Measuring geophysical processes in space from the shifting weight of the
Earth: Old problems, new methods, new results
Prof. Frederik Simons
Princeton University
www.frederik.net
The study of potential fields remains the primary tool in the exploration of the structure and evolution of the surface and interior of Earth and the terrestrial planets Mars, Venus and Mercury, and their moons. In the history of global geophysics and planetary physics, naked-eye, photographic or radar observation and mapping have usually led the way to satellite measurements of topography, gravitational and magnetic anomalies, to provide detailed and comprehensive insight into the inner workings of planetary bodies. Only on Earth and Moon thus far have we enjoyed the luxury of seismology to provide additional, and often more direct, constraints on interior structure.
Planetary exploration has now entered its second century by adding the fourth dimension to our monitoring. A case in point is GRACE, the Gravity Recovery And Climate Experiment, which, since 2002, has been delivering models of Earth's time-variable gravity anomalies that best fit the evolving distance between two identical satellites orbiting the Earth at 400--500 km altitude. These models take the user-friendly form of so-called Level-2 products: time series of spherical harmonic expansion coefficients with a temporal resolution of one-month. The analysis of time-variable gravity has resolved hydrological mass fluxes across large river basins, global mean ocean mass variations, ocean tides, ice sheet mean mass fluxes, and solid-earth mass movements and density changes, to name but a few applications of this remarkable growing data set.
In the past several years, we have completed several detailed theoretical studies designed to improve mass estimation from space. I will introduce the problems, discuss the solutions, and show examples from geodesy/seismology (detecting the Sumatra-Andaman earthquake from space) as well as other fields.