Characterization of sulfates and salty soils in the phyllosilicate-rich layered outcrops at Mawrth Vallis, Mars

Remote characterization of salty materials in the phyllosilicate-rich layered outcrops at Mawrth Vallis, Mars were performed using recently developed algorithms to enhance surface features in CRISM spectra by minimizing noise and atmospheric contributions. A unique spectral “doublet” feature was documented in between the upper Al-rich phyllosilicates and lower Fe/Mg-smectite horizon that was attributed to the presence of sulfates or acid alteration of the clays [1]. For this study we are investigating the shape and position of the spectral “doublet” bands near 2.22-2.23 µm and 2.26-2.27 µm to constrain the presence of these salty materials.

Previous analyses identified “doublet” type units with a stronger band in the 2.26-2.27 µm region, more consistent with the Fe sulfate mineral jarosite, and other “doublet” type units with a stronger band in the 2.22-2.23 µm region, more consistent with the Ca sulfate mineral gypsum [2]. All outcrops containing the spectral “doublet” feature also contain a water band near 1.9 µm, indicating that these salty materials are also hydrated. Visible/near-infrared (VNIR) spectra of lab mixtures and natural samples containing jarosite, gypsum, opal, and clay minerals [3,4] were used to ground truth the types of “doublet” signatures possible for reasonable analogs for these salty outcrops at Mawrth Vallis.

Tracking these small and thin salty units (~50–100 m across and possibly only 10-20 m thick) imbedded in the ~200-300 m thick and >1000 km wide phyllosilicate-rich outcrops supports understanding changes in the aqueous geochemical environment at Mawrth Vallis. The “doublet” type units are observed in multiple locations across this region and represent mixtures of various hydrated minerals and phases. We are mapping associations of the two most common “doublet” type outcrops with assemblages of nontronite, montmorillonite, hydrated silica, jarosite, and Ca sulfates in order to define geochemical transitions over time at Mawrth Vallis.

1. Bishop J.L. et al. (2020) Multiple mineral horizons in layered outcrops at Mawrth Vallis, Mars, signify changing geochemical environments on early Mars. Icarus, 341, 113634.

2. Danielsen J.M. et al. (2019) Characterization of outcrops containing “doublet” spectra at Mawrth Vallis, Mars. 50th LPSC #3017.

3. Usabal G.S. et al. (2019) Characterization of jarosite-bearing outcrops northwest of Mawrth Valles. 50th LPSC #2234.

4. Perrin S. et al. (2018) Mars evaporite analog site containing jarosite and gypsum at Sulfate Hill, Painted Desert, AZ. 49th LPSC #1801.