectable with orbital or landed VNIR spectrometers. Furthermore, we find that the presence of clay minerals can spectrally mask the presence of Mg sulfates under a range of hydration states if the clay minerals are above ∼10 wt% abundance. Random noise was added to the laboratory spectral data to simulate orbital-quality reflectance data, and it is observed that expected changes related to hydration state and crystallinity are likely difficult to detect in current orbital VNIR data such as CRISM and OMEGA. This highlights the importance of future in situ NIR reflectance observations to accurately determine the extent to which hydrous minerals exposed as the surface cycle water with the martian atmosphere under present-day environmental conditions and to properly assess the role of hydrous minerals in the martian water budget.