[1] Univ London Birkbeck Coll, Sch Earth Sci, London WC1E 7HX, England
[2] Univ London Birkbeck Coll, Res Sch Earth Sci, London WC1E 7HX, England
[3] Open Univ, Ctr Earth Planetary Space & Astron Res, Dept Phys & Astron, Milton Keynes MK7 6AA, Bucks, England
[4] Macquarie Univ, Australian Ctr Astrobiol, N Ryde, NSW, Australia
[5] Atom Energy Canada Ltd, Mat & Mech Branch, Chalk River, ON K0J 1J0, Canada