Could a comet be sent on such a trajectory through the galaxy that its surface no longer reflects the world from which it came? New observations of the comet 3I/ATLAS by the James Webb Space Telescope hint that may indeed be the case-and the findings are rewriting the way scientists decode the chemistry of interstellar visitors.
Data from JWST, together with sophisticated computer simulations, show that 3I/ATLAS has been bombarded for billions of years by galactic cosmic rays-high-energy particles originating outside the solar system. In the 7-billion-year lifetime of the comet, this constant irradiation has converted much of its original carbon monoxide ice into carbon dioxide, resulting in the unusually high abundance of CO₂ in the comet. “It’s very slow, but over billions of years, it’s a very strong effect,” says lead author Romain Maggiolo of the Royal Belgian Institute for Space Aeronomy.
The process is well understood in laboratory analogs. Experiments simulating interstellar space conditions show that when CO-rich ice is exposed to cosmic ray particles, ionization and radiolysis break molecular bonds, allowing oxygen atoms to recombine into CO₂. In the absence of the Sun’s protective heliosphere the vast magnetic bubble that shields solar system bodies from most cosmic rays this chemistry can penetrate deep into a comet’s surface. The team’s models suggest that for 3I/ATLAS, the altered layer extends 15 to 20 meters below the surface, effectively sealing away any pristine material from its birth environment.
Such depth-dependent processing mirrors results from multi-layer comet ice chemistry models, which follow how cosmic ray energy deposition drives slow, but cumulative chemical alterations over geological timescales. Here, the outer layers play the role of a sacrificial shield, while deeper strata preserve original molecular inventories. For 3I/ATLAS, the challenge is the shield is now so thick, only significant erosion-either from intense solar heating near perihelion-could expose unaltered material.
The comet reached perihelion on 29 October 2025 at 1.36 astronomical units from the Sun. As with all comets, the heating by the Sun triggered sublimation: volatile ices went directly to gas, entraining dust into space and creating the coma and tail. Before perihelion, the escaped gases likely came entirely from the irradiated crust. After perihelion, Maggiolo adds, monitoring may reveal subtle compositional changes, if deeper layers are penetrated. “Maybe by looking at these differences, we can have some indication about its initial composition,” he said.
The scientific stakes are high. Interstellar comets like 3I/ATLAS are rare-only two others, ‘Oumuamua and 2I/Borisov, have been observed-and they offer direct samples of planetary systems beyond the Sun. Yet the new findings imply that what telescopes detect may be dominated by processed, not primordial, chemistry. This complicates efforts to use such objects as probes of their natal environments.
The irradiation history of 3I/ATLAS also stresses the role of the heliosphere as a cosmic radiation shield. Inside its boundary, the flux of galactic cosmic rays is reduced, which decelerates CO-to-CO₂ conversion. Outside, in the interstellar medium, there is no such protection. Researches on the shape and extent of the heliosphere, driven by Voyager data and the proposed interstellar probe missions, are necessary for understanding how this long-term exposure may vary on different trajectories.
These processes are simulated in the laboratory by irradiating stratified ice mixtures with charged particles under ultra-cold, high-vacuum conditions-for example, using the MAGICKAL (Model for Astrophysical Gas and Ice Chemical Kinetics And Layering) framework. These experiments confirm that cosmic ray penetration depths can reach tens of meters in cometary ice, producing complex organics and altering volatile ratios over gigayear timescales. For 3I/ATLAS, the elevated CO₂-to-H₂O ratio-measured to be about 8:1-is consistent with such prolonged exposure. Moreover, it moves on a hyperbolic trajectory with an eccentricity of 6.14, while its inbound velocity relative to the Sun was over 61 km/s. Such characteristics definitely indicate that the object comes from outside the solar system, most likely from the thick disk of the Milky Way. Given its probable age of 3 billion years older than that of the solar system, the irradiation must have started long before Earth formed. Future observations, especially during the months after perihelion when the comet reemerges from the Sun’s glare, will test whether any unprocessed material becomes detectable. If so, astronomers will be able to directly compare the chemistry of ancient interstellar ice with that of the heavily altered crust, thereby offering a rare before-and-after view of cosmic ray processing across deep time.
