PERSPECTIVES ON AMAZONIAN WEATHERING ON MARS

Near-infrared detections of clay minerals have revolutionized our understanding of alteration processes on Mars. The diversity and widespread occurrence of clay minerals in Noachian terrains tell of a relatively vigorous global alteration environment early in planet's history. The conditions that drove alteration in the Noachian ultimately gave way to more sluggish conditions in the Amazonian. Though sluggish, alteration still occurred during the Amazonian. Gypsum in the north polar regions, weathering rinds on rocks at Gusev Crater, clay minerals in the Martian meteorites, and the high-silica materials found across the northern plains each attest to alteration processes occurring over the last few billion years of Mars history. Despite the fact that weathering processes have been seemingly vapid over this time, the mere fact that there has been so much time for weathering to occur means that these could be important processes that significantly affect remotely sensed observations of the surface.

Strict oxidative weathering, the influence of subsurface brines, ice weathering, and thin films of liquid water have been suggested as possible drivers of alteration during the Amazonian. The relative importance of these processes remains unclear, but the fact that there are substantial geographic differences in the characteristics of the Martian surface indicates that multiple processes have been at play. But which processes are most important and precisely how have they acted upon the surface? Observations indicate the presence of amorphous materials from several landing sites. Are all of these amorphous materials related to each other, and might they have formed during the Amazonian? It is known that surface frosts form at the surface, that ground ice forms in the shallow subsurface, and that where icy deposits occur changes with time and is largely driven by obliquity cycles. Periodic and transient melting of surface and near subsurface icy deposits and their subsequent evaporation or refreezing may, overtime, drive a long-lived pedogenic weathering system on Mars, which is likely conducive to formation of amorphous alteration products. We suggest that this basic system is central to Amazonian weathering and must be better constrained to understand the diversity of Amazonian surfaces on Mars.