Neptune's Spatial Brightness Temperature Variations from the VLA and ALMA
We present spatially resolved ($0.1'' - 1.0''$) radio maps of Neptune taken from the Very Large Array and Atacama Large Submillimeter/Millimeter Array between $2015-2017$. Combined, these observations probe from just below the main methane cloud deck at $\sim 1$ bar down to the NH$_4$SH cloud at $\sim50$ bar. Prominent latitudinal variations in the brightness temperature are seen across the disk. Depending on wavelength, the south polar region is $5-40$ K brighter than the mid-latitudes and northern equatorial region. We use radiative transfer modeling coupled to Markov Chain Monte Carlo methods to retrieve H$_2$S, NH$_3$, and CH$_4$ abundance profiles across the disk, though only strong constraints can be made for H$_2$S. Below all cloud formation, the data are well fit by $53.8^{+18.9}_{-13.4}\times$ and $3.9^{+2.1}_{-3.1}\times$ protosolar enrichment in the H$_2$S and NH$_3$ abundances, respectively, assuming a dry adiabat. Models in which the radio-cold mid-latitudes and northern equatorial region are supersaturated in H$_2$S are statistically favored over models following strict thermochemical equilibrium. H$_2$S is more abundant at the equatorial region than at the poles, indicative of strong, persistent global circulation. Our results imply that Neptune's sulfur-to-nitrogen ratio exceeds unity as H$_2$S is more abundant than NH$_3$ in every retrieval. The absence of NH$_3$ above 50 bar can be explained either by partial dissolution of NH$_3$ in an ionic ocean at GPa pressures or by a planet formation scenario in which hydrated clathrates preferentially delivered sulfur rather than nitrogen onto planetesimals, or a combination of these hypotheses.