Probing the Depths of Neptune

The authors present spatially resolved (0.1-1.0 arcsec) radio maps of Neptune acquired with the Jansky Very Large Array and the 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 K to 40 K brighter than the mid-latitudes and northern equatorial region. The authors 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^+18.9_−13.4 x and 3.9^+2.1_−3.1 x 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. These 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.

Image Caption: Examples of  VLA and ALMA images of Neptune.

Publication: Joshua Tollefson (University of California, Berkeley) et al., Neptune's Spatial Brightness Temperature Variations from the VLA and ALMA, 2021, The Planetary Science Journal, 2, Issue 3, L105.