[1] Univ Edinburgh, UK Ctr Astrobiol, Sch Phys & Astron, Edinburgh EH8 9YL, Midlothian, Scotland
[2] CALTECH, Jet Prop Lab, Pasadena, CA 91125 USA
[3] CALTECH, Seismol Lab, Div Geol & Planetary Sci, Pasadena, CA USA
[4] Univ Toronto, Dept Earth Sci, Toronto, ON, Canada
[5] Cornell Univ, Carl Sagan Inst, Ithaca, NY USA
[6] Cornell Univ, Dept Astron, Ithaca, NY USA