Voyager radio occultation by Uranus' rings: I. Observational results

Abstract

The radio occultation of Voyager 2 by the rings of Uranus on January 24, 1986 provided the first observations of this system at microwave wavelengths ( λ = 3.6 and 13 cm). Fine resolution (50 and 200 m) optical depth profiles at λ = 3.6 cm depicting the nine pre-Voyager rings, including tenuous companiong of Rings η and δ, have been obtained at two azimuths separated by ∼145°. No other rings have been detected. The profiles were reconstructed from diffraction-limited observations assuming Keplerian ring dynamics. The rings are highly opaque, with optical depth τ typically comparable to or exceeding unity and sometimes at least as large as 6–8, in Rings γ and ϵ. Despite the narrow width of most rings, ∼1–10 km, the profiles reveal a system rich in structural details which significantly evolve with longitude. Ring ϵ has the most prominent structure, though it is nowhere convincingly regular. Highly regular wave-like fluctuations are observed at the inner edge of Ring δ. Rings ϵ and γ are the only two rings possessing extremely sharp inner and outer edges (edge gradient ⪆10 τ/ km ), the latter despite a width ∼-1.6 km and τ> 6. Most edges are abrupt, with 28 out of 36 edges characterized by a gradient ⪆1 τ/ km . Rings 6, 5, 4 and δ share common morphology of sharp outer, quasi-exponential inner edge transition. Ring η has similar but reversed behavior. Ring β is distinguished by diffuse edges and the smallest opacity observed ( τ⋍0.4). In all cases, radial edge location is determined to within ±1 km and ring widthto within ±200 m. The integrated 3.6-cm opacity at the two observation longitudes is conserved to within ∼10%, except for Ring γ where the difference is ∼26%. Little convincing evidence for differential extinction between 3.6 and 13 cm is found. The measurements constrain the integrated differential opacity to be ⪅3% in Ring ϵ (241° true anomaly), ⪅15% in Rings α, β, and δ ingress, and ⪅40% for other very narrow rings. Observed differential phase delay divides the rings into two distinct groups, with the five closest to Uranus differentially retarding the 13-cm phase more efficiently than the four outer ones (40 to 75 vs −17 to 31 × 10 −3 cycles/unit-τ).