Uranus from JWST: First Results and Comparison to Ground-Based Thermal Imaging

We aim to present anticipated initial results from the James Webb Space Telescope (JWST) observations of Uranus, which will provide the first spatially resolved, infrared spectra of the planet's atmosphere spanning from 1.66 to 28.6 µm. We evaluate these unprecedented JWST NIRSpec (1.66–3.05 µm, 2.87–5.14 µm) and MIRI (4.9-28.6 μm) spectra in the context of existing observations and open questions concerning Uranus' stratospheric chemistry and thermal structure [1].

Owing to its frigid atmospheric temperatures, Uranus' infrared spectrum is extremely weak. Much of the spectrum has never been spatially resolved, while some has never been clearly observed at all.

From the ground, spatially resolved observations of Uranus' mid-infrared emission are limited to imaging observations targeting the brighter regions of the infrared spectrum (i.e. ~13 µm emission from stratospheric acetylene, and 17-25 µm from the H2 continuum). Images from the Very Large Telescope VISIR instrument at 13 µm show a stratospheric structure distinct to Uranus, with elevated radiance at high latitudes. The physical nature of this structure–-whether produced by chemical or thermal gradients–-is unclear given previously available data [1].

From space, the Spitzer Space Telescope observed Uranus' mid-infrared spectrum between ~7 and 36 µm, but it lacked the spatial resolution necessary to resolve potential thermal and chemical structure across the disk [2].

Now, with its exceptional sensitivity and outstanding spatial and spectral resolution, JWST can reveal Uranus' stratospheric temperature and chemistry with exquisite detail. JWST will observe Uranus between 2022-08-05 and 2022-09-26, supported by observations from the Hubble Space Telescope and ground-based observatories. These data will place new constraints on hydrocarbon abundances and temperature structure across the disk, and determine the nature of Uranus' puzzling stratospheric structure for the first time.

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.

[1] Roman, M.T, et al. "Uranus in northern..." AJ 159.2 (2020): 45.

[2] Rowe-Gurney, N., et al. "Longitudinal variations..." Icarus 365 (2021): 114506.