Rapporteur Paper on the Compositon of Comets

The ISSI workshop on “Origin and evolution of comet nuclei” had the goal to put together recent scientific findings concerning the “life” of a comet from the formation of the material in a dark molecular cloud to the accretion in the early solar system, from cometesimals to comet nuclei which were shaped and altered by cosmic rays, by radioisotopic heating, to their sublimation in the inner solar system. Astronomers, space researchers, modelers and laboratory experimentalists tried to draw the coherent picture. However, it became clear that there are still a lot of open questions, findings which seem to contradict each other, missing laboratory data, and experimental biases not taken into account. The Rosetta mission will make a big step forward in cometary science, but it will almost certainly not be able to resolve all questions. The main outcome of this workshop was the fact that comets are much more diverse than commonly thought and they are not only different from comet to comet but may consist of morphologically and chemically inhomogeneous cometesimals which may even have different places of origin.