An unidentified absorption feature at 5.11 μm on the surfaces of Titan and Pluto from JWST spectroscopy

¹ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
² Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁴ Université Grenoble Alpes, CNRS, IPAG, 38041 Grenoble, France
⁵ Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK
⁷ LEATP, Campus Sciences Exactes et Naturelles – BP 1039, Université de Reims Champagne-Ardenne, CNRS, 51687 Reims, France
⁸ Université Paris Cité, Institut de Physique du Globe de Paris (IPGP), CNRS, 75005 Paris, France
⁹ CNRM, Météo-France, CNRS, Université de Toulouse, Toulouse, France
¹⁰ Department of Astronomy & Astrophysics, University of California, San Diego, La Jolla, CA 92093, USA
¹¹ Florida Space Institute, University of Central Florida, Orlando, FL 32826, USA
¹² Institut d’Astrophysique de Paris, Sorbonne Université, CNRS, UMR7095, Paris, France
¹³ Space Telescope Science Institute, Baltimore, MD, USA
¹⁴ Institute of Space Sciences and Technologies of Asturias (ICTEA), University of Oviedo, 33004 Oviedo, Spain
¹⁵ Department of Physics, University of Oviedo, 33007 Oviedo, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 May 2026
Accepted: 9 June 2026

Abstract

Context. Titan possesses a thick N₂–CH₄ atmosphere that makes its surface difficult to study spectroscopically. The chemical composition of Titan’s solid surface therefore remains highly uncertain.

Aims. By leveraging JWST’s high sensitivity and large spectral coverage, we searched for signatures 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.

Methods. We selected JWST NIRSpec and MIRI spectra around Titan’s disk center and compared the average NIRSpec spectrum with a radiative transfer model that includes gas and haze opacity.

Results. We detect an unidentified absorption in both the 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 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 we present a few plausible candidates. A 4–5% deep absorption is also present in the MIRI spectrum of Pluto, but is about three times broader than on Titan’s trailing side.