1. Columbia Astrophysics Laboratory, Columbia University 1 , New York, New York 10027, USA
2. NASA Goddard Space Flight Center 2 , Greenbelt, Maryland 20771, USA
3. Faculty of Engineering and Applied Science, Memorial University 3 , Newfoundland and Labrador A1B 3X7, Canada
4. Lamont-Doherty Earth Observatory, Columbia University 4 , Palisades, New York 10964, USA
5. Space Telescope Science Institute 5 , 3700 San Martin Dr., Baltimore, Maryland 21218, USA
6. Planetary Science Institute 6 , Tucson, Arizona 85719, USA
7. Laboratory for Astrophysics and Surface Physics, University of Virginia 7 , Charlottesville, Virginia 22904, USA
8. American Museum of Natural History 8 , New York, New York 10024, USA
9. Université Catholique de Louvain 9 B-1348 Louvain-la-Neuve, Belgium