Methane Formation Efficiency on Icy Grains: Role of Adsorption States

TL;DR

This study investigates how different adsorption states of carbon atoms on icy dust grains influence methane formation in space, revealing that only physisorbed C atoms efficiently produce methane, with a maximum conversion rate of 30%.

Contribution

It demonstrates that adsorption states significantly impact methane synthesis, challenging the assumption of barrierless reactions in astrochemical models.

Findings

Only physisorbed C atoms produce CH4 on ice.

Maximum C to CH4 conversion efficiency is 30%.

Adsorption states critically influence methane formation in space.

Abstract

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%.