Delayed Methane Detection in 3I/ATLAS: GCR-Driven Subsurface Stratification and Interstellar Exposure Age Constraints

Interstellar comet 3I/ATLAS shows unusual volatile activity, including strong H 2 O, CO, and CO 2 production and a delayed post-perihelion detection of CH 4. In this work, we investigate the delayed CH 4 detection in an H 2 O, CO 2, 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 CO 2 and depletes near-surface CH 4 through radiolytic conversion. The delayed CH 4 detection can be explained by the survival of CH 4 trapped within the deeper H 2 O matrix and its subsequent release as subsurface heating progresses after perihelion. The timing of CH 4 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.