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

TL;DR

The study presents JWST/NIRSpec, SPHEREx, and Swift observations of the interstellar object 3I/ATLAS, revealing an extreme CO2 enrichment (CO2/H2O ≈ 7.6) and elevated CO (CO/H2O ≈ 1.65) with red spectral slopes. Radiolysis experiments and GCR-transport modeling indicate that galactic cosmic rays over gigayear timescales can convert CO to CO2 and produce organic-rich crusts, confining processing to the outer ~15–20 m of the nucleus. Erosion calculations show that pre-perihelion outgassing samples mostly processed crust, with a small chance of pristine interior exposure only in certain high-erosion scenarios; post-perihelion exposure of pristine material is unlikely for larger nuclei. Collectively, the results argue for a paradigm shift where ISOs bear signatures of long-term GCR processing rather than pristine formation environments, making perihelion-time observations crucial to test this processing pathway and to generalize its prevalence across ISO populations.

Abstract

Spectral observations of 3I/ATLAS (C/2025 N1) with JWST/NIRSpec and SPHEREx reveal an extreme CO2 enrichment (CO2/H2O = 7.6+-0.3) that is 4.5 sigma above solar system comet trends and among the highest ever recorded. This unprecedented composition, combined with substantial absolute CO levels (CO/H2O = 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 CO2 while synthesizing organic-rich crusts, suggesting that the outer layers of 3I/ATLAS consist of irradiated material which properties are consistent with the observed composition of 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 represents a paradigm shift: long-residence interstellar objects primarily reveal GCR-processed material rather than pristine material representative of their primordial formation environments. With 3I/ATLAS approaching perihelion in October 2025, immediate follow-up observations are critical to confirm this interpretation and establish GCR processing as a fundamental evolutionary pathway for interstellar objects.

Figures (5)

• Figure 1: Energy dose deposited by GCRs in a nucleus after an exposure time of 1 Gyr in the local interstellar medium, adapted from Gronoff2020. The simulation assumes a bulk density of 0.5 g cm$^{-3}$ and a nucleus composed of H$_2$O (of isotopic composition $^{16}$O and $^{1}$H) and SiO$_2$ with a SiO$_2$/H$_2$O mass ratio of 4.
• Figure 2: Depth profiles of chemical abundances relative to H$_2$O in a nucleus irradiated by galactic cosmic rays, adapted from Maggiolo2020. The simulation assumes a bulk density of 0.5 g cm$^{-3}$, an exposure time of 1 Gyr, and mixed H$_2$O--CO ices. Radiolysis yields are taken from laboratory experiments by Hudson1999, while the depth-dependent energy deposition is based on Monte Carlo radiation transport results from Gronoff2020. CO (black) is progressively converted into CO$_2$ (red) within the outer $\sim$15--20 m, while other products (CH$_4$, CH$_3$OH, H$_2$CO, HCO) decrease with depth as irradiation effects diminish.
• Figure 3: Depth-dependent transformation of ice structure under galactic cosmic rays irradiation, adapted from Maggiolo2020. The simulation assumes a bulk density of 0.5 g cm$^{-3}$ and an exposure time of 1 Gyr, with energy deposition rates from Gronoff2020. Transformation yields from Dartois2013 for porous amorphous ice and Dartois2015 for crystalline ice are applied to compute the compact amorphous fraction (in %), showing the progressive conversion of crystalline H$_2$O ice (blue) and porous amorphous ice (red). The conversion is most efficient within the outer tens of meters and decreases with depth.
• Figure 4: Cumulative erosion depth of 3I/ATLAS as a function of time during its solar system passage, scaled with heliocentric distance from the object’s orbital parameters. Mass-loss rates are modeled as power laws in heliocentric distance ($Q \propto r_{\mathrm{h}}^{-2}$ and $Q \propto r_{\mathrm{h}}^{-4}$) and anchored to gas production from Cordiner2025 and dust production from Jewitt2025. Solid curves represent maximum dust-loss scenarios (120 kg s$^{-1}$ at 3.83 AU), while dashed curves represent minimum dust-loss scenarios (12 kg s$^{-1}$ at 3.83 AU). Two nucleus radii are considered: 500 m and 2200 m.
• Figure 5: Schematic illustration of interstellar comet 3I/ATLAS showing the expected stratigraphy of an irradiated nucleus. Galactic cosmic ray irradiation over several Gyr alters the outer $\sim$15--20 m, producing a CO$_2$-rich crust through CO-to-CO$_2$ conversion, ongoing CO release from the breakdown of irradiated organics, an organic-rich refractory mantle with a red spectral slope, and compact amorphous ice characterized by low diffusivity and high thermal conductivity. Beneath this layer lies a pristine, unprocessed interior shielded from GCRs, whose composition and structure remain unknown but are expected to differ markedly from the irradiated crust.