NATURE AND ORIGIN OF HYDROUS MINERALS IN SEDIMENTS ON MARS (Invited Presentation)

Spectrometers onboard orbital probes and rovers have discovered a huge diversity of minerals, such as sulfate and clay minerals, derived from the aqueous alteration of magmatic mineral phases. The current paradigm states that hydrated sulfates formed probably in contact with an acidic sulfur-rich atmosphere whereas hydrated silicates limited to outcrops of the ancient crust are best explained by aqueous alteration at depth. In addition, the correlation between these minerals and fluvial and lacustrine landforms remains unclear. For instance, clay minerals formation seems to predate fluvial landforms and lacustrine deposits questioning the role of surface water in these processes. All of this raises doubts for the need of a warmer wetter early Mars. In such a context, hydrous minerals detected in sediments play a huge role for evaluating the role of weathering and ambient temperature alteration versus subsurface and hydrothermal alteration. Are clay minerals in sediments transported from the altered crust in cold wet conditions unable to generate significant alteration? Is there a climatic component in the overall clay signatures observed in sedimentary deposits? What would be the mineralogical signature of weathering in a CO₂-rich reducing atmosphere? In this talk, I will present the geology and diversity in composition of some key regions, such as Mawrth Vallis and north Hellas Planitia, discuss and the origin of the alteration, and I will compare these results to new experimental data of aqueous alteration under Mars-like simulated primitive atmosphere. Conclusions suggest that periods of alteration under ambient conditions in a somehow warmer wetter climate are still required for a better understanding of the mineralogical assemblages in the sedimentary record.