Ice surface reactions: A key to chemical evolution in space

Chemical reactions on the surface of cosmic ice dust play an important role in chemical evolution in space. Among the many kinds of molecules observed, the abundances of some major species such as hydrogen molecules cannot be explained by gas-phase synthesis; therefore, surface reactions on cosmic dust are considered for the synthesis of such molecules. Experimental research on surface reactions relevant to cosmic ice dust has been conducted since the late 1980s. Early experiments were rather qualitative, and so the details of reactions were not clear. In the last decade, many scientists from various fields including surface science have been motivated to extend the knowledge in this area of research, and have performed sophisticated experiments regarding surface reactions on cosmic ice dust. The aim of this article is to review the recent progress in surface astrochemistry. In particular, focus is made on experiments concerning the synthesis of abundant molecules such as H2, H2O, CO2, H2CO and CH3OH on ice surfaces at very low temperatures.