Mid- and far-infrared spectroscopic studies of the influence of temperature, ultraviolet photolysis and ion irradiation on cosmic-type ices

Infrared (IR) studies of laboratory ices can provide information on the evo lution of cosmic-type ices as a function of different simulated space envir onments involving thermal, ultraviolet (UV), or ion processing. Laboratory radiation experiments can lead to the formation of complex organic molecule s. However, because of our lack of knowledge about UV photon and ion fluxes , and exposure lifetimes, it is not certain how well our simulations repres ent space conditions. Appropriate laboratory experiments are also limited b y the absence of knowledge about the composition, density, and temperature of ices in different regions of space. Our current understanding of expecte d doses due to W photons and cosmic rays is summarized here, along with an inventory of condensed-phase molecules identified on outer solar system sur faces, comets and interstellar grains. Far-IR spectra of thermally cycled H 2O are discussed since these results reflect the dramatic difference betwee n the amorphous and crystalline phases of H2O ice, the most dominant conden sed-phase molecule in cosmic ices. A comparison of mid-IR spectra of produc ts in proton-irradiated and UV-photolyzed ices shows that few differences a re observed for these two forms of processing for the simple binary mixture s studied to date. IR identification of radiation products and experiments to determine production rates of new molecules in ices during processing ar e discussed. A new technique for measuring intrinsic IR band strengths of s everal unstable molecules is presented. An example of our laboratory result s applied to Europa observations is included. (C) 2001 Elsevier Science B.V . All rights reserved.