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Researchers derived a water D/H ratio of 0.98, plus or minus 0.06 percent, for interstellar comet 3I/ATLAS—more than an order of magnitude higher than ratios in known comets. They also calculated plausible carbon-isotope ratios, 12C to 13C, of 141 to 191 in carbon dioxide and 123 to 172 in carbon monoxide. Those ranges partly reflected the difficulty of defining spectral baselines for the minor isotopologues 13CO2 and 13CO.
A Nature paper published on June 22, 2026, reported James Webb Space Telescope measurements of water, carbon dioxide, carbon monoxide, and their isotopologues in the comet’s coma. The December 2025 measurements produced a carbon-dioxide-to-water ratio of 1.08, plus or minus 0.03, and a carbon-monoxide-to-water ratio of 2.40, plus or minus 0.07. Compared with JWST observations from August 6, 2025, the authors reported that the coma had shifted from carbon-dioxide dominated to carbon-monoxide dominated.
The observations indicated that most carbon dioxide and carbon monoxide escaped directly from the nucleus. Sublimating icy grains in the coma also contributed to water production.
JWST’s NIRSpec observed 3I/ATLAS on December 22 and 23, 2025, during the outbound leg of its 2025 perihelion passage. The comet was then approximately 2.4 astronomical units from the Sun.
The paper interpreted the unusual deuterium and carbon isotope ratios as evidence that 3I/ATLAS formed at temperatures of about 30 kelvin or below in a relatively metal-poor environment. By comparing its carbon-isotope composition with models of Galactic chemical evolution, the authors inferred that the comet may have accreted as long as 12 billion years ago—but its exact age, birthplace, and path through the Galaxy remained highly uncertain.