Abstract
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 μm) 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.
Highlights
In this Letter we present the first millimeter and mid-infrared observations of the Uranian ring system, obtained in 2017 and 2018 with Atacama Large (sub)Millimeter Array (ALMA) at three wavelengths (Band 3; 3.1 mm, Band 4: 2.1 mm, Band 6: 1.3 mm) and the VISIR instrument (Lagage et al 2004) Q2 filter (18.72 μm) on the Very Large Telescope (VLT)
Our observations of the ring agree very well with the Karkoschka (2001c) model, supporting its derivation of the fractional visible area as a function of ring opening angle, and the 3.1 mm brightnesses of the inner main ring groups take the same ratio as observed at visible wavelengths
The consensus between our millimeter and mid-infrared observations and literature visible-wavelength observations shows that the properties of the main rings remain the same at any observed wavelength despite the fact that our observations are not sensitive to micron-sized dust
Summary
Observational data for the Uranian ring system were obtained from a combination of Earthbased stellar occultation measurements (French et al 1991, and references within), visible-light and radio occultation data from Voyager 2 (e.g., Smith et al 1986; Tyler et al 1986; Gresh et al 1989), visible-light HST (e.g., Karkoschka 2001a), and ground-based near-IR observations with adaptive optics (e.g., de Pater et al 2006, 2007, 2013; de Kleer et al 2013) Together, these observations revealed a complex system of ten narrow rings, three broad dusty rings, and at least thirteen associated small satellites (Smith et al 1986; Karkoschka 2001b; Showalter & Lissauer 2006).
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