[astro-ph.EP]. Interstellar objects are interlopers from other planetary systems, and their volatile compositions provide a glimpse into planet formation around their host star.
We present near-infrared spectra of the coma of interstellar object 3I/ATLAS measured with the James Webb Space Telescope. Our results demonstrate an unexpectedly high D/H = (3.33±0.31)% for methane and represent an exceedingly rare detection of deuterated organic molecules in an interstellar object.
This D/H ratio exceeds any other value for methane measured in the solar system, and is a factor of 14±2 higher than that measured in comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft, the only other comet for which CH3D has been detected. Both 3I/ATLAS and 67P/Churyumov-Gerasimenko show a higher degree of methane dueteration compared to water, consistent with trends seen in other solar system bodies and the nearby interstellar medium, where deuteration in organics exceeds that of water by up to an order of magnitude.
The D/H ratio in methane itself is observationally unconstrained in extrasolar sources to date, but the enriched ratio in 3I/ATLAS is similar to those measured in methanol and formaldehyde toward primitive environments. The elevated D/H ratio for methane in 3I/ATLAS is consistent with the hypothesis that its ices formed in a cold, low metallicity interstellar or protostellar environment under a high cosmic ray irradiation rate (Cordiner et al. 2026).
D/H ratios in solar system and extrasolar sources. Ratios are for methane except when noted otherwise. Sources for which the D/H ratio in c−C3H2 was used to estimate the D/H in CH4 are denoted with a ⋆. References and values for each object are given in Extended Data Table 1. — [astro-ph.EP]
Nathan X. Roth, Martin Cordiner, Stefanie Milam, Geronimo Villanueva, Steven Charnley, Nicolas Biver, Dominique Bockelee-Morvan, Dennis Bodewits, Jacques Crovisier, Maria N. Drozdovskaya, Davide Farnocchia, Kenji Furuya, Michael S.P. Kelley, Marco Micheli, John W. Noonan, Cyrielle Opitom, Megan E. Schwamb, Cristina A. Thomas
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2603.20445 [astro-ph.EP] (or arXiv:2603.20445v2 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.20445
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Submission history
From: Nathan Roth
[v1] Fri, 20 Mar 2026 19:20:31 UTC (287 KB)
[v2] Mon, 10 Aug 2026 18:02:44 UTC (289 KB)
https://arxiv.org/abs/2603.20445
Astrobiology, Astrochemistry,
