Viscoelastic evolution of lunar multiring basins



[1] We investigate the evolution of multiring basins on the Moon. The orbital geophysical and geochemical data collected by Lunar Prospector allow three distinct groups of basins to be identified. Group 1 basins show well-preserved topography, a bull's-eye gravity signature, and little mare basalt fill. Group 2 basins contain large amounts of mare basalt and show mare-dominated gravity and topography signatures. Group 3 basins are characterized by low-amplitude topographic relief and weak gravity anomalies. While the crustal structures of group 1 and 2 basins appear to date from the collapse of the transient cavity, group 3 basins are highly degraded and show very little crustal thinning. Using a viscoelastic model, we explore the evolution of the lunar multiring basins, with the goal of explaining the degraded condition of group 3 basins. Our results show that while viscous relaxation is a plausible explanation for the state of group 3 farside basins, requiring heat flux > 25–40 mW m−2, the thin nearside crust renders subsolidus ductile flow in the crust unlikely. The existence of many degraded basins on the nearside points to a weakened, and possibly partially molten, lower crust. That South Pole–Aitken appears not to have relaxed suggests that similar conditions did not exist there and that the nearside was hotter than the farside early on. This is likely a consequence of the nearside-farside crustal thickness dichotomy, which would have caused the farside to reach full crystallization first, leaving the nearside with the remaining liquid and an excess of heat-producing elements.