1. Planetary Science Institute, 1700 East Fort Lowell Road, Tucson, AZ 85719-2395, USA
2. Freie Universität Berlin, Institute of Geological Sciences, Planetary Sciences & Remote Sensing, Malteserstrasse 74-100, 12249 Berlin, Germany
3. Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, TX 77058, USA
4. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA
5. Dipartimento di Geoscienze, Universita' degli Studi di Padova, via G. Gradenigo 6, 35131 Padova, Italy
6. Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany