GLOBAL CRUSTAL CONSEQUENCES OF MEGA IMPACTS ON MARS

Impact cratering events of local consequence have been extensively studied analytically, numerically, and in laboratories. Mega-impacts, in the size class that produces satellites and binary systems have likewise been simulated. Here we examine the geophysical consequences of intermediate size events which may be responsible for global crustal redistribution, and which exhibit sensitivity to the spherical geometry of the planet. On Mars, such events may be important in the formation of the hemispheric dichotomy, as well as Hellas and the Utopia basins. The Caloris basin on Mercury and South Pole Aitken basin on the Moon may have similar global crustal consequences. To test the hypothesis that the martian dichotomy between the northern, lower-elevation, thinner crust lowlands relative to the south was produced by a mega-impact we employ a fully 3 dimensional self-gravitating Smoothed Particle Hydrodynamics (SPH) model. The amount and spatial distribution of melt produced by the event are investigated, and ultimate configuration of crustal material following the initial relaxation is considered and compared to models. The post-impact angular momentum of the planet and the fate of ejected material is described, as a function of initial conditions. We find the projectile energy, and importantly, three-dimensional effects due to non-zero impact parameter, play a significant role in determining the characteristics of the post-impact planet. Results indicate that at constant energy, larger, slower, and low-angle impacts penetrate deepest into the planet. Again at constant energy, melt production depends on impact velocity, but not monotonically. Constraints on impact scenarios for producing the martian crustal dichotomy are hence placed.