Thermal Emission from the Uranian Ring System

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 midinfrared (18.7 µm) imaging from the Very Large Telescope VISIR instrument, we observed the thermal component of the Uranian ring system for the first time. The epsilon 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 NEATM model of nearEarth asteroids is applied to the epsilon 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 epsilon 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.