Tropospheric Composition and Circulation of Uranus with ALMA and the VLA

Abstract

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 $\sim$1 to $\sim$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 $\sim$50${}^{\circ }$ N) are well fit by a deep H${}_2$S mixing ratio of $8.7_{-1.5}^{+3.1}\times 10^{-4}$ ($37_{-6}^{+13}\times$ Solar) and a deep NH${}_3$ mixing ratio of $1.7_{-0.4}^{+0.7}\times 10^{-4}$ ($1.4_{-0.3}^{+0.5}\times$ 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 $\sim$50${}^{\circ }$N, which we attribute to strong depletions extending down to the NH${}_4$SH layer in both NH${}_3$ and H${}_2$S relative to the equatorial region; the model is consistent with an NH${}_3$ abundance of $4.7_{-1.8}^{+2.1}\times 10^{-7}$ and an H${}_2$S abundance of $<$$1.9\times 10^{-7}$ between $\sim$20 and $\sim$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 $\sim$0.1 to $\sim$50 bar. The highest-resolution maps reveal zonal radio-dark and radio-bright bands at 20${}^{\circ }$S, 0${}^{\circ }$, and 20${}^{\circ }$N, as well as zonal banding within the north polar region. The difference in brightness is a factor of $\sim$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.

27 pages, 13 figures, 3 tables. Accepted by Planetary Science Journal (PSJ)