1. Alfvén, H. and Arrhenius, G.: 1976,Evolution of the Solar System, NASA SP-345.
2. Biermann, L.: 1967, in J. L. Weinberg (ed.),The Zodiacal Light and the Interplanetary Medium, NASA SP-150, p. 279.
3. Brownlee, D. E., Tomandl, D., Blanchard, M. B., Ferry, G. V. and Kyte, F.: 1976, NASA TMX-73, p. 152.
4. De, B. R.: 1979,J. Chem. Phys. (in press).
5. Fechtig, H., Grun, E. and Morfill, G.: 1978, Paper read at the XXIth Plenary Meeting of COSPAR, Innsbruck, Austria, June 1978. To appear in theProceedings, M. J. Rycroft (ed.).