Astronomers studying interstellar comet 3I/ATLAS have found evidence that its surface has been altered by galactic cosmic ray (GCR) processing, suggesting the comet’s outer layers are not pristine material from another star system.

Detected in July 2025, 3I/ATLAS is the third confirmed interstellar object, and its unusually high carbon dioxide–to–water ratio (CO₂/H₂O ≈ 7.6) puzzled researchers.

After testing various formation and chemical scenarios, scientists concluded that long-term exposure to cosmic rays likely transformed carbon monoxide into carbon dioxide, forming an organic-rich irradiated crust.

This means the comet’s current outgassing comes only from material processed over billions of years, masking the original composition beneath, IFL Science has reported.

While disappointing for those hoping to study unaltered interstellar material, the comet still offers a unique opportunity to explore how cosmic rays shape objects traveling between the stars.