Biomolecules in the Interstellar Medium and in Comets

We review recent studies of organic molecule formation in dense molecular clouds and in comets. We summarise the known organic inventories of molecular clouds and recent comets, particularly Hale-Bopp. The principal chemical formation pathways involving gas phase reactions, as well as formation by catalytic reactions on grain surfaces or through dust fragmentation, are identified for both dense clouds and cometary comae. The processes leading to organic molecules with known biological function, carbon chains, deuterium fractionation, HNC and S-bearing compounds are described. Observational searches for new interstellar organics are outlined and the connection between observed interstellar organics and those detected in comets Hale-Bopp and Hyakutake are discussed. INTRODUCTION The birth sites of solar-mass stars and their planetary systems are dense interstellar clouds. Such clouds provide the building blocks, in the form of interstellar molecules and dust, for the formation of planets, comets, asteroids, and other macroscopic bodies in protostellar disks (Ehrenfreund & Charnley 2000; Irvine et al. 2000; Lunine et al. 2000). These clouds are the site of an extremely active and complex chemistry; to date over 100 molecules have been detected in them. Many of these molecules are also observed in protoplanetary disks and comets. A central question in the fields of astrochemistry and astrobiology is how much of the material present in primitive bodies, such as comets and asteroids, is pristine interstellar material? In other words, to what extent does the chemical inventory of these objects and the planets reflect chemical processing, firstly in the collapsing cloud, and subsequently in the protoplanetary disk (e.g. van Dishoeck & Blake 1998; Langer et al. 2000)? Answering this question is of fundamental concern to two areas of research. Firstly, chemical abundances, together with isotopic and ortho-para ratios, are sensitive tracers of the molecule formation conditions and so differences between the chemical inventory of the interstellar medium (ISM) and the solar system can be used to constrain models of low-mass star formation, many aspects of which are not well understood. Secondly, the molecules in the early solar nebula provided the seed from which life eventually formed. It is thought that a great deal of organic material was deposited on the young Earth by cometary and asteroid impacts (Chyba et al. 1990). Many of the organic molecules detected in dense interstellar clouds have important functions in terrestrial biochemistry. The presence of extraterrestrial amino acids in meteorites (e.g. Cronin & Pizzarello 1983) shows that extremely complex molecules are able to form in space. In order to assess the implications for life elsewhere in the universe, we need to understand if such complexity is a general feature of interstellar and/or protoplanetary disk chemistry, or if these molecules arose due to specific conditions in our solar system. We can obtain clues to the above question by comparing observations of comets – thought to be the most pristine objects in our solar system – with those of protoplanetary disks, protostellar cores, and interstellar clouds (Ehrenfreund et al. 1997; Ehrenfreund 2000; Ehrenfreund & Charnley 2000; Bockelée-Morvan et al. 2000; Irvine et al. 2000).