¹S.DeAngelis,²F.Tosi,¹C.Carli,²S.Potin,²P.Beck,²O.Brissaud,²B.Schmitt, ¹G.Piccioni,¹M.C.De Sanctis,¹F.Capaccioni
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114165]
¹INAF-IAPS, Institute for Space Astrophysics and Planetology, Via del Fosso del Cavaliere, 100, Rome 00133, Italy
²Université Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Grenoble 38058 Cédex 9, France
Copyright Elsevier

Hydrated sodium sulfates have been suggested to be present in variable amounts in Solar System objects such as Mars and Europa, among the possible others. The presence of these hydrated species is related to current/past aqueous environments, thus has an importance regarding the potential habitability of planetary objects. In this study, we analyzed anhydrous sodium sulfate (thénardite) and the hydrated sodium sulfate (mirabilite) by means of visible-infrared reflectance spectroscopy in the 0.4–5 μm spectral range, at different low temperatures between 80 and 298 K. Each mineral has been analyzed in three different grain sizes, between 36 and 150 μm. The anhydrous compound, thénardite, is characterized by a nearly flat spectrum in the visible and near IR up to 2.6 μm, while in the 3–4 μm region, the spectrum shows a few weak features due to H₂O and SO₄²⁻ overtones/combinations. The first strong SO₄²⁻ overtone is visible at 4.6 μm. Spectra of mirabilite are substantially characterized by H₂O absorption features in the 1–3 μm region, and by sulfate overtone/combination bands occurring at 3.8 and 4.7 μm. A weak feature appearing at 2.18 μm is also putatively attributed to the sulfate ion. The bands show changes as a function of temperature. The hydration absorption features in mirabilite show the strongest dependence with temperature, both in terms of shift in position and change of spectral shape. Bands at 3.1–3.24 μm in thénardite, as well as absorption features located at 1.78 and 2.47 μm in mirabilite, could be used as diagnostic proxies for the detection of these two minerals on planetary bodies.

^1,2Honglei Lin,³J.D.Tarnas,³J.F.Mustard,¹Xia Zhang,²Yong Wei,
²Weixing Wan,⁴F.Klein,⁵J.R.Kellner
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114168]
¹Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100101, China
²Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. 100029, China
³Dept. of Earth, Environmental, and Planetary Sciences, Brown University, RI 02912, The United States of America
⁴Dept. of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, MA 02543, The United States of America
⁵Institute at Brown for Environment and Society, Brown University, RI 02912, The United States of America
Copyright Elsevier

Serpentine and carbonate are products of serpentinization and carbonation processes on Earth, Mars, and other celestial bodies. Their presence implies that localized habitable environments may have existed on ancient Mars. Factor Analysis and Target Transformation (FATT) techniques have been applied to hyperspectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) to identify possible serpentine and Mg-carbonate-bearing outcrops. FATT techniques are capable of suggesting the presence of individual spectral signals in complex spectral mixtures. Applications of FATT techniques to CRISM data thus far only evaluate whether an entire analyzed image (≈3 × 105 pixels) may contain spectral information consistent with a specific mineral of interest. The spatial distribution of spectral signal from the possible mineral is not determined, making it difficult to validate a reported detection and also to understand the geologic context of any purported detections. We developed a method called Dynamic Aperture Factor Analysis/Target Transformation (DAFA/TT) to highlight the locations in a CRISM observation (or any similar laboratory or remotely acquired data set) most likely to contain spectra of specific minerals of interest. DAFA/TT determines the locations of possible target mineral spectral signals within hyperspectral images by performing FATT in small moving windows with different geometries, and only accepting pixels with positive detections in all cluster geometries as possible detections. DAFA/TT was applied to a hyperspectral image of a serpentinite from Oman for validation testing in a simplified laboratory setting. The mineral distribution determined by DAFA/TT application to the laboratory hyperspectral image was consistent with Raman analysis of the serpentinite sample. DAFA/TT also successfully mapped the spatial distribution of serpentine and Mg-carbonate previously detected in CRISM data using band parameter mapping and extraction of ratioed spectra. We applied DAFA/TT to CRISM images in some olivine-rich regions of Mars to characterize the spatial distribution of serpentine and magnesite outcrops.