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

Joshua Tollefson,Statia Luszcz-Cook,Bryan J Butler,Imke De Pater,Robert J Sault,David Deboer,Edward M Molter

doi:10.3847/psj/abf837

Abstract

Abstract We present spatially resolved (0.″1–1.″0) radio maps of Neptune taken from the Very Large Array and Atacama Large Millimeter/submillimeter Array between 2015 and 2017. Combined, these observations probe from just below the main methane cloud deck at ∼1 bar down to the NH4SH cloud at ∼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 H2S, NH3, and CH4 abundance profiles across the disk, though only strong constraints can be made for H2S. Below all cloud formation, the data are well fit by 53.8 − 13.4 + 18.9 × and 3.9 − 3.1 + 2.1 × protosolar enrichment in the H2S and NH3 abundances, respectively, assuming a dry adiabat. Models in which the radio-cold mid-latitudes and northern equatorial region are supersaturated in H2S are statistically favored over models following strict thermochemical equilibrium. H2S 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 H2S is more abundant than NH3 in every retrieval. The absence of NH3 above 50 bar can be explained either by partial dissolution of NH3 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.

Highlights

Neptune is the prototypical ‘ice giant’: a giant planet composed mainly of elements heavier than hydrogen and helium by mass, such as oxygen, nitrogen, carbon, and sulfur
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
We use radiative transfer modeling coupled to Markov Chain Monte Carlo methods to retrieve H2S, NH3, and CH4 abundance profiles across the disk, though only strong constraints can be made for H2S

Summary

Introduction

Neptune is the prototypical ‘ice giant’: a giant planet composed mainly of elements heavier than hydrogen and helium by mass, such as oxygen, nitrogen, carbon, and sulfur. While early radio observations could only obtain disk-averaged measurements of Neptune, the observed high brightness temperatures longward of 10 cm required NH3, a prominent microwave absorber, to be significantly depleted (de Pater & Massie 1985) This is possible if an NH4SH cloud forms at ∼ 50 bar and if the H2S abundance exceeds that of NH3, de Pater et al (2014) presented centimeter maps of Neptune from 2003, finding that the disk-averaged spectrum agreed with the abundances obtained from earlier radio observations. They found that the bright south polar cap must be significantly depleted in H2S down to ∼ 40 bar in order to match the observed brightness temperature at wavelengths of 0.7 − 6.0 cm. This study did not investigate brightness variations at other latitudes, as the sensitivity and resolution were not good enough to detect significant variations apart from those in the south polar cap

Results

Discussion

Conclusion