1. DLR; Institute of Planetary Research; Berlin Germany
2. LESIA, Observatoire de Paris, CNRS; UPMC University Paris 06, University Paris-Diderot-Meudon; Paris France
3. Department of Geology and Geoenvironment; National and Kapodistrian University of Athens; Athens Greece
4. Now at Jet Propulsion Laboratory; California Institute of Technology; Pasadena California USA
5. Westphalian Wilhelms-University; Institute for Planetology; Münster Germany
6. School of Earth and Environmental Sciences; Washington State University; Pullman Washington USA
7. Center for Astronomy and Astrophysics; Technical University of Berlin; Berlin Germany