1. Chyba, C. F., Thomas, P. J., Brookshaw, L. & Sagan, C. Science 249, 366–373 (1990).
2. Yuen, G., Blair, N., Des Marais, D. J., & Chang, S. Nature 307, 252–254 (1984).
3. National Research Council The Search for Life's Origins (National Academy Press, Washington, DC, 1990).
4. Cronin, J. R., Pizzarello, S. & Cruikshank, D. P. in Meteorites and the Early Solar System (eds Kerridge, J. F. & Matthews, M. S.) 819–857 (University of Arizona Press, 1988).
5. Shock, E. L. & Schulte, M. D. Geochim. cosmochim. Acta (in the press).