Recent spectral observations of the interstellar comet 3I/ATLAS (C/2025 N1), conducted using the James Webb Space Telescope and SPHEREx, indicate a remarkable enrichment of carbon dioxide (CO2). The CO2/H2O ratio is measured at 7.6 ± 0.3, which significantly exceeds standard trends observed in solar system comets by 4.5 sigma, marking it as one of the highest levels recorded to date.
This extraordinary chemical composition, when coupled with substantial levels of carbon monoxide (CO) at a CO/H2O ratio of 1.65 ± 0.09 and a red spectral slope, provides compelling evidence for the processing of the comet”s outer layers by galactic cosmic rays (GCR). Laboratory experiments have shown that GCR irradiation can efficiently transform CO into CO2 and produce organic-rich crusts. This suggests that the outer layers of 3I/ATLAS are composed of irradiated materials, the characteristics of which are consistent with both the composition of the comet”s coma and its observed spectral reddening.
Estimates regarding the erosion rate of 3I/ATLAS suggest that the ongoing outgassing is primarily sampling the GCR-processed zone, which reaches a depth of approximately 15-20 meters, without penetrating the unaltered interior material. While there remains a possibility of outgassing pristine material following perihelion, this scenario is considered unlikely. This finding indicates a significant shift in our understanding: long-residing interstellar objects predominantly exhibit GCR-processed materials rather than materials that reflect their original formation environments.
As 3I/ATLAS approaches perihelion in October 2025, timely follow-up observations will be essential to verify this interpretation and to further establish GCR processing as a critical evolutionary pathway for interstellar objects. The insights from this research could reshape our understanding of interstellar materials and their histories.
The study, authored by R. Maggiolo and colleagues, has been submitted to The Astrophysical Journal and spans 13 pages, including 5 figures.
