Interpreting a Weathered Mars: Investigating the Effects of Weathering on Spectroscopic Observations Through Laboratory Study

Infrared spectroscopy is a critical tool for Martian mineralogy. Because it is crucial to evaluate the history of water on Mars, mineralogical study of weathering and alteration is among the most important topics of Mars spectroscopy. The state of alteration of the Martian surface is evaluated by the presence or absence of alteration phases and their overall abundance. Interpretations of Martian weathering processes are based on the types of alteration products, the mineral assemblages, and derived chemistry. The spectroscopy of alteration minerals has been studied in detail for decades; however, detecting and identifying alteration products from remotely- sensed spectra of natural surfaces is complicated by microtextural mixing of rock-forming minerals, alteration products, and void space. We are investigating the effects that low-temperature weathering has on spectral observations in order to facilitate interpretation of spectroscopic data of Martian surfaces that may be weathered. Our approach has been to characterize the infrared spectra, mineral assemblages, and textures of weathering rinds and rock coatings formed on volcanic rocks in a variety of environments. This approach enables us to witness the spectral variability that results from weathering and tie it to differences in texture or mineralogical composition. More importantly, by examining numerous rinds and coatings, we can determine what effects are common to broader weathering phenomena. For instance, basalt weathering typically leads to systematic changes in silicate vibrational absorptions that can hamper spectral modeling techniques used to assess Martian data. In addition, weathered surfaces may show little evidence of hydrated minerals in near-infrared data. Another important component of our research is the use of controlled laboratory experiments designed to simplify, yet emulate, important attributes of the naturally weathered surfaces, in order to better constrain the spectral effects of weathering. Based on our findings regarding weathering mineral detectability, assertions that Mars has lacked aqueous weathering in its latter history may be incorrect. Rather, we suggest that volumetrically small amounts of high-silica weathering products formed in aqueous environments at middle and high latitudes and that this is consistent with spectroscopic observations of Mars.