1. Department of Lagrange, Université de Nice-Sophia Antipolis, Observatoire de la Côte d'Azur, CNRS, 06304 Nice Cedex 4, France;
2. Department of Astronomy, Cornell University, Ithaca, New York 14853;
3. Planetary Science Institute, Tucson, Arizona 85719;
4. Université de Bordeaux, Observatoire Aquitain des Sciences de l'Univers, CNRS, UMR 5804, Laboratoire d'Astrophysique de Bordeaux, F-33271 Floirac Cedex, France;
5. Department of Space Studies, Southwest Research Institute, Boulder, Colorado 80302;