An interstellar visitor that has been under close watch since it entered the solar system is showing scientists something new about what it is made of. Astronomers using the Subaru Telescope in Hawaii found that the comet 3I/ATLAS carries a far lower ratio of carbon dioxide to water than earlier readings suggested, a shift that points to a comet whose chemistry is evolving as sunlight reaches deeper into its icy body.
The comet, formally designated C/2025 N1, is only the third object ever confirmed to have originated outside the solar system, following 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. Because interstellar comets carry material that formed around other stars, each new reading of their composition gives researchers a rare chance to compare the building blocks of a foreign planetary system with those of one closer to home.
What the Subaru observations found
The measurements came from an observing run at the Subaru Telescope on January 7, 2026, more than two months after 3I/ATLAS made its closest approach to the Sun on October 30, 2025. Researchers led by Dr. Shinnaka examined faint emission features known as forbidden oxygen lines, which let astronomers back out the relative amounts of carbon dioxide and water escaping from the comet’s coma, the cloud of gas and dust surrounding its icy nucleus. The resulting carbon dioxide-to-water ratio came out markedly lower than the ratio calculated from earlier data gathered by the James Webb Space Telescope and the SPHEREx mission, according to the National Astronomical Observatory of Japan, which operates Subaru.
Why the ratio matters for a comet’s structure
A comet’s nucleus is not necessarily uniform. Ices trapped near the surface can be different in makeup from ices buried deeper inside, a layering effect that can form when a comet accumulates material unevenly during its formation or gets weathered differently across billions of years of travel. As 3I/ATLAS swept closer to the Sun and warmed, researchers say gas likely began escaping from progressively deeper, previously undisturbed layers of the nucleus. That would explain why a measurement taken well after perihelion looks so different from the readings taken before and during the comet’s closest solar approach, when only the outer layers had been heated enough to sublimate.
Comparing 3I/ATLAS with solar system comets
Carbon dioxide and water are two of the most abundant ices in comets throughout the solar system, and their ratio is often used as a rough proxy for where and how a comet originally formed. A low carbon dioxide-to-water ratio close to the nucleus, followed by a shift as the comet is heated, suggests the interior chemistry of 3I/ATLAS is not fixed but layered and dynamic, more like some of the more volatile-rich comets that formed farther from their parent star. Sci.News reported that this pattern of change over time is itself evidence the comet’s bulk composition is heterogeneous, rather than a single uniform mixture of ices throughout.
The two interstellar objects observed before 3I/ATLAS each behaved very differently once astronomers got a close look. ‘Oumuamua showed no visible coma or dust tail at all, leaving its composition largely a mystery and fueling years of debate over whether it was icy, rocky, or something in between. Borisov, by contrast, displayed an unusually large amount of carbon monoxide relative to water for a comet of its size, a trait that set it apart from typical solar system comets and hinted that it may have formed in an especially cold region around its parent star. Measured against that backdrop, the shifting carbon dioxide-to-water signature now documented in 3I/ATLAS gives researchers a third, distinct data point in an emerging picture of just how varied interstellar comets can be.
Tracking an object that will not return
Unlike comets that loop back through the inner solar system every few years, 3I/ATLAS is on a hyperbolic trajectory that will carry it back out into interstellar space, never to be observed again by telescopes on Earth. That makes every observing window before it fades from view scientifically valuable, and it is part of why observatories around the world, including Subaru, the James Webb Space Telescope and SPHEREx, all trained instruments on the object during its brief passage. Each dataset captures a different moment in the comet’s outgassing history, and stitching them together lets researchers reconstruct how its chemistry evolved from before perihelion through the two months of post-perihelion observations described in the new findings.
What researchers still want to learn
Scientists studying 3I/ATLAS say the composition shift raises new questions about how much of the difference comes from genuine layering inside the nucleus versus changes in how efficiently different ices sublimate at different distances from the Sun. Continued spectroscopic monitoring, for as long as the comet remains bright enough to observe, is expected to refine the picture of its interior structure. Because only two other interstellar objects have ever been confirmed, each additional data point on 3I/ATLAS adds disproportionately to what is known about the diversity of material drifting between star systems, and researchers are treating the coming weeks of visibility as a closing window before the comet becomes too faint and distant to study in detail.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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