Tropospheric composition and circulation of Uranus with ALMA and the VLA

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 8.7+3.1- 1.5 ×10- 4 ( 37+13-6 × solar) and a deep NH3 mixing ratio of 1.7+0.7-0.4 ×10-4 ( 1.4+0.50.3× 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+2.1- 1.8 ×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.