Astrochemistry

Methane Formation Efficiency on Icy Gains: Role of Adsorption States

By Keith Cowing
Status Report
astro-ph.GA
July 25, 2024
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Methane Formation Efficiency on Icy Gains: Role of Adsorption States
Schematic of the evolution of C atoms accreted onto icy grains at a low temperature (~10 K) under molecular cloud conditions. The fraction represents the probability of each reaction pathway and not the distribution of binding sites. — astro-ph.GA

Methane (CH4) is one of the major components of the icy mantle of cosmic dust prevalent in cold, dense regions of interstellar media, playing an important role in the synthesis of complex organic molecules and prebiotic molecules.

Solid CH4 is considered to be formed via the successive hydrogenation of C atoms accreting onto dust: C + 4H -> CH4. However, most astrochemical models assume this reaction on the ice mantles of dust to be barrierless and efficient, without considering the states of adsorption. Recently, we found that C atoms exist in either the physisorbed or chemisorbed state on compact amorphous solid water, which is analogous to an interstellar ice mantle.

These distinct adsorption states considerably affect the hydrogenation reactivity of the C atom. Herein, we elucidate the reactivities of physisorbed and chemisorbed C atoms with H atoms via sequential deposition and co-deposition processes.

The results indicate that only physisorbed C atoms can produce CH4 on ice. Combining this finding with a previous estimate for the fraction of physisorbed C atoms on ice, we determined the upper limit for the conversion of C atoms into CH4 to be 30%.

Masashi Tsuge, Germán Molpeceres, Yuri Aikawa, Naoki Watanabe

Comments: 24 pages, 4 figures, 1 table; accepted for publication in ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2407.13057 [astro-ph.GA] (or arXiv:2407.13057v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2407.13057
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Submission history
From: Masashi Tsuge
[v1] Wed, 17 Jul 2024 23:58:22 UTC (649 KB)
https://arxiv.org/abs/2407.13057

Astrobiology, Astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻