NEW PERSPECTIVES ON THE ANCIENT MARTIAN CLIMATE: CHEMICAL WEATHERING OF VOLCANIC ASH UNDER TRANSIENT, REDUCED GREENHOUSE CONDITIONS (Invited Presentation)

Climate models suggest that any early martian greenhouse sufficient to allow warm, wet surface conditions likely required reduced greenhouse gases such as CH₄ and/or H₂ in addition to CO₂, but until recently this hypothesis has been difficult to test. The global occurrence of Al-rich clay minerals overlying Fe/Mg-rich clay minerals on Mars has long been considered evidence of leaching during chemical weathering, but recent results revealed a more surprising trend within these deposits around the planet. The Al-rich, upper portion (~10 meters) of the weathering profiles are also strikingly Fe-poor, which is why the rocks appear blue-toned in visible/near infrared HiRISE images. Strong separation of Fe from Al throughout the surface indicates that the weathering fluids were reduced, implicating a reduced greenhouse atmosphere during weathering. While these trends are observed in many locations via remote sensing, deposits already visited by the Curiosity rover in Gale crater amazingly provide ground truth for these global spectroscopic trends. Blue-toned patches with strong weathering index values in infrared remote sensing data are demonstrated in rover data to be Fe-poor and rich in Al and Si. These blue patches, which occur several 100s of meters above the crater floor are only part of a thick weathering profile visited by the rover. While rocks composing the stratigraphic section within Mt Sharp are generally considered to be lacustrine, a closer look at rock textures and geochemical trends (e.g. loss of Fe and Mg, enrichment in Al and Si) suggests that the largest fraction of the stratigraphic section can be explained as eolian and/or volcaniclastic materials weathered subaerially. Gale crater perhaps contained a lake, but sediments that are clearly lacustrine are only recorded in the lowermost part of the crater floor and were visited in the earliest days of the mission (Yellowknife Bay). Similar weathering trends are observed at high elevations on the summits of stratovolcanoes elsewhere on Mars, an observation that can perhaps only be explained by subaerial, precipitation driven chemical weathering. Mafic volcanic ash might be a fundamental component of the martian sedimentary rock record given that volcanism should be more explosive overall on Mars than Earth.