Longitudinal variations in the stratosphere of Uranus from the Spitzer infrared spectrometer

Naomi Rowe-Gurney, Leigh N. Fletcher, Glenn S. Orton, Michael T. Roman, Amy Mainzer, Julianne I. Moses, Imke de Pater, Patrick G.J. Irwin

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


NASA's Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5–37 μm) disc-averaged spectra of Uranus very near to its equinox in December 2007. A mean spectrum was constructed from observations of multiple central meridian longitudes, spaced equally around the planet, which has provided the opportunity for the most comprehensive globally-averaged characterisation of Uranus' temperature and composition ever obtained (Orton et al., 2014a,b). In this work we analyse the disc-averaged spectra at four separate central meridian longitudes to reveal significant longitudinal variability in thermal emission occurring in Uranus' stratosphere during the 2007 equinox. We detect a variability of up to 15% at wavelengths sensitive to stratospheric methane, ethane and acetylene at the ~0.1-mbar level. The tropospheric hydrogen‑helium continuum and deuterated methane absorption exhibit a negligible variation (less than 2%), constraining the phenomenon to the stratosphere. Building on the forward-modelling analysis of the global average study, we present full optimal estimation inversions (using the NEMESIS retrieval algorithm, Irwin et al., 2008) of the Uranus-2007 spectra at each longitude to distinguish between thermal and compositional variability. We found that the variations can be explained by a temperature change of less than 3 K in the stratosphere. Near-infrared observations from Keck II NIRC2 in December 2007 (Sromovsky et al., 2009; de Pater et al., 2011), and mid-infrared observations from VLT/VISIR in 2009 (Roman et al., 2020), help to localise the potential sources to either large scale uplift or stratospheric wave phenomena.

Original languageEnglish (US)
Article number114506
StatePublished - Sep 1 2021


  • Atmosphere
  • Composition
  • Radiative transfer
  • Retrieval theory
  • Stratosphere
  • Uranus

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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