Observations with the James Webb Space Telescope have revealed a distinctive chemical signature in interstellar comet 3I/ATLAS that may help explain where its frozen material formed. A research team led by Nathan Roth of American University found that methane in the comet is unusually enriched in deuterium, a heavier form of hydrogen.
Webb observed the comet on December 22 and 23, 2025, when it was about 2.4 times farther from the Sun than Earth. The telescope measured infrared light from the coma, the cloud of gas surrounding the comet’s icy nucleus, and detected a deuterium-to-hydrogen ratio of about 3.2% in its methane.
That ratio is around 13 times higher than the level measured in comet 67P/Churyumov–Gerasimenko, which is thought to have originated in our solar system. The result is especially useful for comparison because 67P is the only other comet in which deuterium-bearing methane has been detected. The findings were published in Nature Astronomy.
Deuterium has one more neutron in its nucleus than ordinary hydrogen. When it takes the place of hydrogen in a methane molecule, it changes the molecule’s infrared signature, allowing astronomers to distinguish between the two forms.
The methane’s deuterium-to-hydrogen ratio is also about three times higher than the value previously measured in water from 3I/ATLAS. The researchers suggest the methane may have formed later, after the chemical environment had more time to become enriched in deuterium.
The results are consistent with the possibility that 3I/ATLAS formed in a very cold cloud or near a young star, in a region with relatively few elements heavier than helium and intense cosmic radiation. Such high-energy particles can drive chemical reactions that enrich molecules with deuterium. The measurements offer clues to the comet’s formation, but do not identify its birthplace.
Sources: t3n.