1. Department of Earth, Atmospheric and Planetary Sciences , Massachusetts Institute of Technology, Cambridge, MA 02139 , USA
2. Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology , Cambridge, MA 02139 , USA
3. Department of Physics and Astronomy, University of Victoria , Victoria, BC V8W 2Y2 , Canada
4. Département de Physique , Université de Montréal, Montréal, QC H3C 3J7 , Canada
5. Department of Aeronautics and Astronautics, Massachusetts Institute of Technology , Cambridge, MA 02139 , USA
6. Department of Astronomy, Tsinghua University , Beijing 100084 , China