Methyl Detections in the Spitzer Spectra of Neptune and Uranus

The methyl radical (CH₃) is a key intermediate species that is formed during the process that recombines photodissociated methane into ethane (C₂H₆), one of the most abundant complex hydrocarbons in Ice Giant atmospheres. Understanding methyl is crucial to understanding the chemical processes that dominate the stratospheres of these planets. Observations of CH₃ can also be used to constrain the location of the homopause, an important variable in atmospheric chemical models.

The Spitzer Space Telescope Infrared Spectrometer observed Uranus and Neptune multiple times between 2004 and 2007 in the mid infrared between 5 and 37 microns. The best quality Neptune data from 2005 have never been investigated until now. CH₃ has been detected in previous epochs of Spitzer data. We have modelled it using the NEMESIS optimal estimation retrieval algorithm (Irwin et al., 2008: 10.1016/j.jqsrt.2007.11.006) and successfully fit the main double feature at 16.5 microns. We have found that multiple, previously unknown features identified in the 15–19 micron band can also be explained by this radical.

The Uranus data from 2007 have been used for multiple investigations but the methyl radical has only been tentatively detected (Orton et al., 2014: 10.1016/j.icarus.2014.07.012). We used the same methodology used for Neptune at Uranus and have found the main 16.5-micron band of CH₃ is present. This is the first time the methyl radical has been definitively detected in the atmosphere of Uranus at these wavelengths.

We have used the photochemical models from Moses et al. (2005: 10.1029/2005JE002411) as priors and allowed the model to scale to the data. For Neptune, results suggest a stratospheric abundance of CH₃ of twice that predicted by photochemical modeling. Results for Uranus remain poorly constrained given much lower signal to noise consistent with significantly lower abundances.

With the JWST scheduled to observe both ice giants in the next year (including Uranus observations in August/September 2022), we discuss how JWST data will be used to refine these measurements. In particular, the MIRI instrument will be able to take global maps of the planets in the mid infrared and show spectroscopy at much higher spectral resolution than Spitzer. It will also have higher sensitivity and therefore better signal to noise. This will mean that we can further constrain the upper limits of CH₃, as well as show its global distribution.