Interstellar comet 3I/ATLAS has brought into the Solar System a chemical fingerprint of the place where its ice formed. Astronomers measured an unusually high share of heavy hydrogen - deuterium - in its methane. Deuterium has an extra neutron in its nucleus, so it is heavier than ordinary hydrogen. Such a result indicates that the comet's ice could have formed in a very cold environment around another star or in the cloud from which that star arose.
The observations were carried out by a team led by Nathan Roth of American University and NASA's Goddard Space Flight Center. The scientists used the James Webb Space Telescope. The results were published in the journal Nature Astronomy.
Methane consists of one carbon atom and four hydrogen atoms. Deuterium can take the place of ordinary hydrogen, and then the infrared "signature" of the molecule changes. Astronomers distinguish the two forms of methane by this radiation, although the signal from the deuterated version is far weaker.
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 studied infrared light from the coma - the cloud of gas around the comet's icy nucleus. The researchers modeled and subtracted the light of other gases, and then looked for faint traces of heavy methane between the much brighter lines of ordinary methane. As a result, they obtained a deuterium-to-hydrogen ratio (D/H) in methane of about 3.2%.
"This D/H ratio exceeds any other value measured for methane in the Solar System," the authors write.
For comparison, the scientists took comet 67P/Churyumov-Gerasimenko. This is a Jupiter-family comet that probably originated in the Kuiper Belt. It is important because it is the only other comet in which methane with deuterium has been detected. That measurement required close study by the Rosetta spacecraft. Webb, by contrast, detected the molecule from afar at 3I/ATLAS - a visitor born outside our Solar System.
Why can an extra neutron tell us about the comet's homeland? In very cold gas, some chemical reactions promote the incorporation of deuterium into molecules. At a temperature of about 10 degrees above absolute zero, the reactions that would destroy such enrichment work more weakly. Ice rich in deuterium can then build up on interstellar dust grains, and later it becomes part of the comet.
The new measurement also adds a clue about timing. The D/H ratio in methane is about three times higher than that previously measured in the water of 3I/ATLAS. The researchers suggest that the methane could have formed later, when the surrounding chemistry had managed to concentrate heavy hydrogen more strongly.
The team believes the result is consistent with the theory that 3I/ATLAS originated in a cold cloud or in the environment around a forming star, where there are relatively few elements heavier than helium and cosmic-ray bombardment is high. These energetic particles help trigger reactions that enrich molecules with deuterium.
However, this is a plausible story, not an unambiguously established birthplace. Direct measurements of the D/H ratio in methane from distant star-forming regions are lacking for comparison, and the few chemical models give estimates below the measured value. So the comet poses a difficult task for models: any description of how its ingredients came together must now explain why its methane contains so much heavy hydrogen.