Temperature, Composition, and Cloud structure in Atmosphere of Uranus from MIRI-MRS and NIRSpec-IFU Spectra
(2025)
Authors:
Michael Roman, Leigh Fletcher, Heidi Hammel, Patrick Irwin, Oliver King, Naomi Rowe-Gurney, Julianne Moses, Glenn Orton, Imke de Pater, Henrik Melin, Jake Harkett, Matthew Hedman, Simon Toogood, Stefanie Milam
Abstract:
Introduction: Due to Uranus’ weak thermal radiance, the thermal and compositional structures of its atmosphere have remained poorly characterised. Here, using the unprecedented sensitivity of JWST's MIRI and NIRSpec instruments, we present an analysis of Uranus' spatially resolved spectrum spanning the near- and mid-infrared, revealing how temperatures, composition, and clouds vary across the planet's northern hemisphere.Observations: JWST observed Uranus on 8--9 January 2023 (program1248) as part of the Solar System Guaranteed Time Observations (GTO). Integral field spectroscopy (IFS) with the Near-Infrared Spectrograph (NIRSpec) and the Mid-Infrared Instrument/Medium Resolution Spectrometer (MIRI/MRS) were combined to provide nearly simultaneous and continuous spatial and spectral data between 1.66 and 28.70 microns.Temperatures: The nearly continuous spectral coverage offered by the combination of NIRSpec and MIRI provide constraints on the temperature structure from the stratosphere down to several bars. The average temperature-pressure vertical profile is largely consistent with that determined from Spitzer [1], but the spatially resolved JWST reveal how these temperatures vary with latitude in the stratosphere and cloud layer for the first time [2]. They also suggest the possibility of a sub-adiabatic cloud layer.Chemistry: Our radiative transfer analysis of MIRI-MRS spectra 1) provide new constraints on minor species in Uranus’ stratosphere and 2) reveals how various hydrocarbons vary as a function of latitude. The observed distributions are indicative of a combination of seasonal photochemistry [3] and dynamical processes, as we will briefly discuss.Clouds and hazes: Finally, we briefly examine the vertical cloud structure and its latitudinal variation as sensed by NIRSpec data. The data reveal the opacity of Uranus clouds and hazes spanning the transition from scattered sunlight to thermal emission for the first time. The overall vertical structure suggested by these new data largely agrees with that of prior work [3,4,5], but a comparison between observed and model spectra reveal interesting discrepancies and possibly a need for additional sources of opacity. [1] Orton, G.S., Fletcher, L.N., Moses, J.I., Mainzer, A.K., Hines, D., Hammel, H.B., Martin-Torres, F.J., Burgdorf, M., Merlet, C., Line, M.R.: Mid-infrared spectroscopy of uranus from the spitzer infrared spectrometer: 1. determination of the mean temperature structure of the upper troposphere and stratosphere. Icarus 243, 494–513 (2014)[2] Roman, M.T., Fletcher, L.N., Orton, G.S., Rowe-Gurney, N., Irwin, P.G.: Uranus in northern midspring: persistent atmospheric temperatures and circulations inferred from thermal imaging. The Astronomical Journal 159(2), 45 (2020)[3] Moses, J.I., Fletcher, L.N., Greathouse, T.K., Orton, G.S., Hue, V.: Seasonal stratospheric photochemistry on uranus and neptune. Icarus 307, 124–145 (2018)[4] Sromovsky, L.A., Karkoschka, E., Fry, P.M., Pater, I., Hammel, H.B.: The methane distribution and polar brightening on uranus based on hst/stis, keck-nirc2, and irtf/spex observations through 2015. Icarus 317, 266–306 (2019)189[5] Irwin, P.G., Teanby, N.A., Fletcher, L.N., Toledo, D., Orton, G.S., Wong, M.H.,Roman, M.T., Perez-Hoyos, S., James, A., Dobinson, J.: Hazy blue worlds:A holistic aerosol model for uranus and neptune, including dark spots[6] Roman, M.T., Banfield, D., Gierasch, P.J.: Aerosols and methane in the ice giant atmospheres inferred from spatially resolved, near-infrared spectra: I. uranus, 2001–2007. Icarus 310, 54–76 (2018)