1. Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08540
2. Institute of Lunar and Planetary Sciences, School of Physics, University of Newcastle upon Tyne, Newcastle upon Tyne, England
3. McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63160
4. NASA Langley Research Center, Hampton, Virginia 23665