1. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
2. Malin Space Science Systems, San Diego, CA 92191, USA.
3. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA.
4. Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.
5. Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA.
6. Physikalisches Institut, Universität Bern, Bern, Switzerland.