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Methane is one of the simplest stable molecules that is both abundant andwidely distributed across space. It is thought to have partial origin frominterstellar molecular clouds, which are near the beginning of the starformation cycle. Observational surveys of CH$_4$ ice towards low- and high-massyoung stellar objects showed that much of the CH$_4$ is expected to be formedby the hydrogenation of C on dust grains, and that CH$_4$ ice is stronglycorrelated with solid H$_2$O. Yet, this has not been investigated undercontrolled laboratory conditions, as carbon-atom chemistry of interstellar iceanalogues has not been experimentally realized. In this study, we successfullydemonstrate with a C-atom beam implemented in an ultrahigh vacuum apparatus theformation of CH$_4$ ice in two separate co-deposition experiments: C + H on a10 K surface to mimic CH$_4$ formation right before H$_2$O ice is formed on thedust grain, and C + H + H$_2$O on a 10 K surface to mimic CH$_4$ formedsimultaneously with H$_2$O ice. We confirm that CH$_4$ can be formed by thereaction of atomic C and H, and that the CH$_4$ formation rate is 2 timesgreater when CH$_4$ is formed within a H$_2$O-rich ice. This is in agreementwith the observational finding that interstellar CH$_4$ and H$_2$O formtogether in the polar ice phase, i.e., when C- and H-atoms simultaneouslyaccrete with O-atoms on dust grains. For the first time, the conditions thatlead to interstellar CH$_4$ (and CD$_4$) ice formation are reported, and can beincorporated into astrochemical models to further constrain CH$_4$ chemistry inthe interstellar medium and in other regions where CH$_4$ is inherited.

An experimental study of the surface formation of methane in interstellar molecular clouds