1. Department of Astronomy, University of Maryland, College Park, Maryland, USA.
2. Planetary Environments Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
3. Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA.
4. Department of Astronomy, Cornell University, Ithaca, New York, USA.
5. NASA Ames Research Center, Moffett Field, California, USA.