Gravity and tectonic patterns of Mercury: Effect of tidal deformation, spin-orbit resonance, nonzero eccentricity, despinning, and reorientation

摘要

We consider the effect of spin-orbit resonance, nonzero eccentricity, despinning, and reorientation on Mercury's gravity and tectonic pattern. Large variations of the gravity and shape coefficients from the synchronous rotation and zero eccentricity values, J 2/C 22 = 10/3 and (b ? c)/(a ? c) = 1/4, arise because of nonsynchronous rotation and nonzero eccentricity even in the absence of reorientation or despinning. Reorientation or despinning induces additional variations. The large gravity coefficients J 2 = (6 ± 2) × 10?5 and C 22 = (1 ± 0.5) × 10?5 estimated from the Mariner 10 flybys cannot be attributed to Caloris alone since the required mass excess in this case would have caused Caloris to migrate to one of Mercury's hot poles. Similarly, a large remnant bulge due to a smaller semimajor axis and spin-orbit resonance can be dismissed since the required semimajor axis is unphysically small (<0.1 AU). Reorientation of a large remnant bulge recording an epoch of faster rotation (without significant semimajor axis variations) can explain the large gravity coefficients. This requires initial rotation rates 20 times the present value and a positive gravity anomaly associated with Caloris capable of driving ～10°–45° equatorward reorientation. The required gravity anomaly can be explained by infilling of the basin with material of thicknesses 7 km or an annulus of volcanic plains emplaced around the basin with an annulus width ～1200 km and fill thicknesses 2 km. The predicted tectonic pattern due to these despinning and reorientation scenarios, including some radial contraction, is in good agreement with the lobate scarp pattern observed by Mariner 10. We also predict that lobate scarps will follow a NE–SW orientation in the eastern hemisphere and a positive gravity anomaly (of a few hundred mGal) associated with Caloris.