Gas-surface interactions and heterogeneous chemistry on interstellar grains analogues

Detailed laboratory studies and progress in surface science technique, have allowed in recent years the first experimental confirmation of surface reaction schemes, as introduced by Tielens, Hagen and Charnley (1, 2). In this paper, we review few heterogeneous processes which give routes to form elementary molecules considered as precursors for explaining the variety and richness of molecular species in the interstellar medium. Adsorption, diffusion and reaction processes are discussed. With emphasis on the experimental approaches, but also supported by theoretical developments, progresses in the understanding of the "catalytic role" of a dust grain surface in various physical conditions are described. Recent advances made on few important species (H2 ,H 2O, CH3OH) are used to illustrate basic properties and raise open questions. Astrochemistry is fundamental to understand the formation of molecules in many locations in the universe, and especially in star forming regions where a wealth of molecular species is present. More than 150 molecules have been observed in interstellar and circumstellar environments, from the simplest and most abundant molecule of the universe H2, to traces of organic species with sizes up to 15 atoms. For more than 30 years, astronomers and chemists have investigated the gas-phase formation and destruction channels of various molecules from atomic matter. The understanding of the chemical evolution of the interstellar medium (ISM) has been a great stimulus to develop intense research programs around ion-molecule and neutral-neutral reactions. Theoretical models including a huge network of gas phase reactions are now able to explain most of the molecules detected in the gas phase (3, 4). It is now well recognized that gas-grain interactions are playing additional important role, in many regions of the interstellar space, enabling another - but still largely unknown - chemistry to occur through heterogeneous interaction between the gas and the surface of grains. This lack of knowledge about gas-grain interactions appears as a limitation for the interpretation of observations, especially in dense star forming regions, where the gas-dust interaction dominates any purely gas-phase process. Heterogeneous chemistry gives routes to form elementary molecules that are precursors for explaining the variety and richness of molecular species in the ISM. Important new efforts have been made to understand the interaction of atoms and molecules with surfaces of astrophysical interest, in physical conditions that are extreme and difficult to reproduce in the laboratory (low pressure, low temperature, intense radiation fields and fast protons and electrons). In this paper, we discuss some progress done in this area with emphasis made on experimental approaches.