# Thermal Emission from the Uranian Ring System

## TL;DR

This study presents the first thermal imaging observations of Uranus's ring system, revealing particle temperatures, composition insights, and confirming the absence of micron-sized dust, thereby enhancing understanding of ring particle properties.

## Contribution

First thermal emission measurements of Uranus's rings using millimeter and mid-infrared imaging, providing new constraints on particle temperature, composition, and size distribution.

## Key findings

- The epsilon ring is strongly detected and visible in thermal images.
- Ring particle temperature is approximately 77 K, indicating low thermal inertia or slow rotation.
- The ring lacks micron-sized dust, consistent with optical and infrared observations.

## 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 $\mu$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.