2019AJ....157..251T

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 H₂S, CH₄, and ortho-/para-H₂ 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 H₂S profile that range from sub- to supersaturations at altitudes above the 10 bar pressure level, while variations in CH₄ improve the quality of fit near the equator. At the south polar cap, our best-fit model has a depleted deep atmospheric abundance of H₂S from 30 to only 1.5 times the protosolar value, while simultaneously depleting the CH₄ abundance. This pattern of enhancement and depletion of condensible species is consistent with a global circulation structure where enriched air rises at the midlatitudes (32°-12°S) and north of the equator (2°-20°N), and dry air descends at the poles (90°-66°S) and just south of the equator (12°S-2°N). Our analysis finds more complex structure near the equator than accounted for in previous circulation models.