1 , B. Ray Hawke 2 , W. B. Garry 1 , Bruce A. Campbell 3 , T. A. Giguere 2 , and D. B. J. Bussey 4 , 1 NASA Goddard Space Flight Center, Planetary Geodynamics Lab Code 698, Greenbelt, MD 20771, lynn.m.carter@nasa.gov, 2 Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu HI 96822, 3 Center for Earth and Planetary Studies, Smithsonian Institution, Washington DC 20013, 4 The Johns Hopkins University Applied Physics Lab, Laurel, MD, 20723.