Astrochemistry

An Unidentified Absorption Feature At 5.11 μm On The Surface Of Titan And Pluto From JWST Spectroscopy

By Keith Cowing
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astro-ph.EP
June 12, 2026
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An Unidentified Absorption Feature At 5.11 μm On The Surface Of Titan And Pluto From JWST Spectroscopy
NIRSpec average spectrum of Titan (black) compared with a radiative transfer calculation in which the surface albedo decreases smoothly with wavelength beyond 4.9 µm (red). The blue line shows the difference between the observed and synthetic spectra, shifted by 0.005 for clarity. The 5.11-µm absorption is clearly missing in the model. The small mismatch below 4.93 µm is due to a component of the non-LTE CO emission not included in the model. — astro-ph.EP

Titan possesses a thick N2-CH4 atmosphere that makes it difficult to study its surface spectroscopically. The chemical composition of the solid surface of Titan thus remains very uncertain.

By leveraging JWST’s high sensitivity and large spectral coverage, we searched for any signature from Titan’s surface in the broad and less explored 5-μm atmospheric window. We also investigated the JWST spectrum of Pluto which has a thin Titan-like atmosphere.

We made selections of JWST NIRSpec and MIRI spectra around Titan’s disk center and compared the NIRSpec average spectrum with a radiative transfer model including gas and haze opacity.

We detected an unidentified absorption in both NIRSpec and MIRI spectra of Titan centered at 5.113 μm (1956 cm−1) and 6-7% deep. The width of the feature is 0.024±0.0008 μm (9.2±0.3 cm−1) in the NIRSpec spectrum recorded on the trailing side and is possibly 25% narrower in the MIRI spectrum of the leading side.

This absorption most likely originates from the surface. We could not identify this signature among published laboratory spectra of ices relevant to Titan’s atmospheric compounds but present a few plausible candidates.

A 4-5% deep absorption is also present in the MIRI spectrum of Pluto but is about 3 times broader than on Titan’s trailing side.

B. Bézard, E. Lellouch, M. Camarca, J. I. Lunine, E. Quirico, C. A. Nixon, N. A. Teanby, P. Rannou, S. Rodriguez, M. Es-Sayeh, S. K. Trumbo, A. C. Souza-Feliciano, P. Lavvas, T. Bertrand, I. Wong, N. Pinilla-Alonso, G. L. Villanueva

Comments: Accepted for publication in Astronomy & Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2606.13350 [astro-ph.EP] (or arXiv:2606.13350v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2606.13350
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Submission history
From: Bruno Bézard
[v1] Thu, 11 Jun 2026 13:39:36 UTC (66 KB)
https://arxiv.org/abs/2606.13350
Astrobiology, Astrochemistry,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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