Third interstellar comet 3I/ATLAS carries water with deuterium ratio 40 times Earth’s
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The interstellar comet 3I/ATLAS, only the third confirmed visitor from beyond the Solar System, carries water with a deuterium-to-hydrogen ratio about 40 times higher than Earth’s oceans, a study published in Nature Astronomy reports. The ratio, detected by the ALMA telescope in Chile, suggests the comet formed in an environment far colder and less irradiated than the disc that birthed our own Sun and planets.
The Discovery of 3I/ATLAS
The ATLAS sky survey in Chile first detected the comet on 1 July 2025 as a faint moving point of light. Archived images later pinned its first appearance to 14 June 2025. Unlike native comets that follow elliptical orbits, 3I/ATLAS travels a hyperbolic path, confirming its interstellar origin. It made its closest approach to the Sun on 30 October 2025 at roughly 210 million kilometers, inside Mars’s orbit, and never came within 270 million kilometers of Earth. NASA estimates its icy nucleus spans between 440 meters and 5.6 kilometers across.
Deuterium Ratio in Water
Researchers led by Luis Salazar Manzano and Teresa Paneque-Carreño of the University of Michigan used the MDM Observatory in Arizona and the ALMA telescope in Chile to analyze gas from the comet. They measured the ratio of deuterium, a heavy form of hydrogen, to ordinary hydrogen in the comet’s water. The value is about 30 times higher than in comets that formed in our own solar system and roughly 40 times higher than in Earth’s ocean water. The findings were published in Nature Astronomy in May 2026, with support from NASA, the U.S. National Science Foundation and Chile’s ANID.
Implications for Formation
A water molecule’s deuterium ratio tends to reflect the temperature and radiation environment where the ice originally formed. A ratio as high as 3I/ATLAS’s points to a birthplace far colder and less bathed in radiation than the protoplanetary disk that created our Sun and planets. If confirmed, the measurement provides a rare direct window into the conditions of a distant planetary system. The researchers caution that this is a single study and not settled consensus, but the interpretation aligns with models of interstellar ice formation.
What's Next
Astronomers plan further observations of 3I/ATLAS as it recedes from the Sun, though its hyperbolic trajectory means it will leave the Solar System forever. It remains unclear whether future interstellar visitors will show similarly extreme deuterium ratios or if 3I/ATLAS is an outlier.
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Third interstellar comet 3I/ATLAS carries water with deuterium ratio 40 times Earth’s
