Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic-Ray Processing

Abstract Spectral observations of 3I/ATLAS (C/2025 N1) with JWST/NIRSpec and SPHEREx reveal an extreme CO 2 enrichment (CO 2 /H 2 O = 7.6 ± 0.3) that is 4.5 σ above solar system comet trends and among the highest ever recorded. This unprecedented composition, combined with substantial absolute CO levels (CO/H 2 O = 1.65 ± 0.09) and red spectral slopes, provides direct evidence for galactic cosmic-ray (GCR) processing of the outer layers of the interstellar comet nucleus. Laboratory experiments demonstrate that GCR irradiation efficiently converts CO to CO 2 while synthesizing organic-rich crusts, suggesting that the outer layers of 3I/ATLAS consist of irradiated material whose properties are consistent with the observed composition of the 3I/ATLAS coma and with its observed spectral reddening. Estimates of the erosion rate of 3I/ATLAS indicate that current outgassing samples the GCR-processed zone only (depth ∼15–20 m), never reaching pristine interior material. Outgassing of pristine material after perihelion remains possible, though it is considered unlikely. This points to a paradigm shift: long-residence interstellar objects likely reveal primarily GCR-processed material rather than pristine material representative of their primordial formation environments. With 3I/ATLAS approaching perihelion in 2025 October, immediate follow-up observations are critical to confirm this interpretation and establish GCR processing as a fundamental evolutionary pathway for interstellar objects.