Thermal Emission from the Uranian Ring System

Abstract

The narrow main rings of Uranus are composed of almost exclusively centimeter- to meter-sized particles, with a very small or nonexistent dust component; however, the filling factor, composition, thickness, mass, and detailed particle size distribution of these rings remain poorly constrained. Using millimeter (1.3–3.1 mm) imaging from the Atacama Large (sub-)Millimeter Array and mid-infrared (18.7 μ m) imaging from the Very Large Telescope VISIR instrument, we observed the thermal component of the Uranian ring system for the first time. The ϵ ring is detected strongly and can be seen by eye in the images; the other main rings are visible in a radial (azimuthally averaged) profile at millimeter wavelengths. A simple thermal model similar to the Near-Earth Asteroid Thermal Model (NEATM) of near-Earth asteroids is applied to the ϵ ring to determine a ring particle temperature of 77.3 ± 1.8 K. The observed temperature is higher than expected for fast-rotating ring particles viewed at our observing geometry, meaning that the data favor a model in which the thermal inertia of the ring particles is low and/or their rotation rate is slow. The ϵ ring displays a factor of 2–3 brightness difference between periapsis and apoapsis, with 49.1% ± 2.2% of sightlines through the ring striking a particle. These observations are consistent with optical and near-infrared reflected light observations, confirming the hypothesis that micron-sized dust is not present in the ring system.