Viscoelastic response to surface and tidal loading - applications to glacial isostatic adjustment of the Earth and tidal deformation of the icy satellites.

摘要

Observations of glacial isostatic adjustment (GIA), the viscoelastic relaxation of the Earth induced by deglaciation following the last glacial maximum, have provided valuable constraints on late Pleistocene ice history and on the internal viscoelastic structure of the solid Earth. The GIA signal is also a significant source of noise for other applications. For example, errors in GIA models due to errors in the assumed ice deglaciation history and mantle viscosity structure, are generally assumed to be the largest source of uncertainty when using GRACE time-variable satellite gravity data to estimate present-day thinning rates of the Antarctic ice sheet.;The same physical law that governs the Earth's viscoelastic deformation is also applicable to the tidal deformation on Jupiter's icy moons. One of the long-sought objectives of an orbiter or fly-by mission to one of Jupiter's icy moons, has been to use observations of tides on the moon to help determine the existence of a liquid ocean and characteristics of the overlying icy shell.;For the first part of this study, we develop a 3-D finite-element model to study the viscoelastic response of a compressible Earth to surface loads. By forcing our model with the ICE-5G global ice loading history, and computing GIA results for a 3-D viscosity profile derived from a realistic seismic tomography model, we study the effects of 3-D viscosity structure on several GIA observables, including relative sea level measurements in Canada, and present-day time-variable gravity and uplift rates in Antarctica. We also apply our semi-analytic method to the southern Greenland ice sheet (sGrIS). Using a newly developed ice elevation change history along with different 1-D viscosity structures, we study the influence of GIA effects caused by the Post-Little Ice Age (Post-LIA) deglaciation for the last century on GRACE and GPS present-day observables in sGrIS. In general, we find that the effects of a 3-D viscosity profile and the Post-LIA deglaciation play a minor role when using GRACE to study the present-day ice loss, but they could have a significant impact on GPS present-day surface motion estimates.;For the second part of this study, we apply the same finite-element model to solve for the response of Ganymede and Europa to Jupiter's tidal forcing, using various icy shell models with 3-D structure. We find that the presence of 3-D shell thickness and shear modulus would probably not affect future attempts to detect a liquid ocean and to determine the mean shell thickness. The inference of a possible 3-D shell structure from the tidal measurements would be challenging. Grounded ice, if existed, might be detected from tidal measurements, but its existence might lead to an overestimate of the floating icy shell's thickness.