APL Colloquium

November 8, 2002

Colloquium Topic: Time-Variable Gravity From Space: A Quarter Century of Observations, Mysteries, and Prospects

Any large mass transport in the Earth system produces changes in the gravity field. Via the space geodetic technique of satellite-laser ranging in the last quarter century, the Earth's dynamic oblateness J2 (the lowest-degree harmonic component of the gravity field) has been observed to undergo a slight decrease ­ until around 1998, when it switched quite suddenly to an increase trend which continues to date. The secular decrease in J2 has long been attributed primarily to the post-glacial rebound in the mantle; the present increase signifies an even larger change in global mass distribution whose J2 effect overshadows that of the post-glacial rebound, at least over interannual timescales. Intriguing evidences have been found in the ocean water distribution, especially in the extra tropical Pacific basins, that may be responsible for this J2 change. New techniques based on satellite-to-satellite tracking will yield greatly improved observations for time-variable gravity, with much higher precision and spatial resolution (i.e., much higher harmonic degrees). The most important example is the GRACE mission launched in March 2002, following the success of the CHAMP mission. Such observations are becoming a new and powerful tool for remote sensing of geophysical fluid processes that involve larger-scale mass transports.

Colloquium Speaker: Benjamin F. Chao

Dr. Benjamin F. Chao received his B.S. in physics from National Taiwan University in 1973 and his Ph.D. in earth sciences from the University of California, San Diego in 1981. From 1975-81, he worked as a research assistant at the University of California ­ Scripps Institution of Oceanography. In 1981, he joined NASA Goddard Space Flight Center and is currently Head of the Space Geodesy Branch and also the Mission Scientist for the GRACE mission. Dr. Chao is a member of the International Earth Rotation Service, and he is president-elect of the Geodesy Section of the American Geophysical Union. He serves on the editorial board and committees of several prominent professional journals. His research interests include Earth and planetary rotation dynamics and gravitational variations, global changes in geophysical fluids, global geophysics and seismology.