1. Institute of Geophysics and Planetary Physics; University of California; Los Angeles California 90095-1567 USA
2. Department of Chemistry; Tokyo Metropolitan University; Hachioji Tokyo 192-0397 Japan
3. Department of Polar Science; School of Multidisciplinary Science; SOKENDAI (The Graduate University for Advanced Studies); Tokyo 190-8518 Japan
4. NASA Marshall Space Flight Center; Huntsville Alabama 35812 USA