ALMA Detects Record Deuterated Water in Interstellar Comet 3I/ATLAS

Comet 3I/ATLAS, the third interstellar object observed passing through the solar system, was first identified in July 2026. After its perihelion exit in December, astronomers pointed the Atacama Large Millimeter/submillimeter Array (ALMA) at the comet in early November when it was about 126 million miles (203 million km) from the Sun. ALMA’s radio receivers detected deuterated water (HDO) for the first time in an interstellar body, while ordinary water (H2O) remained below the instrument’s sensitivity.

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Spectral analysis revealed that the comet’s water contains deuterium at levels more than 40 times higher than Earth’s oceans and over 30 times higher than water in Solar System comets. The researchers infer a formation temperature below 30 K (-243 °C), characteristic of dense molecular clouds and the outer zones of a protoplanetary disk. The high deuterium fraction, together with an estimated age up to 11 billion years, points to an origin in a very cold, early-generation planetary system.

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The study was led by Luis Eduardo Salazar Manzano, a doctoral candidate in astronomy at the University of Michigan and co-author of a *Nature Astronomy* paper. He said the detection opens a “time capsule” revealing the comet’s birth conditions. Dr. Theodore Kareta, a planetary astronomer at Villanova University, noted the community is only beginning to sample interstellar comets and that the results imply a shift in comet chemistry as the Milky Way ages.

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Deuterium enrichment in cold molecular clouds means the comet’s water records Milky Way chemistry over 10 billion years ago, when heavy elements were scarce. Comparing deuterium ratios among interstellar objects lets astronomers trace how planet-building material has changed, informing models of planetary system evolution.

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• “Deuterium is generally found in the water of Solar System comets and in Earth’s oceans in the form of deuterated water, HDO, also called semi-heavy water,” — Luis Eduardo Salazar Manzano, lead study author

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• “This does not mean that 3I/ATLAS did not have ordinary water; it only means that it was below the sensitivity of our observations,” — Luis Eduardo Salazar Manzano

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• “The enrichment in deuterium generally happens when water forms in cold molecular clouds in interstellar space, which is generally around the same time that solar systems around other stars form,” — Luis Eduardo Salazar Manzano

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• “We’re very clearly only seeing the top of the iceberg when it comes to studying these interstellar comets,” — Dr. Theodore Kareta, Villanova University

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The Vera C. Rubin Observatory, now operating in Chile, is expected to discover interstellar objects more often, allowing systematic comparison of deuterium abundances. Determining whether 3I/ATLAS is typical of a class of highly deuterated comets and identifying its parent planetary system remain key goals for upcoming studies.

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