1. Department of Physics and Astronomy , Dartmouth College, Hanover, NH 03755 , USA
2. Atmospheric and Environmental Research, Inc. , Lexington, MA 02421 , USA
3. Carl Sagan Institute , Space Science Building 311, Ithaca, NY 14850 , USA
4. Department of Astronomy, Cornell University, Astronomy and Space Sciences Building , Ithaca, NY 14850 , USA