Temperature Structure, Chemistry, and Clouds in the Atmosphere of Neptune Revealed by JWST

Abstract

We present observations and analysis of Neptune’s atmosphere from JWST, providing new constraints on hydrocarbon abundances, cloud properties, and temperature structure across the planet’s disk.  Spatially-resolved spectra from JWST NIRSpec (1.6–5.2 µm) and MIRI (4.9–28.5 µm) were acquired in June 2023 and amount to the most comprehensive infrared observations Neptune’s atmosphere since Voyager 2.  We compare these observations and results to similar observations of Uranus made six months prior.From the ground, spatially resolved observations of Neptune’s mid-infrared emission are limited to imaging targeting the brighter regions of the infrared spectrum (i.e. 8-µm emission from stratospheric methane, 12-µm emission from stratospheric ethane, and 17-25 µm thermal emission from the hydrogen continuum). From space, Voyager provided infrared spectroscopy of Neptune at close proximity in 1989 (after similarly observing Uranus 3 years earlier), but lacked the sensitivity needed to adequately measure mid-infrared emission from stratospheric hydrocarbons.  Between 2004 and 2006, the Spitzer Space Telescope observed both planets' mid-infrared spectra between 7 and 36 µm, but Spitzer lacked the spatial resolution necessary to resolve potential thermal and chemical structure across the disk.Now, with its exceptional sensitivity and outstanding spatial and spectral resolution, JWST reveals Neptune's stratospheric temperature and chemistry with exquisite new detail, placing new constraints on hydrocarbon abundances, cloud properties, and temperature structure across the disk. In this talk, we introduce these new data along with results of an initial radiative transfer analysis.  We briefly compare and contrast these finding with those of our recent similar analysis of Uranus.With a projected lifetime of over a decade, JWST promises to continue providing exciting new insights into the atmospheric structure, composition, and variability of the ice giants for years to come.