  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%.
