Geological processes and evolution

Geological mapping and establishment of stratigraphic relationships provide s an overview of geological processes operating on Mars and how they have v aried in time and space. Impact craters and basins shaped the crust in earl iest history and as their importance declined, evidence of extensive region al volcanism emerged during the Late Noachian. Regional volcanism character ized the Early Hesperian and subsequent to that time, volcanism was largely centered at Tharsis and Elysium, continuing until the recent geological pa st. The Tharsis region appears to have been largely constructed by the Late Noachian, and represents a series of tectonic and volcanic centers. Global ly distributed structural features representing contraction characterize th e middle Hesperian. Water-related processes involve the formation of valley networks in the Late Noachian and into the Hesperian, an ice sheet at the south pole in the middle Hesperian, and outflow channels and possible stand ing bodies of water in the northern lowlands in the Late Hesperian and into the Amazonian. A significant part of the present water budget occurs in th e present geologically young polar layered terrains. In order to establish more firmly rates of processes, we stress the need to improve the calibrati on of the absolute timescale, which today is based on crater count systems with substantial uncertainties, along with a sampling of rocks of unknown p rovenance. Sample return from carefully chosen stratigraphic units could ca librate the existing timescale and vastly improve our knowledge of Martian evolution.