  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 $\mu$m) 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 near-Earth asteroids is applied to the
$\epsilon$ ring to determine a ring particle temperature of $77.3 \pm 1.8$ 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 $49.1 \pm 2.2$\% 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.
