This award supports a theoretical and modeling effort to determine the effects of the Greenland ice sheet on the rotation and gravity field of Earth. To accomplish the goals, the secular and interannual changes in the geopotential, the position in the Earth's rotation axis and certain other parameters, such as center-of-mass motion and length of day will be calculated from mass balance changes in ice. The largest gravitational effects of ice thickness change shown in simple models are up to many times higher than the observed effects. This suggests that the viscous response of the mantle to long term changes in ice loading may be cancelling some of the elastic response. Current changes in gravity and crustal motion depend critically on the time lagged response of the Earth's mantle to previous loading histories, lasting as long as tens of thousands of years. Spatial and temporal changes in mass balance from late Pleistocene deglaciation and the subsequent interglacial period will be combined with current mass balance estimates to assemble time histories for ice thickness throughout the ice sheet. From these histories, the elastic results for gravity, rotation and deformation can be corrected by the use of glacial rebound models, such as the visco-elastic spherical Earth model. The viscous correction to the elastic loading for several cases of late Pleistocene deglaciation and more recent thickness change will be calculated for various upper and lower mantle viscosity profiles. A 20 kilometer by 20 kilometer grid will provide means of projecting the viscous rebound rates for the point mass model into the non-pointlike structure of the Greenland ice sheet. In addition to gravity, the components of crustal deformation and tilt are important in geodetic measurements from space. This project will also assess the impact that future observational improvements will have on understanding the mass balance of Greenland. The observational improvements are anticipated fr om global positioning system (GPS) studies and studies involving satellites that use laser ranging. The use of existing gridded meteorological, accumulation, ablation, and ice thickness data, along with ice flow models to predict future ice thickness changes in Greenland is expected to greatly reduce the difficulty in calculating gravity coefficients and crustal motion for both the elastic and viscous cases. The effects of the possible changes in the position of the upper boundary of appreciable summer snow melt that roughly follows the periphery of the ice sheet called the Benson line can be used to predict vertical motion near the ice sheet boundary and the results may be checked against sea level records, where they exist. Greenland strain and velocity data may be used to estimate retention time of new accumulation and the rates of discharge at the perimeter of the ice sheet. The sensitivity of ice model contributions to the total mass loss (or gain) to the oceans and to changes in other ice systems will also be investigated. Anticipated results include high resolution surface models of gravity, deformation, and tilt for the Greenland Ice Sheet, based on refinements of existing models of ice thickness change, and corrected for the viscous response of the mantle. Since these results are expected to fall within the detectable range of GPS receivers and possibly absolute gravity meters, they may be tested against space and ground based geodetic observations. This research will lead to a better understanding of the dynamic relationship between ice sheets and Earth's crust.
