1. Planetary Science Institute, Tucson, AZ 85719, USA.
2. School of Geography and the Environment, University of Oxford, Oxford OX1 3QY, UK.
3. Applied Physics Lab, John Hopkins University, Laurel, MD 20723, USA.
4. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
5. Department of Earth and Environmental Science, Rutgers University, Newark, NJ 07102, USA.
6. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
7. U.S. Geological Survey, Flagstaff, AZ 86001, USA.
8. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA.
9. Space Research and Planetology Division, University of Bern, CH-0132 Bern, Switzerland.