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

• Published in: Nature astronomy Vol. 4; no. 8; pp. 781 - 785
• Main Authors: Qasim, D., Fedoseev, G., Chuang, K.-J., He, J., Ioppolo, S., van Dishoeck, E. F., Linnartz, H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.08.2020; Nature Publishing Group
• Subjects: 639/33/34/4117; 639/33/34/4125; 639/33/34/865; Astronomy; Astrophysics and Cosmology; Dust; Hydrogenation; Ice formation; Methane; Physics; Physics and Astronomy
• ISSN: 2397-3366; 2397-3366
• DOI: 10.1038/s41550-020-1054-y

Methane is one of the simplest stable molecules that is both abundant and widely distributed across space. Observational surveys of CH 4 ice towards low- and high-mass young stellar objects showed that much of the CH 4 is expected to be formed by the hydrogenation of C on dust grains, and that CH 4 ice is strongly correlated with solid H 2 O. However, this has not been investigated under controlled laboratory conditions. Here, we successfully demonstrate with a C-atom beam implemented in an ultrahigh vacuum apparatus the formation of CH 4 ice in two separate co-deposition experiments: C + H on a 10 K surface to mimic CH 4 formation directly before H 2 O ice is formed on the dust grain, and C + H + H 2 O on a 10 K surface to mimic CH 4 formed simultaneously with H 2 O ice. We confirm that CH 4 can be formed by the reaction of atomic C and H, and that the CH 4 formation rate is twice as high when CH 4 is formed within a H 2 O-rich ice. This is in agreement with the observational finding that interstellar CH 4 and H 2 O form together in the polar ice phase. The conditions that lead to interstellar CH 4 (and CD 4 ) ice formation are reported, and can be incorporated into astrochemical models to further constrain CH 4 chemistry in the interstellar medium and in other regions where CH 4 is inherited. Methane ice has been presumed to form via the sequential hydrogenation of carbon atoms on dust grains for many years, but now Qasim et al. have performed the experiment, with and without the presence of water. Methane forms more rapidly in the polar ice phase.