1. The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
2. Planetary Science Institute, Tucson, AZ 85719, USA.
3. Institut de Recherche en Astrophysique et Planétologie, Université Paul Sabatier–CNRS–Observatoire Midi-Pyrénées, Toulouse, France.
4. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
5. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.