ISO Observations of Interstellar Ices and Implications for Comets

We report on first ISO observations of interstellar ices in the direction of the protostellar object RAFGL 7009S. Due to its extreme extinction this source represents a unique target for the detection of interstellar ices. Identified molecules include H2O, CO, and CO2together with13CO2, CH4, OCS, and H2CO. Other less firmly identified features are observed that appear in laboratory spectra of interstellar ice analogs. The evolution of interstellar dust grains plays an active role in interstellar chemistry and determines solid state and gas phase abundances. During their lifetime dust grains cycle between dense and diffuse clouds and undergo considerable metamorphism. Comets are likely the least evolved bodies in the Solar System and comet nuclei may be low density aggregates of interstellar dust. An important constraint for the origin and evolution of comets can be derived from the presence of pre-solar ices and organics. To study volatiles and grains in the cometary coma is one of the future goals of the ROSETTA comet rendezvous mission. In comparison with new ISO data we present laboratory studies on interstellar ice analogs which reveal the composition and structure of ices in dense molecular clouds. We discuss the ubiquitous presence and high abundances of interstellar CO2ice in the cometary context and estimate column densities of molecules such as CH4, H2CO, and OCS which provide important constraints on the origin of cometary ices and for cometary evolution.