Study Finds Interstellar Comet 3I/ATLAS Contains Extremely High Deuterium Levels
Observations with the James Webb Space Telescope reveal that comet 3I/ATLAS has a deuterium-to-hydrogen ratio in methane far exceeding that of known Solar System bodies, suggesting an origin in an ancient, low-metallicity environment.
Astronomers using the James Webb Space Telescope observed interstellar comet 3I/ATLAS in late December 2025 as it passed through the asteroid belt between Mars and Jupiter. The comet’s coma expanded under solar radiation, allowing scientists to measure its chemical composition, particularly the deuterium-to-hydrogen (D/H) ratio in methane.
The analysis, published in Nature Astronomy, shows that 3I/ATLAS contains a D/H ratio in methane that is significantly higher than any object in the Solar System and about 13 times greater than that measured in comet 67P/Churyumov-Gerasimenko.
"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," the research team explained.
Such isotopic signatures imply that the comet’s ices likely formed in a cold, dense region of a young star-forming disk or a prestellar core, environments that preserve heavy isotopes like deuterium. Low carbon-13 content further supports an origin before the Galaxy was enriched by multiple generations of stars.
If the comet originated more than 7 billion years ago, it would predate the formation of the Sun and could provide a rare glimpse of the chemical conditions in the early Milky Way.