THE ISOSTATIC STATE OF MARTIAN TOPOGRAPHY - REVISITED

We present a spatial analysis of free air gravity and topography for Mars using new, higher resolution gravity data and the latest digital elevation data, both represented by spherical harmonics up to degree and order 50 and, separately, for harmonic degrees 4-19. Interpreting the gravity vs gravity-from-topography scatter diagram in terms of isostatic compensation for small regions, we find that most of Mars older than Amazonian is fully compensated at shallow depths. Exceptions to this include the ancient Tempe Terra region and the central portion 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 Amazonian-age plains (and perhaps Late Hesperian ridged plains) near the young volcanic constructs, like Olympus Mons and the Tharsis and Elysium volcanoes themselves, are clearly not isostatically compensated at shallow depth. Central Alba, an Early Ammazonian volcanic of great size, also appears not to be compensated at shallow depth.