An enriched methane D/H ratio in the interstellar object 3I/ATLAS
Nathan X. Roth, Martin Cordiner, Stefanie Milam, Geronimo Villanueva, Steven Charnley, Nicolas Biver, Dominique Bockelée-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
Nature Astronomy, Published: 23 October 2026
“Interstellar objects are interlopers from other planetary systems, and their volatile compositions provide a glimpse into planet formation around their host star. Here 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 = (3.18 ± 0.30)% 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 13 ± 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 deuteration compared with 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 like those measured in methanol and formaldehyde towards pre- and protostellar 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.”
