Tropospheric Composition and Circulation of Uranus with ALMA and the VLA

Abstract

Abstract We present Atacama Large Millimeter/submillimeter Array (ALMA) and Very Large Array (VLA) spatial maps of the Uranian atmosphere taken between 2015 and 2018 at wavelengths from 1.3 mm to 10 cm, probing pressures from ∼1 to ∼50 bar at spatial resolutions from 0.″1 to 0.″8. Radiative transfer modeling was performed to determine the physical origin of the brightness variations across Uranus’s disk. The radio-dark equator and midlatitudes of the planet (south of ∼50°N) are well fit by a deep H2S mixing ratio of ( solar) and a deep NH3 mixing ratio of ( solar), in good agreement with models of Uranus’s disk-averaged spectrum from the literature. The north polar region is very bright at all frequencies northward of ∼50°N, which we attribute to strong depletions extending down to the NH4SH layer in both NH3 and H2S relative to the equatorial region; the model is consistent with an NH3 abundance of and an H2S abundance of <1.9 × 10−7 between ∼20 and ∼50 bar. Combining this observed depletion in condensible molecules with methane-sensitive near-infrared observations from the literature suggests large-scale downwelling in the north polar vortex region from ∼0.1 to ∼50 bar. The highest-resolution maps reveal zonal radio-dark and radio-bright bands at 20°S, 0°, and 20°N, as well as zonal banding within the north polar region. The difference in brightness is a factor of ∼10 less pronounced in these bands than the difference between the north pole and equator, and additional observations are required to determine the temperature, composition, and vertical extent of these features.