1. McKay, D. S., et al., Science, 167, 654–656 (1970).
2. Chernyak, Yu. B., and Nussinov, M. D., Preprint 189 (Space Res. Inst., Acad Sci. USSR, Moscow, 1974).
3. Mueller, D., and Hinch, G., Nature, 228, 5268 (1970).
4. Shima, M., et al., in COSPAR Space Research XII (Academic, Berlin, 1972).
5. Cloud, T., et al., Science, 167, 3918 (1970).