The Mg sulfate-bearing unit of Gale Crater, Mars from a ChemCam perspective

Across Mars, stratigraphy consistent with wetter, more clement conditions appear to transition to stratigraphy consistent with drier, more saline conditions, hinting at a global drying and likely cooling process eventually leading to the cold, dry planet we observe today. Such transitional stratigraphy—clay-bearing strata underlying sulfate-bearing strata—is observable in Gale crater, Mars, currently explored by the Curiosity rover. Curiosity has now made its way through a significant section of the Mg sulfate-bearing unit of Gale crater, permitting classification of the rocks originally observed by orbit and critical to the selection of Gale crater as the landing site for the Mars Science Laboratory mission.

In situ, the Mg sulfate-bearing unit has indeed proven home to copious Mg sulfates, identified chemically via instruments like the Alpha Particle X-Ray Spectrometer (APXS) and the Chemistry & Camera (ChemCam) instrument, and verified via the rover’s X-ray diffractometer, CheMin. These sulfates are largely present in aeolian bedrock, defined sedimentologically as the Mirador formation. Yet additional salts have been identified as well, suggesting complex, possibly multicomponent fluids were extensively present throughout the region. Here, we consider the geochemistry of these sulfate-bearing rocks via the ChemCam instrument onboard Curiosity. ChemCam has observed three major salt components to the aeolian bedrock blocks: halite, Mg-bearing sulfate, and Ca-sulfate. Siderite (FeCO3) has been identified via CheMin and is challenging to confirm via ChemCam, though is likely extensively present throughout the sulfate-bearing unit. These salts are often in diagenetic features, or highly intimately mixed with a silicate component, inferred to be the grains transported in the ancient aeolian environments. Salts are likely acting as cements for the silicate component, and may have been deposited via saline groundwater fluids. The origin and duration of such fluids remain open questions, that will be answered as Curiosity continues to explore this intriguing region.