![Surviving CH4 fraction as a function of depth below the surface and effective GCR exposure time. The colour scale gives the CH4 abundance relative to its initial value, while the dashed contours indicate 90%, 50%, and 10% remaining CH4. Vertical dashed lines mark the 3σ viable GCR exposure range of 0.87 - 3.12 Gyr. Increasing GCR exposure progressively extends CH4 depletion to greater depths while more deeply shielded layers retain a larger fraction of their primordial CH4. --[astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/09/Delayed.png?fit=1200%2C669&ssl=1)
[astro-ph.EP] Interstellar comet 3I/ATLAS shows unusual volatile activity, including strong H2O, CO, and CO2 production and a delayed post-perihelion detection of CH4.
In this work, we investigate the delayed CH4 detection in an H2O, CO2, and CO ice matrix using a shape model coupled with a thermophysical nucleus model that incorporates interstellar processing by Galactic Cosmic Rays.
We track the sublimation, trapping, depletion, and release of these volatiles as a function of heliocentric distance and explore the relevant parameter space through 12000 model runs to identify viable combinations of primordial volatile inventory and effective GCR exposure age that reproduce the observed production rates of these volatiles.
The viable models favour a volatile-rich primordial inventory consistent with formation in an ultra-cold (T<20 K) region beyond the CO snowline, followed by an effective GCR processing exposure of 0.87–3.1 Gyr during interstellar passage. GCR processing produces a continuous compositional gradient and converts part of the CO reservoir into secondary CO2 and depletes near-surface CH4 through radiolytic conversion.
The delayed CH4 detection can be explained by the survival of CH4 trapped within the deeper H2O matrix and its subsequent release as subsurface heating progresses after perihelion. The timing of CH4 sublimation can be used to estimate the effective GCR processing age of 3I/ATLAS.
The posterior distributions also reveal a degeneracy between primordial dust content and GCR exposure, where dust-rich, weakly processed and ice-rich, strongly irradiated evolutionary pathways produce processed subsurface layers with similar thermophysical properties.
Akshat Rawat, Kinsuk Acharyya
Comments: Accepted for publication in ApJ (total 22 pagesand 8 figures, including appendix)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2609.19694 [astro-ph.EP] (or arXiv:2609.19694v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.19694
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Submission history
From: Kinsuk Acharyya
[v1] Thu, 17 Sep 2026 04:42:58 UTC (3,997 KB)
https://arxiv.org/abs/2609.19694
Astrobiology, Astrochemistry,
