Paul K. Byrne, Sean C. Solomon, Francis Nimmo, Thomas R. Watters, Brett W. Denevi, Carolyn M. Ernst, Maria E. Banks, Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA, cklimczak@ciw.edu; Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA; 3 Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064, USA; Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560, USA; The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.