The SLR secular gravity variations and their impact on the inference of mantle rheology and lithospheric thickness

A long history of SLR (Satellite Laser Ranging) observations of the geodetic satellites LAGEOS-I, LAGEOS-II, Starlette and Stella have been analyzed in order to estimate the time series of the low degree zonal coefficients in the Earth gravity field, up to degree six, and derive their secular drifts. The paper will point out the critical aspects of the analysis process and will compare the estimated zonal rates with other published results. Comparison of these zonal rates with the results of global, viscoelastic Earth models forced by Pleistocenic deglaciation, shows that the SLR retrieved zonals equation image and the lumped odd zonals can be used to infer the upper mantle viscosity and lithospheric thickness. Discrepancies in the viscosity profiles, required to reproduce the different zonals, seem to indicate ongoing mass redistribution over the Earth.