Webb Finds Record Heavy Hydrogen in Methane of Interstellar Comet 3I/ATLAS
Measured at about 3.2 percent, the methane's deuterium ratio is 13 times that of comet 67P and points to a birthplace colder and poorer in heavy elements than ours.
The interstellar comet 3I/ATLAS carries methane with more heavy hydrogen than any methane ever measured in the solar system, according to a new study in Nature Astronomy. The finding, based on observations from the James Webb Space Telescope, gives astronomers a chemical clue to the cold, distant place where the comet was born.
A team led by Nathan Roth of American University and NASA's Goddard Space Flight Center measured the ratio of deuterium to ordinary hydrogen in the comet's methane. Deuterium is a heavy form of hydrogen with an extra neutron in its nucleus. The ratio came out at about 3.2 percent, roughly 13 times higher than in comet 67P/Churyumov-Gerasimenko, a Jupiter-family comet that likely formed in our own solar system. The authors write that the value "exceeds any other value for methane measured in the solar system."
Webb observed 3I/ATLAS on December 22 and 23, 2025, when the comet was about 2.4 times as far from the Sun as Earth is. The telescope recorded infrared light from the coma, the gas cloud around the comet's icy nucleus. The team modeled and subtracted light from other gases, including methanol, formaldehyde, ethane and hydroxyl emission, to find the faint signal of deuterium-bearing methane between the much brighter features of ordinary methane.
The detection was hard. Methane is one carbon atom bonded to four hydrogen atoms, and when deuterium replaces one of them the molecule's infrared signature shifts and becomes much fainter. Only one other comet has ever had deuterated methane detected, 67P, and that took the close-up Rosetta spacecraft. Webb saw it in 3I/ATLAS from a great distance.
The explanation lies in extreme cold. At around 10 kelvin, some chemical reactions favor putting deuterium into molecules, and the reactions that would undo the enrichment become less effective. Chemistry on interstellar dust grains can then build deuterium-rich ice that later ends up in a comet. The methane ratio is also about three times the ratio previously measured in the comet's water, which the researchers suggest means the methane formed later, after the surrounding chemistry had more time to concentrate heavy hydrogen.
The team argues that the results fit an origin for 3I/ATLAS in a cold cloud or in the environment around a forming star with relatively few elements heavier than helium and a high rate of cosmic-ray bombardment. Energetic cosmic rays help start the reactions that enrich molecules in deuterium. That dovetails with earlier work reported on the comet's heavy water, which also suggested a metal-poor birthplace.
The authors are careful about the limits. The result gives a plausible history, not a uniquely identified birthplace. Direct measurements of methane's deuterium ratio in distant star-forming regions do not exist for comparison, and the few chemical models that estimate such values fall short of what 3I/ATLAS shows. The comet therefore becomes a demanding test for those models, and a rare sample of material from around a young star that telescopes normally cannot reach.
