We present ALMA and 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 8.7−1.5+3.1×10−4 (37−6+13× Solar) and a deep NH3 mixing ratio of 1.7−0.4+0.7×10−4 (1.4−0.3+0.5× Solar), in good agreement with literature models of Uranus's disk-averaged spectrum. 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 4.7−1.8+2.1×10−7 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.
@article{arxiv.2010.11154,
title = {Tropospheric Composition and Circulation of Uranus with ALMA and the VLA},
author = {Edward M. Molter and Imke de Pater and Statia Luszcz-Cook and Joshua Tollefson and Robert J. Sault and Bryan Butler and David de Boer},
journal= {arXiv preprint arXiv:2010.11154},
year = {2020}
}