The interstellar comet 3I/ATLAS, recently passing its closest point to the sun, shows unusually high levels of carbon dioxide that could stem from billions of years of cosmic ray exposure. This alteration might prevent scientists from tracing its home star system. Upcoming observations could reveal more pristine material beneath its surface.
Astronomers first spotted the interstellar comet 3I/ATLAS in July 2025. Since then, observations have revealed puzzling properties, including carbon dioxide levels in its coma—a plume of gas and dust—at least 16 times higher than those in typical solar system comets, making it one of the most CO2-rich comets ever detected.
After reaching perihelion, its closest approach to the sun, on or around November 2025, the comet is now moving away from our star at approximately 210,000 kilometers per hour, eventually escaping the solar system. However, its journey has sparked debate over its origins.
Researchers, led by Romain Maggiolo at the Royal Belgian Institute for Space Aeronomy in Uccle, Belgium, suggest that high-energy cosmic rays have radically transformed the comet's outer layers over billions of years. This process could explain the elevated CO2 without invoking an exotic home system or improbable extraterrestrial involvement. Laboratory studies simulating cosmic rays on ice composed of water and carbon monoxide—similar to comet material—show that the particles generate abundant CO2 and leave a red, carbon-rich residue matching observations of 3I/ATLAS.
“Somehow, this process has been a bit overlooked or taken as a secondary process, because it’s very slow. But in the end, for objects like comets or interstellar objects, it has a strong effect,” says Maggiolo. He adds, “Very slowly, [cosmic rays] will break molecules and produce reactive radicals, fragments of molecules that will recombine, and so they will slowly change the chemical composition of the [comet’s] ice.”
This alteration challenges the view of interstellar comets as pristine "cold fossils" preserving details of distant star systems. A spacecraft mission is impossible due to the comet's high speed. Yet, hope remains: as 3I/ATLAS passed close to the sun, it became temporarily out of view from Earth but is expected to reappear in December 2025. The heat may melt its outer ice crust, potentially exposing less-altered material underneath, depending on the amount of ice lost and crust thickness—details currently unknown.
Cyrielle Opitom at the University of Edinburgh, UK, emphasizes the importance of forthcoming observations. “We have a very exciting few months coming,” she says, referring to studies planned with the James Webb Space Telescope and ground-based telescopes to probe for pristine subsurface material. Their findings, detailed in a preprint (arXiv:2510.26308), underscore the need for caution in interpreting such visitors.