3I/ATLAS has an extreme taste for heavy water

More and more details about 3I/ATLAS are filtering through the scientific process as time goes on. Our third known interstellar visitor attracted the attention of some of the world’s most powerful observatories when it was discovered in July 2025, and some of those telescopes found something peculiar—the isotopes it contained appeared different. A new paper submitted to The Astrophysical Journal Letters (and available as a preprint on arXiv) by Kenji Furuya of the RIKEN Pioneering Research Institute in Japan and his co-authors shows that the isotopic discrepancy is likely due to the “low metallicity” of the stellar nursery in which 3I/ATLAS was born.

In everyday language, we think of metals as having certain properties. But to astronomers, “metals” include anything with an atomic number higher than helium that was created in supernovae after the first generation of stars died. Hence, the universe has slowly become more metallic over time, at least according to astronomical standards, as more and more stars explode in supernovae and create “crazier space dust,” as a famous YouTube video once put it.

Some of that crazier space dust takes the form of carbon, which is crucial in organic chemistry but also present in 3I/ATLAS. Critically, it was present in an isotope ratio that didn’t match anything seen in our own solar system. The ratio of C-12 (i.e., a carbon atom with 12 protons and neutrons) to C-13 (a carbon atom with one more neutron) for objects in our solar system hovers around 90. The ratio for the interstellar medium averages around 68. In 3I/ATLAS, that ratio was between 123 and 191—much higher than anywhere the interstellar object has been traveling in the past few billion years.

Another strange isotope signature scientists have found in 3I/ATLAS was that of water. Hydrogen has a “heavy” form, known as deuterium, which carries a neutron in addition to the single proton that makes up hydrogen itself. When astronomers measured this deuterium/hydrogen ratio in 3I/ATLAS’s water, they found a ratio of around 1%. Compared with 0.015% to 0.03% for a typical comet in the solar system, it becomes clear that our interstellar visitor carries a lot more “heavy water” than is typical in any object in our own solar system.

So what caused these isotopic discrepancies? Scientists have long thought that a high C-12-to-C-13 ratio was caused by formation in a low-metal environment. However, according to the paper, the D/H ratio would also be explained by that origin story. To prove their point, the researchers simulated the complete life cycle of water ice—from its formation in an interstellar cloud through its collapse into a collection of gas and dust that will eventually form a star. Most importantly, they modeled a specific step in that life cycle that happens in the depths of interstellar space.

Out in the void, interstellar gas is heated only to around 10 Kelvin (-263℃)—far too cold for typical chemical reactions to take place. One of those typical chemical reactions is called deuterium enrichment—or “deuterium fractionation,” as the paper calls it—which, in the depths of interstellar space, will take place only based on energy differences.

Deuterium fractionation is driven by cosmic rays hitting the gas and forming triatomic hydrogen atoms (H₃+). These then occasionally collide with a deuterated hydrogen molecule (HD) to create H₂D+, regular H₂, and a little excess energy. At such cold temperatures, this reaction, which is normally bidirectional, doesn’t have enough energy to kick the H₂D+ back into the neutral, inert HD form—essentially trapping the deuterium in a much more unstable form.

Eventually, the H₂D+ molecules get hit by an electron, breaking apart and creating atomic deuterium. Those atomic deuterium atoms eventually meet up with oxygen atoms, creating “heavy” water at ratios much higher than expected in our own solar system.

There are three main reasons for this deuterium boom, according to the paper, all of which have to do with the low metallicity of its formative environment. First, there are fewer carbon monoxide molecules in the area. CO destroys H₂D+, so having less of it around means that H₂D+ itself can survive longer to create atomic deuterium.

Second is a lack of a “hydrogen deluge.” In metal-rich environments, UV photons break apart water into OH and H, creating a flood of atomic hydrogen atoms and thereby diluting the D/H ratio. In metal-poor environments, there isn’t as much water breaking apart, meaning the area is flushed with less atomic hydrogen, keeping the ratio relatively high.

Third is a relatively low cosmic ray ionization rate. In gas clouds with lots of high-energy cosmic rays, there is enough excess energy to kick the chemical process that creates the H₂D+ back into the more stable form of regular HD, thereby lowering the amount of H₂D+ available to be split into atomic deuterium. Lower cosmic ray energies mean less energy feeding that chemical reversal reaction, thereby keeping deuterium levels higher than expected.

To validate these processes, the authors ran a validation check using another common cometary material—methane. Previous studies had found that the D/H ratio in the methane in 3I/ATLAS was around 3%—still an order of magnitude above the 0.2% found in other solar system comets such as 67P/Churyumov-Gerasimenko. However, the relative ratio between the D/H ratio in methane and that in water is remarkably similar (3.4 in 3I/ATLAS and 4.8 in comet 67P). So while the absolute deuteration of a comet skyrockets in low-metallicity conditions, the ratio of methane deuteration to water deuteration remains constant no matter the environment in which the comet formed.

As more and more scientific papers come out discussing the various nuances of 3I/ATLAS, it seems clearer that this visitor offered an up-close look at the environment during a very early period of the solar system. And while it’s great that we got to study it while we did, it has now moved beyond our observational capabilities. Maybe the next interstellar visitor will have the same deuteration and carbon isotopic features, or maybe not, but either way, we should continue to take every chance we get to study these unique windows into our galaxy’s history.

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Gaby Clark

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Andrew Zinin

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