Delayed Methane Detection in 3I/ATLAS: GCR-Driven Subsurface Stratification and Interstellar Exposure Age Constraints
Abstract
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.
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BibTeX
@misc{rawat2026delayed,
title = {{Delayed Methane Detection in 3I/ATLAS: GCR-Driven Subsurface Stratification and Interstellar Exposure Age Constraints}},
author = {Akshat Rawat and Kinsuk Acharyya},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.19694},
eprint = {2609.19694},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.19694},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Delayed Methane Detection in 3I/ATLAS: GCR-Driven Subsurface Stratification and Interstellar Exposure Age Constraints
AU - Akshat Rawat
AU - Kinsuk Acharyya
PY - 2026
DA - 2026/09/17
PB - arXiv
DO - 10.48550/arxiv.2609.19694
UR - https://arxiv.org/abs/2609.19694
AB - Interstellar comet 3I/ATLAS shows unusual volatile activity, including strong H_2O, 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_2O, 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_2O 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.
ER -