Laboratory measurements of anhydrous minerals mixed with hyperfine hydrated minerals to support interpretation of infrared reflectance observations of planetary surfaces

Identification of water in our Solar System is a key point to understanding the formation and evolution of planetary bodies as well as for astrobiological studies. Scientists identified hydrated minerals as a prime source of H2O in our Solar System. Minerals such as clays, serpentines and other phyllosilicates were discovered by orbiter and lander spacecraft and ground observations on a large variety of rocky surfaces from Mars to small asteroids using InfraRed (IR) spectroscopy as primary technique. It has already been observed that in the presence of large amounts of hydrated minerals in mixtures with anhydrous minerals, the IR spectra can be dominated by the features of hydrated minerals. However, it is still poorly studied how the IR spectra change in presence of different grain size of the two components.The goal of this study was to investigate the infrared spectroscopic features of anhydrous mineral spectra in presence of low amounts of small grain size hydrated hyperfine particles. We prepared several mixtures using 1 wt% and 5 wt% of very small grain size (< 10 mu m) hydrated minerals and 95 wt% and 99 wt% of larger grain size (200-500 mu m) anhydrous minerals. We measured the IR reflectance spectrum of these mixtures in the range 8000-400 cm-1 (1.25-25 mu m). Results presented here show how the presence of a very limited amount of hy-drated minerals with grain size one order of magnitude smaller than the anhydrous component is sufficient to change the IR spectrum, especially in the Near-InfraRed (NIR) region where some of the major hydrated features manifest. On the contrary, the Mid-InfraRed (MIR) part of the spectrum (also identified as thermal infrared) is definitely less affected and anhydrous mineral features continue to be dominant with slight modifications. This result is of pivotal importance for correctly interpreting the IR reflectance observations of planetary bodies such as Mars or asteroids where a mixing of anhydrous and hydrated minerals can be observed. The presence of strong spectroscopic features due to hydrated minerals can be misinterpreted as a large abundance of this material instead of a spectroscopic effect.