Experimental evidence for interstellar methane ice formation in the water-rich ice phase of molecular clouds

Methane (CH4) is one of the most abundant and widespread molecules in the universe. Observational studies of methane ice towards low- and high-mass young stellar objects suggest that much of the observed methane should originate from grain surface chemistry [1]. Particularly, its peak in formation is thought to occur at the edge of interstellar molecular clouds, where atomic C and O are largely available to accrete onto dust grains [1]. These atoms become barrierlessly hydrogenated to form simple hydrides, such as methane and water, respectively. Although this formation route to methane in water-rich ices has been observationally constrained and is included in astrochemical models, such a pathway has not been experimentally confirmed, thus causing ambiguity to the idea of interstellar methane formation. From an experimental perspective, this is not surprising, as the study of C-atom chemistry in interstellar ice analogues is technically challenging. In this contribution, the formation of methane starting from atomic C and H in a water-rich ice at 10 K under ultrahigh vacuum conditions is overviewed. It is demonstrated that methane can be formed by both: sequential hydrogenation of C and sequential hydrogenation of C in a water-rich ice, in which the latter has twice the formation rate. This supports ice observations that methane is formed also when water is formed. For the first time, values such as the formation yields and rates obtained under various experimental conditions are provided, and can be included into astrochemical models to enhance the overall understanding of methane, from its origin in interstellar molecular clouds to regions where it is distributed. [1] Oberg, K.I., Boogert, A.C.A., Pontoppidan, K.M., et al 2008, ApJ 678 1032