ABSTRACT
We have seen that interstellar space is both the burial ground for remnants of stellar systems which have expired and a place where stars and planets are born. Interstellar matter is reprocessed in the course of successive generations of stars in violent events at extremely high pressures and temperatures.
An essentially different type of chemistry is considered in this chapter. It concerns the rarefied mixture of gas and dust between the stars , which represents several percent of the mass of a galaxy. This chemistry takes place at extremely low densities of matter and low temperatures. Measuring equipment set up on the Earth and in satellites is presently supplying an ever-increasing store of information on the presence of fairly complex molecules , and about 100 species comprising ions and free radicals have been reported. The latter are produced to a large extent by ionizing radiation and are mainly responsible for the interstellar syntheses.
Radiation and radioactivity must also be taken into account in the investigation of comets and meteorites. Comets are bodies composed of ice and dust with a diameter up to a few kilometers. They travel through the confines of the Solar System as a cometary cloud until some of them approach the inner Solar System. Meteorites are bodies , with a size ranging from micrometers to several meters , which reach the Earth's ’s surface after revolving around the Sun. As relics of events that took place almost 5000 million years ago , they provide us with information from the era prior to the formation of the Solar System from presolar nebulas.
The spectacular close encounter with Halley ’s comet in 1986 provided a wealth of data. They suggest that prolonged exposures to cosmic rays have led to important chemical changes in primordial cometary material. Laboratory experiments show that radiation-induced synthesis can lead to significant amounts of fairly complex organic molecules. In the early period of the Solar System when cometary impacts occurred , organic compounds may have reached the primitive Earth. Cosmic rays also induce the formation of radionuclides , which are of particular help for the radioactive dating of meteorites and which provide reliable information on the time scale of various events in the life of the Solar System.
