Neptune's Spatial Brightness Temperature Variations from the VLA and ALMA

Joshua Tollefson; Imke de Pater; Edward M. Molter; Robert J. Sault; Bryan J. Butler; Statia Luszcz-Cook; David DeBoer

paper

Neptune's Spatial Brightness Temperature Variations from the VLA and ALMA

arXiv:2104.06554

Abstract

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 $\sim 50$ 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 $53. 8_{- 13.4}^{+ 18.9} \times$ and $3. 9_{- 3.1}^{+ 2.1} \times$ 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.