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

Journal article

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.

Authors: Rowe-Gurney, N; Fletcher, LN; Orton, GS; Roman, MT; Mainzer, A

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Elsevier
• Journal: Icarus
• Volume: 365
• Article number: 114506
• Publication date: 2021-05-03
• Acceptance date: 2021-04-26
• DOI: 10.1016/j.icarus.2021.114506
• EISSN: 1090-2643
• ISSN: 0019-1035
• Language: English
• Keywords: FFR; Uranus; stratosphere; retrieval theory; composition; radiative transfer; atmosphere