Neptune's Latitudinal Variations as Viewed with ALMA

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.