1. Campins, H., Rieke, G.H., and Lebofsky, M.J. 1983, Nature, 301, pp 405–406
2. Hanner, M.S. 1984, Ap.J. Lett. 277, pp L75–78
3. Whipple, F.L. 1950, Ap.J., Ill, pp 375–394
4. Huebner, W.F. and Weigert, A. 1966, Zs. Astrophys., 64, pp 185–201
5. Delsemme, A.H. and Wenger, A. 1970, Planet. Space Sci., 18, pp 709–715