Interstellar Comet 3I/ATLAS: Galactic Cosmic Ray Effects—New Research Study
This schematic illustration shows the expected stratigraphy of the irradiated nucleus interstellar comet 3I/ATLAS. Galactic cosmic ray (GCR) irradiation over several circular motions (gyr) alters the outer ∼15–20 m, producing a carbon dioxide CO2-rich crust through CO-to-CO2 conversion, ongoing carbon monoxide 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.
Comet 3I/ATLAS is the third known object from outside our solar system to be discovered passing through our celestial neighborhood. Astronomers have categorized this object as interstellar because of the hyperbolic shape of its orbital path. (It does not follow a closed orbital path about the Sun.) When the orbit of 3I/ATLAS is traced into the past, the comet clearly originates from outside our solar system.
Comet 3I/ATLAS poses no threat to Earth and will remain far away. The closest it approached our planet was about 1.8 astronomical units (about 170 million miles, or 270 million kilometers). 3I/ATLAS and its closest point to the Sun was around Oct. 30, 2025, at a distance of about 1.4 au (130 million miles, or 210 million kilometers) — just inside the orbit of Mars.
The interstellar comet’s size and physical properties are being investigated by astronomers around the world. 3I/ATLAS remained visible to ground-based telescopes through September 2025, after which it will pass too close to the Sun to observe. It will reappear on the other side of the Sun by early December 2025, allowing for renewed observations.
Spectral observations of 3I/ATLAS (C/2025 N1) with NASA's James Webb Space Telescope (JWST) near infrared spectrograph (NIRSpec) and spectro-photometer for the history of the universe, epoch of reionization, and ices explorer (SPHEREx) reveal an extreme carbon dioxide (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 carbon monoxide (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 with properties consistent with the observed composition of the 3I/ATLAS coma demonstrating 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.
Follow-up observations of 3I/ATLAS are critical to confirm the above interpretation and to confirm if GCR processing is a "fundamental evolutionary pathway" for interstellar objects.
Read this new research paper "Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic Ray Processing": https://arxiv.org/pdf/2510.26308
Release Date: Oct. 31, 2025
Source: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
https://doi.org/10.48550/arXiv.2510.26308
https://science.nasa.gov/solar-system/comets/3i-atlas/
Thanks to Cornell University and Astrobiology[dot]com
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