THE ISOSTATIC STATE OF MARTIAN TOPOGRAPHY REVISITED

We present a spatial analysis of free air gravity and topography for M ars using new, higher resolution gravity data and the latest digital e levation data, both represented by spherical harmonics up to degree an d order 50 and, separately, for harmonic degrees 4-19. Interpreting th e gravity vs gravity-from-topography scatter diagram in terms of isost atic compensation for small regions, we find that most of Mars older t han Amazonian is fully compensated at shallow depths (< 50 km). Except ions to this include the ancient Tempe Terra region and the central po rtion of the Isidis impact basin (a known mascon). Most Hesperian-age and Amazonian-age volcanic plains not located near Tharsis and Elysium volcanoes are also compensated isostatically at shallow depths, but A mazonian-age plains near the young volcanic constructs, like Olympus M ons and the Tharsis and Elysium volcanoes themselves, are not. Both Ol ympus Mons and Alba have isostatic compensation depths > 300 km.