Tropospheric Composition and Circulation of Uranus with ALMA and the VLA

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 ${8.7}_{-1.5}^{+3.1}\times {10}^{-4}$ ( ${37}_{-6}^{+13}\times $ solar) and a deep NH₃ mixing ratio of ${1.7}_{-0.4}^{+0.7}\times {10}^{-4}$ ( ${1.4}_{-0.3}^{+0.5}\times $ 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 ${4.7}_{-1.8}^{+2.1}\times {10}^{-7}$ 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.