Sequence and timing of conditions on early Mars

The geological record of early Mars displays a variety of features that indicate fundamental differences from more recent conditions. These include evidence for: (1) widespread aqueous alteration and phyllosilicate formation, (2) the existence of an active magnetic dynamo, (3) the erosion of extensive valley networks, some thousands of kilometers long, (4) a much more significant role of impact cratering, forming structures up to the scale of large basins, and (5) the construction of much of the Tharsis volcanic province. Mars also is likely to have had a much thicker atmosphere during this early period. We discuss and review the temporal relationships among these processes and conditions. Key observations from this analysis suggest the following: (1) the last large impact basins, Argyre, Isidis, and Hellas, all pre-date the end of valley network formation, potentially by several hundred million years, (2) the magnetic dynamo is likely to be ancient (pre-Hellas), since the center of Hellas and other young basins lack magnetic remanence, and (3) the period of phyllosilicate formation is not readily connected to the period of valley network formation. Concepts for the possible formation and evolution of life on Mars should address this time sequence of conditions.