The interstellar comet 3I/ATLAS carries a chemical clue to conditions in the planetary system of its origin, locked away long before it reached our sun. Its methane signature is unusually rich in heavy hydrogen, suggesting that the comet’s frozen ingredients took shape in an intensely cold environment around another star—or in the cloud that preceded it.
Using the James Webb Space Telescope, Nathan Roth of American University and NASA’s Goddard Space Flight Center and colleagues measured a heavy-to-ordinary hydrogen ratio in 3I/ATLAS that’s about 13 times as high as that of comet 67P/Churyumov-Gerasimenko (which likely originated in our own solar system).
Their findings, published in Nature Astronomy, give scientists a rare opportunity to investigate another planetary system’s early chemistry. The challenge is working backward from the escaping gas to the conditions that made its ice.
Finding a faint signal between brighter ones
Heavy hydrogen, called deuterium, has an extra neutron in its nucleus. It can take the place of ordinary hydrogen in a molecule of methane, which consists of one carbon atom joined to four hydrogen atoms. That substitution changes the molecule’s infrared signature, allowing astronomers to distinguish between the two forms, but the deuterium-bearing version produces a much fainter signal.
Webb observed 3I/ATLAS on Dec. 22 and 23, 2025, when it was about 2.4 times as far from the sun as Earth is. The telescope examined infrared light from the coma, the cloud of gas surrounding the comet’s icy nucleus.
The researchers modeled and subtracted light from other gases, then looked between ordinary methane’s much brighter features for the faint traces of its heavier counterpart. Those signals yielded a deuterium-to-hydrogen ratio, or D/H ratio, of approximately 3.2% in methane.
“This D/H ratio exceeds any other value for methane measured in the solar system,” the authors write.
The comparison with comet 67P, which is a Jupiter-family comet that probably originated in the Kuiper Belt, is especially useful because it is the only other comet in which deuterium-bearing methane has been detected. That measurement required a close-up investigation by the Rosetta spacecraft. Webb detected the molecule from afar in 3I/ATLAS, a visitor born outside our solar system.
What the ice remembers
Why should an extra neutron reveal anything about a comet’s birthplace?
In extremely cold gas, some chemical reactions favor the incorporation of deuterium into molecules. At temperatures around 10 degrees above absolute zero, reactions that would undo that enrichment become less effective. Chemical reactions on interstellar dust grains can then build deuterium-rich ice that eventually becomes part of a comet.
The new measurement also adds a clue about timing. Methane’s D/H ratio was about three times the ratio previously measured in 3I/ATLAS’s water. The researchers suggest that the methane may have formed later, after the surrounding chemistry had had more time to concentrate heavy hydrogen.
The team argues that the methane result fits with the theory of proposed origin for 3I/ATLAS in a cold cloud or an environment surrounding a forming star, with relatively few elements heavier than helium and a high rate of cosmic-ray bombardment. Those energetic particles help start the reactions that enrich molecules in deuterium.
That offers a plausible history rather than a uniquely identified birthplace, however. Direct measurements of methane’s D/H ratio in distant star-forming environments for comparison are lacking, and the few chemical models that report such estimates fall short of the value measured here.
The comet therefore gives those models a demanding test. Dense material around young stars can hide the ice chemistry from telescopes, but 3I/ATLAS has carried some of that material within our reach. Any descriptions of how its ingredients formed must now explain why its methane contains so much heavy hydrogen.
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Publication details
Nathan X. Roth et al., An enriched methane D/H ratio in the interstellar object 3I/ATLAS, Nature Astronomy (2026). DOI: 10.1038/s41550-026-03002-5
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Heavy hydrogen in 3I/ATLAS’s methane points to a frigid birthplace (2026, November 1)
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