The sedimentary rock record of Mars as viewed from the last decade of orbital missions

Kurzfassung

Over the last decade, orbital and landed missions have revealed a diverse and extensive sedimentary rock record on Mars. In the absence of plate tectonics, and because of a decline of the geological activity over time, the Martian sedimentary record is well-preserved and much older than terrestrial records. Both clastic and chemical sedimentary rocks occur on Mars, in a wide range of depositional environments. In addition to the expected impact-and volcanically-generated deposits, some sedimentary rocks were formed and deposited in local aqueous environments, i.e., alluvial, fluvial, deltaic, and lacustrine environments. Interestingly, clays are the most common and widespread alteration minerals on Mars. They have been detected by orbital visible and near-infrared spectroscopy in association with sedimentary rocks, supporting intense past chemical weathering under circum-neutral pH conditions. Therefore, more clement conditions have likely prevailed during the first billion years on Mars. Some regionally extensive sedimentary formations with thicknesses of up to several kilometers cover plateaus and fill canyons and other closed basins in the equatorial regions of Mars. Containing a variety of sulfates, iron oxides, amorphous silica, sometimes interbedded with clays, their origin is still under debate but likely results from multiple formation processes including lacustrine evaporation, groundwater alteration, hydrothermalism, and eolian reworking. Numerous exposures of chloride-bearing deposits show the existence of ancient ponding of brines. The mid-latitudes regions of Mars show a variety of ice-related deposits including lobate debris aprons, lineated valley fill and concentric crater fill. Sulfate-bearing sands in the polar regions are inferred to have formed by weathering of dust particles within ancient massive ice deposits. This presentation will provide an overview of those sedimentary formations as seen from orbital observations, and will discuss the implications on the geologic and climatic evolution of Mars.