1. Department of Earth and Planetary Sciences; Birkbeck University of London; Malet Street London WC1E 7HX UK
2. UCL/Birkbeck Centre for Planetary Sciences; UCL; Gower Street London WC1E 6BT UK
3. Department of Earth Sciences; Natural History Museum; Cromwell Road London SW7 5BD UK
4. Department of Physical Sciences; The Open University; Walton Hall Milton Keynes MK7 6AA UK
5. Department of Earth Sciences; University College London; Gower Street London WC1E 6BT UK
6. SETI Institute; Carl Sagan Centre; 189 Bernardo Avenue Mountain View California 94043 USA
7. Department of Physics; University of Khartoum; PO Box 321 Khartoum 11115 Sudan