1. P. T. P. Ho and C. H. Townes, Ann. Rev. Astron. Astrophys. 21, 239 (1983).
2. R. Wildt, Veroff. U.-Sternw. Gottingen 22, 171 (1932).
3. L. P. Giver and H. Spinrad, Icarus 5, 586 (1966).
4. G. P. Kuiper in The Atmospheres of the Earth and Planets, ed. G. Kuiper, The University of Chicago Press, Ch. 12, pp. 306–405 (1952).
5. D. K. Aitken and B. Jones, Nature 240, 230 (1972).