Molecules are fragile objects; they cannot survive long in normal interstellar space because they are usually rapidly destroyed in bombardment by ultraviolet starlight. It follows, therefore, that the molecules we detect were formed only recently and that the dense clouds where we see them are the chemical ‘factories’ where they are produced. For a chemical reaction to take place, the participating atoms or molecules must collide or, at least, approach each other very closely. The clouds where molecules are found must therefore be relatively dense, and collisions in them frequent occurrences. Also, in order to prevent the destruction of molecules by starlight, a cloud rich in molecules must contain a high enough concentration of dust grains that light is excluded from its interior. This dust may serve an additional role hi aiding the actual production of molecules, in that gas atoms landing on the surface of a dust grain may combine with other atoms already there, using the dust surface as a catalyst.

Some clues as to how interstellar molecules are formed may be obtained by examining the classes of molecules that are known to exist and, just as important, which molecules up to now have not been detected. As we would expect if large molecules are made by synthesis from small ones, we find that complicated molecules are in general rarer than simple ones. A more unexpected phenomenon is that in almost every case where two carbon atoms are joined together, it is by a single bond (C—C) or a triple bond (C=C) rather than by a double bond (C=C). Another strange feature is that no oxide of nitrogen, nor any molecule containing a nitrogen-oxygen bond has yet been seen. Similarly ring molecules, such as pyridine (C6Hr)N), although common in terrestrial chemistry, are rare in interstellar space. From clues such as these, astronomers are trying to work out the chemical reactions by which molecules are synthesized. One possibility under discussion is that while the simplest molecules are formed catalytically on the surface of grains, the more complicated molecules are built up by direct collisions among the gaseous molecules and ionized radicals. One of the difficulties in solving the problem of INTERSTELLAR SYNTHESIS is that many of the chemical reactions considered important in interstellar space are insignificant at terrestrial densities, and hence have been ignored until now. Consequently there is nearly as much activity by chemists in their laboratories discovering and measuring new reactions as there is among astronomers in their observatories looking for their products in space.

Although the chemistry of interstellar molecules has turned out to be much richer than anticipated, the complexity of the com¬pounds discovered to date is still many orders of magnitude less than that of even the simplest living organism. Biological cells are made up of vast chains of amino acids, the simplest of which is glycine (NH2.CH2.COOH). Even if, as it not unlikely, some individual amino acids are discovered in interstellar clouds, the implications for the origin of life on planets such as ours are few; any molecules present in the cloud out of which the Solar System formed would probably not have survived the heat given out as the Earth condensed. Thus, although there are some interesting analogies between biology and interstellar chemistry, there is probably no causal relationship between them.

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