Predicting the chemistry happening inside interstellar ice

In the search for sources of complex organic molecules in space, some researchers have turned to hydrocarbon ices, which exist in interstellar space and on bodies in our solar system, such as Saturn’s moon Titan. Experiments have shown that high-energy electrons generated by ionizing radiation can induce formation of complex organic molecules on these ices. However, researchers still aren’t sure which chemicals form inside of the ices and which form on their surface or in nearby gases. Existing models of space chemistry can’t predict the products of such reactions.

At the American Chemical Society national meeting in Boston on Monday, researchers reported that they used tunable lasers to identify which isomers of small hydrocarbon molecules form within methane ice in laboratory experiments simulating interstellar conditions. Matthew Abplanalp, a graduate student in Ralf I. Kaiser’s group at University of Hawaii, Manoa, described their findings during a session in the Physical Chemistry Division.

The researchers held methane ice at 5.5 K inside an ultrahigh vacuum, bombarded it with 5 kV electrons, and then allowed the ice to heat up to room temperature. They developed a tunable vacuum ultraviolet laser to selectively ionize isomers sublimating from the ice. Using fourier-transform infrared spectroscopy and reflectron time-of-flight mass spectrometry, the team identified methyl acetylene, propene, 1,3-butadiene, and vinyl acetylene. These are known precursors of polycyclic aromatic hydrocarbons, species that share structural motifs with some biological molecules.

Kyle Crabtree, an astrochemist at the University of California, Davis, who presided over the session, said the group’s work is pushing forward our understanding of reactions in interstellar ice. Crabtree explained that adding the results of the experiments to theoretical space chemistry models will help astronomers look for these chemicals.

Kaiser’s group is now working on a new version of these experiments using a synchrotron at the University of Science and Technology of China. Abplanalp said the synchrotron will allow them to complete in one week what took him a month and predict more products of the reactions in ice.