ISO OBSERVATIONS OF INTERSTELLAR ICES AND IMPLICATIONS FOR COMETS

We report on first ISO observations of interstellar ices in the direct ion of the protostellar object RAFGL 7009S. Due to its extreme extinct ion this source represents a unique target for the detection of inters tellar ices. Identified molecules include H2O, CO, and CO2 together wi th (CO2)-C-13, CH4, OCS, and H2CO. Other less firmly identified featur es are observed that appear in laboratory spectra of interstellar ice analogs, The evolution of interstellar dust grains plays an active rol e in interstellar chemistry and determines solid state and gas phase a bundances. During their lifetime dust grains cycle between dense and d iffuse 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 f or 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 rendezvo us mission, In comparison with new ISO data we present laboratory stud ies on interstellar ice analogs which reveal the composition and struc ture of ices in dense molecular clouds. We discuss the ubiquitous pres ence and high abundances of interstellar CO2 ice in the cometary conte xt and estimate column densities of molecules such as CH4, H2CO, and O CS which provide important constraints on the origin of cometary ices and for cometary evolution. (C) 1997 Academic Press.