The electronic spectroscopy of resonance-stabilised hydrocarbon radicals

The photochemistry of simple molecules containing carbon, hydrogen, nitrogen, and oxygen atoms in the atmosphere of Titan has been investigated using updated chemical schemes and our own estimates of a number of key rate coefficients. Proper exospheric boundary conditions, vertical transport, and condensation processes at the tropopause have been incorporated into the model. It is argued that he composition, climatology, and evolution of Titan's atmosphere are controlled by five major processes: (a) photolysis and photosensitized dissociation of CH4; (b) conversion of H to H2 and escape of hydrogen; (c) synthesis of higher hydrocarbons; (d) coupling between nitrogen and hydrocarbons; (e) coupling between oxygen and hydrocarbons. Starting with N2, CH4, and H2O, and invoking interactions with ultraviolet sunlight, energetic electrons, and cosmic rays, the model satisfactorily accounts for the concentrations of minor species observed by the Voyager IRIS and UVS instruments. Photochemistry is responsible for converting the simpler atmospheric species into more complex organic compounds, which are subsequently condensed at the tropopause and deposited on the surface. Titan might have lost 5.6 x 10(4), 1.8 x 10(3), and 4.0 g cm-2, or the equivalent of 8, 0.25, and 5 x 10(-4) bars of CH4, N2, and CO, respectively, over geologic time. Implications of abiotic organic synthesis on Titan for the origin of life on Earth are briefly discussed.

Recent advances in the understanding of the chemical processes that occur during all stages of the formation of stars, from the collapse of molecular clouds to the assemblage of icy planetesimals in protoplanetary accretion disks, are reviewed. Observational studies of the circumstellar material within 100–10,000 AU of the young star with (sub)millimeter single-dish telescopes, millimeter interferometers, and ground-based as well as space-borne infrared observatories have only become possible within the past few years. Results are compared with detailed chemical models that emphasize the coupling of gas-phase and grain-surface chemistry. Molecules that are particularly sensitive to different routes of formation and that may be useful in distinguishing between a variety of environments and histories are outlined. In the cold, low-density prestellar cores, radicals and long unsaturated carbon chains are enhanced. During the cold collapse phase, most species freeze out onto the grains in the high-density inner region. Once young stars ignite, their surroundings are heated through radiation and/or shocks, whereupon new chemical characteristics appear. Evaporation of ices drives a “hot core” chemistry rich in organic molecules, whereas shocks propagating through the dense envelope release both refractory and volatile grain material, resulting in prominent SiO, OH, and H2O emission. The role of future instrumentation in further developing these chemical and temporal diagnostics is discussed.