1. Geology and Geochemistry Cluster, Faculty of Science, Vrije Universiteit, de Boelelaan 1085, 1081HV Amsterdam, the Netherlands
2. Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
3. Institut für Planetologie, Westfälische Wilhelms-Universität Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany