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

Introduction: NASA’s Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5-37 μm) disc-averaged spectra of Uranus very near its equinox over 21.7 hours on 16th to 17th of December 2007. A global-mean spectrum was constructed from observations of multiple longitudes, spaced equally around the planet, and have provided the opportunity for the most comprehensive globally averaged characterisation of Uranus’ temperature and composition ever obtained (Orton et al., 2014 a, b). In this work, we analyse the disc-averaged spectra at four separate longitudes to shed light on the discovery of longitudinal variability occurring in Uranus’ stratosphere during the 2007 equinox.

The composition and temperature structure of Uranus’ stratosphere is dominated by methane photolysis in the upper stratosphere (Moses et al., 2018). The complex hydrocarbons produced in these solar-driven reactions are the main trace gases present in the stratosphere and upper troposphere. These species are observable at mid-infrared wavelengths sensitive to altitudes between around one nanobar and two bars of pressure (Orton et al., 2014a).

Due to Uranus’ extremely high obliquity we can only clearly observe its longitudinal variation in disc-averaged observations close to its equinox. The northern spring equinox occurred in December 2007 with the aforementioned Spitzer observations occurring just 10 days after. The Spitzer data have been re-analysed using the most up to date pipeline available from NASA’s Spitzer Science Centre, resulting in minor changes over the previous reduction.

Longitudinal Variation: We assess the variations in discrete channels sensitive to different emission features. The radiances inside each interval are averaged and compared to the mean of all four longitudes. Each instrument module is exposed at a different time causing a spread of data points across the multiple longitudes displayed in Figure 1.

We detect a variability of up to 15% at stratospheric altitudes sensitive to the hydrocarbon species at around the 0.1-mbar pressure level. The tropospheric hydrogen-helium continuum and the monodeuterated methane that also arises from these deeper levels, both exhibit a negligible variation smaller than 2%, constraining the phenomenon to the stratosphere. Observations from Keck II NIRCII in December 2007 (Sromovsky et al., 2009; de Pater et al., 2011) and VLT/VISIR in 2009 (Roman et al. 2020) suggest possible links to these variations in the form of discrete meteorological features. In particular, Roman et al. (2020) identified discrete patches of brightness in 13-μm (acetylene) emission within a broad stratospheric band at mid-latitudes, which could be related to the variability observed by Spitzer.

Optimal Estimation Retrievals: 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 low-resolution spectra at each longitude to distinguish between thermal and compositional variability. The model suggests that variations can be explained solely by changes in stratospheric temperatures. A temperature change of less than 2 K is needed to model the observed variation. This is compounded by results from high-resolution forward models (primarily sounding the ethane and acetylene emission) constructed using the parameters retrieved from the low-resolution spectra.

The data were best reproduced by models with atmospheric mixing via eddy diffusion that was weaker than that assumed by Orton et al. but still within the confines of a realistic fit according to their model. An eddy diffusion coefficient value of 1020 cm²sec^-1 and a tropopause methane mole fraction of 8.0x10^-5 provides the best fit to the temperature structure and the methane vertical profile whilst also maintaining the closest chi-squared value for the spectral fit (Moses et al., 2018).

Conclusion: The longitudinal variation detected at Uranus during the 2007 equinox is an observed physical change in the stratosphere of the planet, most likely a temperature change associated with the band of bright stratospheric emission observed in ground-based images. The Spitzer IRS data can provide much detail but without accompanying spatial resolution it is impossible to come to a definitive conclusion as to the origins of the changes.

The James Webb Space Telescope, when it launches in 2021, will provide much improved spectral and spatial resolution needed in the mid-infrared band to provide answers to the causes of the observed variation.