1. A. L. Lane et al., Science 215, 537 (1982).

2. C. W. Hord et al., ibid. 206, 956 (1979)- M. G. Tomasko, E Karkoschka, S. Marrinek, Icarus 65 218 (1986).

3. Acetylene has been detected in the high atmosphere [A. L. Broadfoot et al., Science 233, 74 (1986), and Atreya and Romani [S. K. Atreya and P. N. Romani, in Planetary Meteorology, G. Hunt, Ed. (Cambridge Univ. Press, Cambridge, 1985), pp. 17-68] have argued that photochemically produced acetylene should be abundant enough to condense at temperatures below 70 K, that is, near pressures of a few millibars for current estimates of the temperature structure [J. Appleby, Icarus 65, 383 (1986)]. Acetylene does not absorb sufficiently at 0.27 micrometers and longer wavelengths to be detected, but acetylene polymers, which have been proposed as possible aerosols on Titan, do [M. Allen, J. P. Pinto, Y. L. Yung, Astrophys. J. 242, L125 (1980)]. Polyacetylene production from gas-phase reactions is slow in the atmospheres of the giant planets and is thought to be negligible compared to N2H4 and P2H4 production on Jupiter and Saturn [R. A. West D. F. Strobel, M. G. Tomasko, Icarus 65, 161 (1986)]. Polyacetylene photochemistry, however, may well be the leading candidate for Uranus and Neptune since the partial pressure of ammonia is extremely low in the stratospheres of those planets, and phosphine has not yet been observed. Polyacetylene production may be stimulated by condensed-phase photochemical reactions occurring in UV-irradiated acetylene aerosols.

4. R. A. West et al. J. Geophys. Res. 88, 8699 (1983).

5. J. B. Pollack et al., Icarus 65, 442 (1986).

6. G. L. Tyler et al., Science 233, 79 (1986).

7. L. W. Esposito et al., J. Geophys. Res. 88, 8643 (1983).

8. J. L. Elliot and P. D. Nicholson, in Planetary Rings, R. Greenberg and A. Brahic, Eds. (Univ. of Arizona Press, Tucson, 1984), pp. 25-72.

9. B. A. Smith et al., Science 233, 43 (1986).

10. J. N. Cuzzi, Icarus 63, 312 (1985).

11. R. G. French, J. L. Elliot, S. E. Levine, in preparation.

12. W. B. Hubbard et al., Nature (London) 319, 636 (1986).

13. P. Goldreich and S. D. Tremaine, Astron. J. 84, 1638 (1979).

14. The upper limit for the surface number density is derived from the epsilon rings surface mass density (13) combined with the minimum of the particle size derived from the radio occultation differential opacity (6). The lower limit is from the PPS-measured optical depth (tau = 0.5 at maximum epsilon ring width; Table 1), and the maximum particle size derived from the PPS upper limit on the ring's vertical thickness (< 150 m, which implies a radius <30 m). The particle separation is derived from the measured optical depth and the range of permissible particle sizes.

15. W. M. Irvine, J. Geophys. Res. 71, 2931 (1966).

16. Theoretical studies, laboratory experiments, and astronomical observations of other objects in the solar system have shown that a determination of the phase curve is important in understanding the physical nature (composition, particle size, and degree of compaction) of a planetary regolith [B. W. Hapke, Astron. J. 71, 333 (1966); J. Veverka, in Planetary Satellites, J. Burns, Ed. (Univ. of Arizona Press, Tucson, 1977), pp. 171-209]. Especially important is the opposition surge at small phase angles (<6 degrees). Previous ground-based observations of the Uranian satellites have been limited, of necessity, to solar phase angles of less than 3.0 degrees [R. H. Brown and D. P. Cruikshank, Icarus 55 83 (1983)], and therefore the full solar phase angle range of the opposition surge region was unknown before this study. The conventional explanation of the opposition surge is that mutual shadowing among the particles in the optically active portion of the regolith disappears as the face of the object becomes illuminated to the observer. In general, fluffy surfaces exhibit greater opposition surges.

17. B. J. Buratti and J. Veverka, Icarus 58, 254 (1984); B. J. Buratti, ibid. 59, 392 (1984).

18. R. M. Nelson et al., in preparation.

19. R. M. Nelson and B. W. Hapke, Icarus 36, 304 (1978).

20. M. Noland et al., ibid. 23, 334 (1974).

21. We thank the members of the Voyager flight team, who made possible the successful encounter of the PPS with Uranus; P. D. Nicholson and J. Colwell for timing of the predicted occultation events; and M. D. Morrison (JPL) and A. Bahrami (LASP) for exceptional software wizardry. We also thank NASA's Office of Information Systems (code EI) Pilot Planetary Data System Project at the Jet Propulsion Laboratory for computational assistance in the rapid transformation of flight telemetry into scientific results.

Last updated 12-19-94

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