Longitudinal Variations in the Stratosphere of Uranus from the Spitzer Infrared Spectrometer

TL;DR

This study analyzes mid-infrared spectra of Uranus taken near its equinox to identify significant longitudinal variability in the stratosphere's temperature and composition, revealing up to 15% variability in thermal emission at specific wavelengths.

Contribution

It presents the first detailed analysis of longitudinal variability in Uranus' stratosphere using Spitzer spectra, distinguishing between thermal and compositional changes with advanced retrieval methods.

Findings

Detected up to 15% variability in stratospheric thermal emission.

Variations are primarily due to temperature changes less than 3 K.

Stratospheric phenomena like uplift or waves are potential sources.

Abstract

NASA's Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5-37 microns) 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., 2014 a [arXiv:1407.2120], b [arXiv:1407.2118]). 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 [10.1016/j.jqsrt.2007.11.006]) 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 [arXiv:1503.01957], de Pater et al., 2011 [10.1016/j.icarus.2011.06.022]), and mid-infrared observations from VLT/VISIR in 2009 (Roman et al., 2020 [arXiv:1911.12830]), help to localise the potential sources to either large scale uplift or stratospheric wave phenomena.