Solar heating of the Uranian mesopause by dust of ring origin

Abstract

We have estimated the magnitude of an equatorial heat source due to submicrometer dust in the Uranian upper atmosphere under the assumption that the dust collects extra solar energy in the visible while being inhibited from radiating in the infrared. Dust at the orbit of Uranus that possesses the combined bulk optical properties of known materials can attain temperatures of up to 200 K. Thus dust is capable of providing a significant heat source at the mesopause level, where temperatures of from 150 to 200 K have been observed in ground-based occultations. Dust heating may explain heat sources observed only near the equator, for which there is some evidence in both the ground-based and UVS-derived temperature profiles. However, such dust is ruled out as a heat source for the thermospheric temperatures of 500–800 K measured by the Voyager UVS (ultraviolet spectrometer). The influx necessary to provide a significant heat source corresponds to the decomposition into submicrometer dust, averaged over the age of the solar system, of some 10 2 moons 10 km in diameter of density 1.5 g cm −3. Recent work indicates that this is a realistic average dust flux for the Uranian rings.