HYDRATED SULFATES ON MARS (Invited Presentation)

The sulfate minerals that have been detected across the Mars record ancient climate and aqueous conditions that led to their precipitation as well as present-day water exchange with the atmosphere. The detections of sulfates at all landing sites and multiple locations from orbit point to the importance of sulfate-bearing fluids. Understanding the sources of sulfur – from local weathering, atmospheric volatiles or local groundwaters carrying magmatic volatiles is an important big picture question. While recent instruments with the ability to determine hydration state have enabled the phase and hydration state of sulfates to be constrained by direct measurements, these have also raised new questions.

Mg sulfates of various hydration states have been identified in situ but notably, no Mg sulfates of high hydration degree (>5 water per formula unit) have been identified with any instrument, implying that if initially formed, these have since dehydrated over persistent arid conditions, despite Martian surface thermodynamic conditions that should their presence, at least ephemerally. Mg sulfates of varying hydration states (<5 water per formula unit) sometimes coexist in locations that are exposed to the same present-day conditions, whether at large scales like the layered sulfate deposits seen from orbit or the small-scale detections in single rock targets at Jezero crater. Thermodynamics predicts phase changes at changing humidity and temperature conditions so the coexistence of both low- and mid-hydration Mg sulfates is a puzzle.

The Ca sulfates observed at Jezero crater so differ from those observed at Gale crater; notably missing from Jezero crater is gypsum. The lack of gypsum, scarcity of bassanite and pervasiveness of anhydrite detections at Jezero crater likely indicates that these anhydrite formed from direct precipitation. While the direct precipitation of anhydrite typically occurs at higher temperatures than gypsum and bassanite, with no geological evidence suggesting a high-temperature environment at the time of anhydrite formation, we favor the hypothesis that anhydrite precipitated from a low temperature, high salinity fluid. Key outstanding questions are why the Gale and Jezero fluids differed and what ions contributed salinity at Jezero.