Methyl Radical Detected on Titan with JWST/MIRI

Saturn’s largest moon Titan has a nitrogen-methane atmosphere and a rich organic photochemistry. Dissociation of Titan’s molecular methane and nitrogen into N and methyl (CH₃) radicals forms the basis of this photochemistry and results in a vast array of hydrocarbon and nitrile species. The abundance of CH₃ is thus of critical importance to understanding Titan’s atmospheric chemistry. CH₃ is predicted by photochemical models and must be present to explain Titan’s trace gas composition, but has never been directly observed. Cassini’s mass spectrometer was unable to make a detection as the extreme reactivity of radicals results in reactions on the instrument wall (e.g. recombination with H) before detection is possible. Emission features in the infra-red are also very weak, so detection from remote-sensing spectroscopy has previously not been possible. Here we use the very high sensitivity of the James Webb Space Telescope’s (JWST) Mid-InfraRed Instrument (MIRI) to detect emission from CH₃ at 16.5 microns. We have used this to validate model predictions that underpin Titan’s rich atmospheric chemistry.

JWST/MIRI observations were taken in Medium Resolution Spectroscopy (MRS) mode on 11^th July 2023 as part of Guaranteed Time Observation programme 1251 [Nixon et al., 2025]. Observations were reduced using the standard pipeline and combined to give a disc-averaged spectrum (Fig 1). The observed spectrum was compared to a forward model generated with a reference Titan atmosphere using the NEMESIS radiative transfer suite [Irwin et al., 2008]. The reference atmospheric temperature profile was based on observation from Cassini half a Titan year previous, augmented with ground-based measurements from ALMA and in-situ measurements from the Huygens probe (Fig 2a). A baseline atmospheric composition was compiled from Cassini/Huygens measurements [Teanby et al., 2019]. For the CH₃ profile, in the absence of measurements, we used the predicted abundance from a photochemical model [Vuitton et al., 2019] (Fig 2a).

The abundance profile of CH₃ is expected to be extremely steep with very high fractional abundances in the thermosphere (100 ppm at 1000km) and much lower abundances in the stratosphere and mesosphere (1 ppb at 300km). Peak emission under conditions of local thermodynamic equilibrium should originate from the mid-thermosphere at an altitude of ~800km (Fig 2b). However, our analysis shows that non-local thermodynamic equilibrium (non-LTE) emission is expected due to very low thermospheric pressures [Nixon et al., 2025]. This supresses emission below that expected from the Planck function and reduces infra-red emission from thermospheric CH₃ to negligible levels. When non-LTE effects are considered, we find that the emission instead originates from the stratopause region (~300km) where CH₃ abundances are predicted to be around 1 ppb (Fig 2c).

Agreement between forward modelled non-LTE emission using the photochemical model profile and the JWST/MIRI observation match very well (Fig 1) – confirming the model predicted abundances are consistent with conditions in Titan’s middle atmosphere. Our initial results were presented in Nixon et al., (2025). Here we present an updated analysis using improved pipeline processing, more in-depth treatment of the disc-averaged nature of the observation, and provide formal limits on the CH₃ abundance profiles. The consistency of our results with predictions from photochemical models gives confidence to current chemical schemes for Titan’s low-order chemistry, which provides a sound basis for a deeper analysis of Titan’s more exotic species such as high-order hydrocarbons and poly-aromatic hydrocarbons.

References

Irwin, P.G.J., et al., 2008. The NEMESIS planetary atmosphere radiative transfer and retrieval tool. Journal of Quantitative Spectroscopy and Radiative Transfer 109, 1136–1150.

Nixon, C.A., et al., 2025., Titan’s Atmosphere in Late Northern Summer from JWST and Keck Observations. Nature Astronomy, in press.

Teanby, N.A., et al., 2019. Seasonal Evolution of Titan’s Stratosphere During the Cassini Mission. Geophysical Research Letters 46, 3079–3089.

Vuitton, V., et al., 2019. Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model. Icarus 324, 120–197.

Fig 1: JWST/MIRI disc-average spectrum compared with forward models with and without CH₃. The model including CH₃ provides a much better fit to the observations.

Fig 2: (a) Titan’s atmospheric temperature structure and uncertainty envelope from Nixon et al. (2025), along with photochemical model prediction of the CH₃ profile from Vuitton et al. (2019). (b) Contribution functions for LTE case with nominal temperature profile (green), hot temperature limit (red) and cold temperature limit (blue). For LTE, peak emission would be from the thermosphere at ~800km, but this is not realistic. (c) Contribution functions for a more realistic non-LTE emission case peak at ~300km around the mesopause as non-LTE effects suppress emission at very low pressures. Our observations are thus most sensitive to abundances around the stratopause.