Ice Giant Stratospheres from the Spitzer Space Telescope's Infrared Spectrometer

The ice giants are the least explored planets in the Solar System. Many questions regarding their entire planetary systems and their role in shaping the solar system remain

unanswered. NASA's Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5 { 37 ?m) disc-averaged spectra of Uranus and Neptune multiple times between 2004 and 2007. Analysing all the sets of data with multiple separate longitudes gives us a unique opportunity to compare the longitudinal variability in the thermal emission of the two planets for the first time. At Uranus, there was a considerable variation in stratospheric emission detected for multiple epochs. A variation is not present at Neptune for the majority of the observed epochs.

The observations of Uranus in 2007 and Neptune in 2005 were used to develop a consistent retrieval framework for ice giant middle atmospheres. Building on the forward-modelling analysis of the global average study of Uranus by Orton et al. (2014), and conducting novel analysis on the Neptune data, we present full optimal estimation inversions (using NEMESIS) of the spectra of both planets. At Uranus,

we perform spectral inversions for each longitude to distinguish between thermal and compositional variability, showing that longitudinal variations in stratospheric temperature are the cause of Uranus' rotational variability. At Neptune, we constrain the temperature profile and the abundances of the stratospheric hydrocarbons, including

the first retrieval of methyl (CH3), and a statistical study of the methane D/H ratio.

This disc-averaged thermal and chemical structure from Spitzer will likely be our best characterisation of ice giant thermal structure until the James Webb Space Telescope acquires spatially-resolved mid-infrared spectroscopy in 2022. We present the first stages of modelling the JWST observations of both planets and discuss the advancements that the observatory's guaranteed time observations will give with respect to Spitzer.