Viscous relaxation on early Mars: A study of ancient impact basins

The improved resolution of recent global models of the gravity field of Mars enables an investigation of the crustal structure of large Martian impact basins. The results reveal that almost all basins between 275-1000 km in diameter show very shallow depths and limited crustal thinning for their size. We also show evidence that Martian basins once had lunar mascon-like gravity highs. We demonstrate that it is unlikely that surface processes alone are responsible for the current state of Martian basins and propose that viscous relaxation of relief by lower crustal flow was the dominant process in modifying Martian basins early in the planet's history. Viscoelastic modeling suggests that the current surface topography and crustal structure of these basins are consistent with this hypothesis. The heat flows required to produce relaxation (> 47-55 mW m(-2)) are in good agreement with previous geophysical constraints on early Martian thermal evolution.