7 - Interstellar grain chemistry

Summary

Introduction

The subject of this article is dust in dense interstellar clouds – its composition and its chemical evolution. The relevant astronomical data include extinction and polarization in the infrared, visible and ultraviolet spectral ranges. Identification of the carriers of the observed spectral features is a non-trivial task. The features appear to be notoriously non-unique; significant fractions of them have been assigned to two or more dissimilar materials. Also, it now appears that grains are made largely of highly disordered and/or composite materials. In disordered mixtures spectral features of molecules can be altered considerably with respect to known spectra of pure crystalline materials, which also complicates the identification.

Clearly, additional sources of information are needed. The sources which I chose to employ are the recently available data on the composition of Halley's comet, and on the structure, the composition and the spectral properties of the interplanetary dust particles (IDPs). I thus assume that cometary and IDP materials preserve many of the characteristics of the dust in the original interstellar cloud. As a rationale I quote a recent review of Geiss (1987) on the results of exploration of Halley's comet: (a)‘… The abundance data show that a large fraction of material in Halley's nucleus condensed at very low temperature’ (b)‘… comets are regular members of the solar system which have preserved the original charactistics of the condensed and accreted matter better than other bodies in this system.’