1. Agarwal, V. K., Schutle, W., Greenberg, J. M., Ferris, J. P., Briggs, R., Conner, S., Van Debult, C. P. E. M., and Baas, F.: 1985,Origins of Life 16, 21.
2. Alfven, H. and Arrhenius, G.: 1976,Evolution of the Solar System, NASA SP-345.
3. Allen, M., Delitsky, M., Huntress, W., Yung, Y., Ip, W., Schwenn, R., Balsiger, H., and Geiss, J.: 1987,Astron. Astrophys. 187, 502.
4. Bar-Nun, A., Bar-Nun, N., Bauer, S.H., and Sagan, C.: 1971, in R. Buvet and C. Ponnamperuma (eds),Chemical Evolution and the Origin of Life, North-Holland Publishing Co., Amsterdam, p. 114.
5. Bernal, J. D.: 1951,The Physical Basis of Life, Routledge and Kegen Paul, London.