Interstellar and Cometary Dust in Relation to the Origin of Life.

Cometary grains are commonly considered to be the precursors of organic materials in early life on the Earth. It is a persistent mystery, however, how chemical evolution within the earliest protocells was triggered. We discuss the type of organics measured from Comet p/Wild-2, using the Cometary and Intersellar Dust Analyzer (CIDA). This instrument is not sensitive to mineralic cores of dust particles due to their low-impact velocities (6 km/s). The CIDA data allows us to analyze the organic fraction of interstellar dust at ~1 AU from the Sun. Surprisingly, it mainly consists of quinone-type polycyclics, some of them similar to pyrrolo-Cquinoline-Cquinone. Strong redox catalysts are apparently required to trigger the earliest chemical evolutionary steps. Our understanding of the thermochemical evolution of accretion from interstellar clouds containing cosmic grains to comets qualitatively and quantitatively requires a comprehensive understanding of the production of water and CO from the comets. The"dirty snowball" model is rejected as a suitable model for "fresh" comets. Apparently the primordial source of water is twofold: (1) a signicant fraction of cometary clathrates is contained in the mineralic and the organic fractions, producing water by evaporation; (2) rich oxygen containing PAH's in the organic phase produces water by solar radiation-induced chemical processes. [ABSTRACT FROM AUTHOR]