4.5 Article

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

Journal

ICARUS
Volume 365, Issue -, Pages -

Publisher

ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.icarus.2021.114506

Keywords

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

Funding

  1. European Research Council Consolidator Grant (European Union) at the University of Leicester [723890]
  2. NASA [NNX16AK14G]
  3. NASA Solar System Workings grant [80NSSC19K0536]
  4. NASA
  5. European Research Council (ERC) [723890] Funding Source: European Research Council (ERC)

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The Spitzer Infrared Spectrometer acquired mid-infrared spectra of Uranus near its equinox in 2007, revealing significant thermal and compositional variability in its stratosphere. Longitudinal analysis showed up to 15% variability in gases sensitive to certain wavelengths at stratospheric levels. Optimal estimation inversions suggested that these variations can be explained by a temperature change of less than 3 K in the stratosphere, with potential sources localized to large scale uplift or stratospheric wave phenomena.
NASA's Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5-37 mu m) 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 similar to 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.

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