SOFTENED IMPACT CRATERS ON MARS - IMPLICATIONS FOR GROUND ICE AND THE STRUCTURE OF THE MARTIAN MEGAREGOLITH

"Terrain softening" is a distinct style of landform degradation observed on Mars. The morphology and distribution of softened terrain have led to the hypothesis that it is the result of quasiviscous relaxation of topology due to the creep deformation of subsurface ice. In order to test this hypothesis, we use a finite element model to investigate the relaxation of impact craters on Mars. For the martian megaregolith we assume a non-Newtonian viscoelastic rheology derived from experiments on frozen soils. Our results indicate that impact craters in softened terrain at midlatitudes on Mars have a morphology consistent with relaxation in a deforming layer no more than ∼1 km deep. We therefore conclude that creep of ground ice is consistent with the morphology of terrain softening. We also use our results to estimate the total quantity of subsurface ice on Mars. We find a quantity of ice equal to ∼17 m, spread over the planet, associated with the terrain softening observed to date. This number increases to 55 m if we include locations for which terrain softening is likely but for which adequate imaging data do not exist and 125 m if the conditions required for terrain softening are assumed to exist everywhere poleward of ±30° latitude. © 1993 by Academic Press, Inc.