Thermal emission studies of sulfates and chlorides; implications for salts on the Martian surface

Discoveries of hydrated minerals on Mars highlight the need to better understand their spectral properties to allow them to be remotely mapped. Hydrated minerals, such as sulfates and chlorides, are considered representative of aqueous depositional environments on Mars and provide insight about past climates when liquid water was present at the surface.; Laboratory thermal infrared (TIR) spectroscopy is a valuable tool for understanding remotely sensed data. Until recently, relatively few minerals representative of aqueous depositional environments were included in spectral libraries used for interpretation of TIR remote sensing data. A technique was developed to measure hydrous minerals that prevents sample dehydration. Applying this technique, TIR spectra for a suite of chloride and magnesium sulfate minerals were measured and characterized for comparison to martian datasets. The spectra of anhydrous chlorides are featureless and transparent over the TIR and hydrous chlorides mostly exhibit features due to H2 O. Additionally, chlorides exhibit maximum emissivity much less than one. The spectra of sulfates systematically shift to higher frequency with decreased hydration. This trend is also present for calcium and iron sulfates, which allows mineral identification based on absorption position and knowledge of sample chemical properties.; The new laboratory spectra were used to analyze Mars Exploration Rover (MER) Miniature Thermal Emission Spectrometer (Mini-TES) spectral data from both MER landing sites as well as from Thermal Emission Imaging System (THEMIS) and Thermal Emission Spectrometer (TES) data. Analyses of Mini-TES spectra suggest that the light-toned outcrop at the MER-B landing site contains a mixture of magnesium and calcium sulfates. Mini-TES analyses of bright soil at MER-A suggests a mixture of calcium, sodium and magnesium sulfates present. However, coordinated analyses of absorption position and chemistry predict that kornelite (Fe2(SO4)3•7H2O) is most likely and abundant. A compositional unit has been identified based on spectral distinctiveness in THEMIS data that is consistent with laboratory measurements of chlorides. Specifically, these spectra are featureless and exhibit a negative slope attributed to non-unit maximum emissivity. The detection of salt minerals often requires the synergy of several techniques to support the detection and identification of salt mineralogy and hydration on Mars.