1. Nelson, R. M. et al. Science 210, 784 (1980).
2. Nelson, R. M., Matson, D. L., Lane, A. L., Motteler, F. C. & Ockert, M. E. Bull. Am. astr. Soc. 12, 713 (1980).
3. Nelson, R. M. & Hapke, B. W. Icarus 36, 304 (1978).
4. Morrison, D. & Morrison, N. D. Planetary Satellites (ed. Burne, J.) 363 (University of Arizona Press, 1977).
5. Johnson, T. V. Icarus 14, 94 (1971).