Abstract
Saturn’s moon Titan undergoes a long annual cycle of 29.45 Earth years. Titan’s northern winter and spring were investigated in detail by the Cassini–Huygens spacecraft (2004–2017), but the northern summer season remains sparsely studied. Here we present new observations from the James Webb Space Telescope (JWST) and Keck II telescope made in 2022 and 2023 during Titan’s late northern summer. Using JWST’s mid-infrared instrument, we spectroscopically detected the methyl radical, the primary product of methane break-up and key to the formation of ethane and heavier molecules. Using the near-infrared spectrograph onboard JWST, we detected several non-local thermodynamic equilibrium CO and CO2 emission bands, which allowed us to measure these species over a wide altitude range. Lastly, using the near-infrared camera onboard JWST and Keck II, we imaged northern hemisphere tropospheric clouds evolving in altitude, which provided new insights and constraints on seasonal convection patterns. These observations pave the way for new observations and modelling of Titan’s climate and meteorology as it progresses through the northern fall equinox, when its atmosphere is expected to show notable seasonal changes.
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Data availability
JWST observational data are accessible from the Barbara A. Mikulski Archive for Space Telescopes (https://mast.stsci.edu). All Keck data, including the Twilight Zone data, are made public after 18 months, and can be retrieved from the Keck Observatory Archive (https://koa.ipac.caltech.edu/UserGuide/about.html).
Code availability
The custom pipeline and data processing code104 used in this study is available from https://doi.org/10.3847/2515-5172/ad045f. NIRC2 distortion files and IDL software to correct the images for distortion can be downloaded from https://www2.keck.hawaii.edu/inst/nirc2/dewarp.html. General software to reduce Keck NIRC2 (Twilight Zone) data can be obtained from https://nirc2-reduce.readthedocs.io/en/latest/. The NEMESIS modelling software used for MIRI analysis is fully described in ref. 60 and available from GitHub (https://github.com/nemesiscode/radtrancode). The radiative transfer code used to generate the NIRSpec LTE spectra is available from B.B. upon reasonable request. The KOPRA radiative transfer code and the GRANADA non-LTE code used in the analysis of the CO2 and CO emissions are available from M.L.-P. upon request.
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Acknowledgements
C.A.N. was funded for this work by JWST Archive Research Project 02524. B.B., E.L., P.R., S.R. and M.E.-S. acknowledge support from the Programme National de Planétologie of CNRS-INSU co-funded by CNES. S.R. also acknowledges financial support from the CNES and the French National Research Agency (grant nos. ANR-21-CE49-0020-04/RAD3-NET and ANR-23-CE56-0008/EOLE). M.L.-P. acknowledges financial support from the Agencia Estatal de Investigación, MCIN/AEI/10.13039/501100011033 (grant nos. PID2022-141216NB-I00 and CEX2021-001131-S). N.A.T. was funded by the UK Science and Technology Facilities Council (grant no. ST/Y000676/1). N.A.L. and J.M.L. were funded by NASA CDAP (grant no. 80NSSC20K0483). H.B.H. and S.N.M. acknowledge support from NASA JWST Interdisciplinary Scientist grant 21-SMDSS21-0013. H.M. was supported by the STFC James Webb Fellowship (ST/W001527/2) at Northumbria University. LNF, ORTK and MTR were supported by STFC Consolidated Grant reference ST/W00089X/1. For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) licence to the Author Accepted Manuscript version arising from this submission. A portion of this work used the ALICE high performance computing facility at the University of Leicester. This work is based in part on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programme 1251. We thank the following staff at the Space Telescope Institute for support with the execution of the JWST observations: K. Murray, B. Hilbert, G. Wahlgren and B. Porterfield. Some of the data presented in this paper were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. Observing time for this project was allocated by all three institutions, in part thanks to the Twilight Zone Program. NASA Keck time is administered by the NASA Exoplanet Science Institute. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We recognize and acknowledge the important cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have had the opportunity to conduct observations from this mountain.
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Contributions
C.A.N. was the principal investigator of the JWST guaranteed time observation (GTO) 1251 analysed in this paper, led the team in analysing the data and formulating the results and conclusions, and was also the lead author of this article, writing and editing key portions of the final paper. B.B., T.C., B.P.C., I.d.P., M.E.-S., H.B.H., E.L., N.A.L., M.L.-P., J.M.L., P.R., S.R., N.A.T. and E.P.T. contributed substantially to the data analysis and writing of the paper. H.B.H. was the lead scientist of the Solar System GTO programme, encompassing GTO 1251 (Titan), and contributed to the observing proposal and interpretation of the results. B.B., E.L. and M.L.-P. were members of the GTO proposal team and also jointly modelled the NIRSpec data (Fig. 3). Y.N.-P. worked on non-LTE modelling of NIRSpec data (Fig. 3). T.C. was a member of the GTO 1251 observing team and was involved in the calibration, quality control and navigation of the returned data. N.A.T. was a member of the GTO 1251 proposal team and analysed and modelled the MIRI MRS data (Fig. 2), with modelling code support from P.G.J.I. B.P.C. and M.E.-S. calibrated and plotted the spectral data from NIRSpec and MIRI (Fig. 1) and assessed the noise level. S.R., C. Sotin, E.P.T. and R.A.W. were members of the GTO 1251 proposal team and were involved with the interpretation of the NIRCam data. S.R. and J.S. made images of the NIRCam and NIRC2 data (Figs. 4 and 5). S.N.M. and J.A.S. provided guidance on the formulation of the observing proposal GTO 1251, including observational parameters. R.K.A. and A.G.H. contributed to the formulation of the observing proposal GTO 1251, were co-investigators on the proposal and contributed to the discussion of the results in this paper. Technical advice on the observational settings was provided by J.A.S. N.R.-G., B.J.H., O.R.T.K., L.N.F. and M.T.R. contributed to the calibration of the MIRI spectra, especially the non-standard pipeline processing to improve the calibration of bright or extended objects. S.C.R.R., N.W.K., J.M.L. and N.A.L. contributed to interpreting the Titan cloud patterns and the comparison to Titan climate models. In addition, J.M.L. made Extended Data Fig. 7. P.R. modelled the light penetration depth in different NIRCam and NIRC filters and made Fig. 6. I.d.P., K.d.K. and A.G.D. wrote competing proposals to their three institutions for the Twilight Zone Program, which provided some of the Keck data. I.d.P. observed, reduced and calibrated the Keck data with the assistance of T.C.M. and E.M.M. and contributed to the technical sections of the paper. J.O’M. and C.J. assisted with the scheduling of additional Keck observations. C.A., G.D. and A.R. provided observing support. J.O’D., C. Schmidt, H.M., L.M. and T.S.S. wrote a competed observing proposal for Keck time that provided some observing time for this project.
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Extended data
Extended Data Fig. 1 Seasons on Titan seen by various spacecraft missions.
Extended observations by Cassini (June 2004 to September 2017) may be compared to JWST (from November 2022). The early JWST observations probe a much different season to Cassini (late northern summer), last seen in the early 1990s.
Extended Data Fig. 2 MIRI spectroscopic modeling inputs for the CH3 analysis.
(a) Nominal and end-member temperature profiles and CH3 profile. Stratospheric and mesospheric temperatures are based on Cassini CIRS temperature inversions from 2008.8 (half a Titan year earlier for seasonal correspondence in the latitude range ± 45∘N). Thermospheric temperatures are highly variable and end-member profiles are based on low latitude results from Cassini UVIS and INMS, the Huygens probe entry profile at 15∘S, and ALMA equatorial observations. b) non-LTE emission ratio for the range of k1 values and temperatures considered. The grey region shows the uncertainty in the non-LTE effect and the vertical dotted line shows the LTE case for comparison. (c–f) Normalised contribution functions (dR/dx where R is radiance and x is log(VMR) for the LTE and non-LTE cases). The non-LTE effect suppresses the thermospheric emission peak so most emission is from the stratopause region in the nominal temperature and k1=7 × 10−13 cm3 s−1 case. Note the slightly negative contribution functions in the thermosphere originate from weak CH3 absorption due to the high abundance, combined with negligible non-LTE emission at those altitudes.
Extended Data Fig. 3 Nominal kinetic temperature profile and vibrational temperature profiles for CO and CO2.
Left: nominal kinetic temperature (Tk, black) profile and vibrational temperature profiles for the fundamental bands of the CO isotopologues (colors) computed by using the non-LTE model. Right: the vibrational temperatures of the higher energy levels, CO(2) and CO(3), and also that of CO(2) calculated by Fabiano et al.30.
Extended Data Fig. 4 Contribution to the nadir radiances for the different bands of CO and the ν3 fundamental band of CO2.
CO bands: FB = Fundamental Band; FH = First Hot Band; ISO = Isotopic (13CO(1 → 0) and C18O(1 → 0)) Bands; SH = Second Hot Band. Contribution function amplitude indicates the atmospheric regions where emission is arising. The radiances have been integrated in the 4.45–5.00 μm spectral interval for the CO bands and in the 4.20–4.35 μm range for CO2.
Extended Data Fig. 5 Penetration depth of the direct and scattered flux for NIRSpec as a function of wavelength.
Calculations at three emission angles: θe = 0∘ (nadir viewing, green), θe = 44∘ (red) and θe = 90∘ (limb viewing, blue).
Extended Data Fig. 6 One-way transmission through Titan’s atmosphere.
Calculations at emission angles θe = 44∘ (left) and at the limb θe = 90∘ (right) for light at different wavelengths probed by JWST NIRCam and Keck NIRC2. JWST filters (wavelengths): F165N (1.65 μm); F187N (1.87 μm); F212N (≃ 2.12 μm). Keck NIRC2 filters (wavelengths): Brγ filter (2.157 μm); H2 1–0 (≃ 2.12 μm). The dots indicate for each filter and viewing orientation the altitude where the effective transmission reaches e−1, and on each curve, the crosses indicate the altitude where the one-way effective transmission reaches 90% (upper cross) and 10% (lower cross). The latter altitude is also used as the threshold altitude of detection.
Extended Data Fig. 7 Seasonal distribution of methane clouds in Titan’s troposphere.
(a) Latitudinal distribution of clouds observed from Cassini and ground-based observatories6,54, overlain on the zonal-mean precipitation distribution averaged over 40 Titan years of a simulation with TAM57. Vertical dotted lines indicate the timing of solstices (northern winter solstice and northern summer solstice, NWS and NSS) and equinoxes (northern vernal equinox and northern autumnal equinox, NVE and NAE); thin contours denote isolines of diurnally averaged top-of-atmosphere insolation, and grey shaded regions denote latitudes and times of polar night. (b) A simplified illustration of the expected atmospheric circulation during late northern summer leading to upwelling at northern mid-latitudes, which promotes cloud formation there, as observed by JWST and Keck most recently (see Figs. 4 and 5).
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Data processing details, Figs. 1–4 and Tables 1–5.
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Nixon, C.A., Bézard, B., Cornet, T. et al. The atmosphere of Titan in late northern summer from JWST and Keck observations. Nat Astron 9, 969–981 (2025). https://doi.org/10.1038/s41550-025-02537-3
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DOI: https://doi.org/10.1038/s41550-025-02537-3
