3I/ATLAS has an extreme taste for heavy water

Hubble image of interstellar comet 3I/ATLAS, which was used in the study. Credit: NASA, ESA, D. Jewitt (UCLA); Image Processing: J. DePasquale (STScI) 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.

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 so

AI-generated summary of the original article — see the source for the full story.