Thermal Measurements of the Ring System of Uranus

Abstract:

The narrow main rings of Uranus are composed of primarily centimeter- to meter-sized particles, with a very small or nonexistent dust component. This property presents a challenge to visible and near-infrared instruments observing the rings, which are largely unable to differentiate large particles from dust; thus, the thickness, mass, filling factor, and detailed particle size distribution of these rings remain poorly constrained. In this work we observed the thermal component of the Uranian ring system for the first time, making use of millimeter (1.3-3.1 mm) imaging from the Atacama Large (sub-)Millimeter Array and mid-infrared imaging from the Very Large Telescope VISIR instrument. The ε ring was readily seen by eye in the images; the other main ring groups were 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 was leveraged to determine a ring particle temperature of 77.3 ± 1.8 K for the ε ring. This 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 displayed a factor of 2-3 brightness difference between periapsis and apoapsis, and an average fractional visible area (the two-dimensional projection of the filling factor) of 49.1% ± 2.2%. These observations are consistent with optical and near-infrared reflected light observations, confirming the hypothesis that micron-sized dust is not present in Uranus's main rings.