The James Webb Space Telescope detected a 15- to 20-meter-thick radiated crust on the surface of the interstellar comet 3I/Atlas. This layer results from billions of years of exposure to galactic cosmic rays during their journey through the Milky Way. The observation occurred in August 2025, when the comet was 4.45 billion kilometers from Earth.
Comet 3I/Atlas, the third confirmed interstellar object, was discovered in July 2025 by the ATLAS system in Chile. It follows a hyperbolic orbit, indicating origins outside the Solar System. Spectroscopic analysis from Webb’s NIRSpec instrument identified high concentrations of carbon dioxide in the coma.
Researchers from the Royal Belgian Institute of Space Aeronomy lead the study. The data shows that cosmic rays converted carbon monoxide to carbon dioxide at the surface. This transformation occurred without the protection of the solar heliosphere.
The crust dominates the surface, but inner layers preserve primitive materials. The comet’s perihelion occurred on October 29, 2025, 1.36 astronomical units from the Sun.
Altered chemical composition
3I/Atlas’ coma exhibits a CO2 to H2O ratio of 7.6, the highest recorded in comets. This dominance arises from the sublimation of the crust irradiated by solar heating.
Simulations based on the Rosetta mission to comet 67P indicate that a billion years of radiation forms such a layer. The chemical process remodels surface ices without affecting the core.
Comet’s Interstellar Journey
The 3I/Atlas travels at more than 210 thousand kilometers per hour. Its reddish color, similar to D-type asteroids, results from irradiated organic compounds.
Hubble observations in July 2025 estimate the core to be between 0.32 and 5.6 km in diameter, probably smaller than 1 km. Jets of gas have been detected at cold distances.
The absence of a heliosphere allowed intense bombardment by high-energy particles. This differentiates the comet from solar objects, which maintain primitive compositions.
Crust formation by cosmic rays
Galactic cosmic rays, high-energy particles, have hit the ices of the 3I/Atlas for billions of years. Laboratory experiments simulate these effects in water ice and carbon monoxide.
Chemical conversion raises CO2 at the surface. The 15 to 20 meter crust masks the comet’s natal stellar origin.
Initial remarks and activity
The comet showed a marginal coma with an elongation of 3 arcseconds in telescopes such as Canada-France-Hawaii. Ultraviolet spectroscopy detected cyanide and atomic nickel in the coma.
Fine dust of 1 micrometer ejected at 22 m/s forms the coma.Grains larger than 100 micrometers come out at 2 m/s, totaling 66 kg per second in July.
Brightness increased post-perihelion, with release of volatiles. Ground-based telescopes monitor the waning tail.
- Estimated core diameter: less than 1 km.
- Orbital speed: greater than 210 thousand km/h.
- Dominant composition in coma: carbon dioxide.
- Estimated radiation exposure: up to 7 billion years.
- Distance at perihelion: 1.36 AU from the Sun.
Implications for interstellar objects
The discovery changes the study of visitors like 2I/Borisov. Irradiated surfaces can hide original chemistry, requiring in-depth analysis.
Webb and SPHEREx observed in August 2025. Preliminary data indicates water ice and carbonyl sulfide in the core.
Maximum approach to Earth occurs in December 2025. Continuous observations refine models of chemical evolution.
The blue hue of coma results from heated carbon molecules. This reinforces the distinction between interstellar and solar comets.
Spectroscopic detection details
Webb’s NIRSpec instrument captured near-infrared spectra. High resolution revealed CO2 lines and traces of CO and H2O.
MIRI complemented this with thermal imaging, confirming the thickness of the crust. Analysis indicates that radiation processed material for at least 1 billion years.