1. Andronico, G., Baratta, G. A., Spinella, F. and Strazzulla, G.: 1987, Astron. Astrophys. 184, 333.
2. Apt, J., Carleton, N. P. and Mackay, C. D.: 1983, Astrophys. J. 270, 342.
3. Bell, J. F., Cruikshank, D. P. and Gaffey, M. J.: 1985, Icarus 61, 19.
4. Bibring, J. P. and Rocard, F.: 1984, Adv. Space Res. 4, 103.
5. Brown, R. H.: 1983, Icarus 56, 414.