Relaxation, contraction, and polar wander: A study of the evolution of crustal and lithospheric thickness variations on the Moon, Mars, Mercury, and Ganymede.

摘要

The majority of the surfaces of the Moon, Mars, Mercury, and Ganymede were shaped during the first 500-1000 Myr of the Solar System. As a result, they provide great insight into the processes that must have occurred on most, if not all, terrestrial planets and moons during that time period. In this study, a semi-analytic, self-gravitating, viscoelastic model of planetary deformation is developed and applied to the evolution of variations in their mechanical properties. First, the plausibility of viscous relaxation of large multi-ring lunar basins is investigated. This is found to be likely to have occurred during the first few hundred million years of lunar history, which places constraints on the timing and mechanism of crystallization of the lunar magma ocean. Second, the physical parameters of the largest martian basins are analyzed and found to be consistent with the occurrence of viscous relaxation throughout the period of heavy bombardment. The viscoelastic model is then employed to place constraints on the thermal state of early Mars. Third, the model is expanded to include lateral variations in viscosity and applied to the early contraction of Mercury. The results confirm the hypothesis that the amount of radial contraction has previously been underestimated. In addition to its expression through thrust faults, some fraction of the compressive stress was possibly taken up by long-wavelength folding of the mercurian lithosphere. Finally, an explanation of the anomalous cratering asymmetry between the leading and trailing hemispheres of Ganymede is proposed. Rotational dynamics calculations show that the thickness variations induced by the pole-to-equator temperature contrast was likely sufficient to make the axis of rotation unstable and cause the poles to exchange positions with the leading and trailing points.