Uranian ring photometry: Results from Voyager 2

Abstract

We have investigated seven images of the Uranian rings taken with the clear filter of the Voyager 2 cameras during the January 1986 encounter. The images are at phase angles of 15.5°, 21.0°, 89.5°, and 172.5°. We obtained an average reflectivity (I/F) for each of the nine main rings (6, 5, 4, α, β, η, γ, δ, and ε) at each phase angle. We compared the observed phase angle variation of I/F with model calculations for various combinations of macroscopic (backscattering) and microscopic (forward scattering) particles. The phase angle variation of I/F for all of the main rings is dominated by backscattering. The phase functions for the ring particles resemble that of the Moon; however, the observed phase functions decrease more rapidly between 21.0° and 15.5° than predicted by the lunar phase function. We obtained a single‐scattering or Bond albedo for the large particles using the best fit backscatter phase function. The average Bond albedo of the particles for all the rings was 0.014 ± 0.004. The shape of the phase variation of I/F around 172.5° phase angle was used to constrain the size distribution of the microscopic “dust‐sized” particles. The fractional area in dust‐sized particles in and around the main rings did not exceed 2 × 10−3. The observations are consistent with an average dust particle size of 1.0 ± 0.3 µm in a Hansen‐Hovenier distribution and with a power law distribution for spherical particles with spectral index (p) = 2.5 ± 0.5. The phase function of 1986U1R was not modeled, because of an anomalous azimuthal darkening in the highest phase angle image that could be due to structural variation.