SCATTERING PROPERTIES OF NATURAL SNOW AND FROST - COMPARISON WITH ICY SATELLITE PHOTOMETRY

The recent success in fitting Hapke's 1986 photometric function to Voyager observations of the icy satellites of Uranus and Saturn enables a comparison between their derived regolith parameters and those for terrestrial snow and frost surfaces. Middleton and Mungall (1952, J. Opt. Soc. Amer. 42, 572-579) provide excellent in situ goniophotometric data extending to 154° in phase angle for a variety of snow and frost surfaces. We fit Hapke's 1986 equation to determine the single-scatteringalbedo ω0 and the one-term Henyey-Greenstein particle phase function g for each of the Middleton and Mungall samples and find very high single-scattering albedos (w0 ∼ 0.99) and strongly forward scattering particle phase functions (g = +0.52 to +0.79), in accordance with theoretical considerations based on Mie theory, but in contrast to the observed backscattering (negative g behavior of icy satellites. We suggest that on icy satellites frost grains are aggregated into particles of complex texture which produce the remarkable backscattering behavior. The extremely low temperatures prevalent in the outer Solar System may be significant in preserving such highly textured aggregates. © 1990.