Viscous topographic relaxation on Mercury revisited: Improved constraints on Mercury's crustal thickness

Kurzfassung

The crustal thickness of Mercury is only poorly constrained. Current estimates based on the observed center of mass - center of figure offset yield large crustal thickness values of 100-300 km. Further constraints on Mercury's crustal thickness may be obtained from the degree of topographic relaxation observed on the surface. Isostatically compensated loads induce lateral pressure gradients which can drive flow in the lower crust if the temperatures there are sufficiently high. The efficiency of the crustal flow and the degree of topographic relaxation depend on the temperature structure of the crust and therefore the crustal thickness, which may thus be constrained from the presence or absence of long wavelength topography. Impact structures on Mercury like the Caloris basin have been shown to be in a state close to total isostatic compensation for diameters larger than ~ 800 km. Since these structures do not show evidence for large scale topographic relaxation, crustal flow on Mercury cannot have been significant since the time of their emplacement in the pre-Tolstojan era around 4 Gyr. This stability of long-wavelength topography has been used by Nimmo (2002) to constrain Mercury's crustal thickness. Depending on crustal rheology and the distribution of heat sources in the interior, he could tighten the bounds on Dc to 100-200 km. We have reanalyzed the conditions under which the temperatures in Mercury's lower crust are sufficient to induce large scale flow, thus relaxing isostatically supported topography. Contrary to the previous study by Nimmo (2002), we use parametrized thermal evolution models to calculate the crust's thermal structure, taking secular cooling and a poorly conducting regolith layer into account. The results show that the survival of long-wavelength topography over the last 4 Gyr implies a crustal thickness of less than 100 km. Furthermore, if the thickness of the regolith layer approaches that of the lunar regolith, the average crustal thickness cannot be larger than 85 km. Nimmo, F., 2002. Constraining the crustal thickness on Mercury from viscous topographic relaxation, Geophys. Res. Lett., 29, doi:10.1029/2001GL013883.