Neptune's Latitudinal Variations as Viewed with ALMA

Abstract

We present spatially resolved millimeter maps of Neptune between 95 and 242 GHz taken with the Atacama Large Millimeter/submillimeter Array (ALMA) in $2016-2017$. The millimeter weighting functions peak between 1 and 10 bar on Neptune, lying in between the altitudes probed at visible/infrared and centimeter wavelengths. Thus, these observations provide important constraints on the atmospheric structure and dynamics of Neptune. We identify seven well-resolved latitudinal bands of discrete brightness temperature variations, on the order of $0.5-3$K in all three observed ALMA spectral bands. We model Neptune's brightness temperature using the radiative transfer code Radio-BEAR and compare how various H${}_2$S, CH${}_4$, and \textit{ortho/para} H${}_2$ abundance profiles can fit the observed temperature variations across the disk. We find that observed variations in brightness temperature with latitude can be explained by variations in the H${}_2$S profile that range from sub- to super-saturations at altitudes above the 10-bar pressure level, while variations in CH${}_4$ improve the quality of fit near the equator. At the south polar cap, our best fit model has a depleted deep atmospheric abundance of H${}_2$S from 30 to only 1.5 times the protosolar value, while simultaneously depleting the CH${}_4$ abundance. This pattern of enhancement and depletion of condensable species is consistent with a global circulation structure where enriched air rises at the mid-latitudes ($32^{\circ }-12^{\circ }$S) and north of the equator ($2^{\circ }-20^{\circ }$N), and dry air descends at the poles ($90^{\circ }-66^{\circ }$S) and just south of the equator ($12^{\circ }$S$-2^{\circ }$N). Our analysis finds more complex structure near the equator than accounted for in previous circulation models.

21 pages, 17 Figures, accepted into the Astronomical Journal May, 2019

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