Comets are thought to have accreted in the cold, outer portions of the protosolar nebula when our Solar System was forming, and they clearly contain a higher proportion of volatiles than materials formed closer in to the Sun. Storage of most comets in the cold, outer regions of the Solar System since their formation has probably helped minimize any subsequent parent body processing of their contents. As a result, cometary materials may represent samples that best preserve the original components from which our Solar System was made. Comparison of cometary and asteroidal materials (which formed much closer to the Sun) can also provide insights into large-scale heterogeneity and transport of materials in the early solar nebula. Comets are also of considerable interest since they may have delivered volatiles, like H2O, to the early cooling Earth, thereby playing a key role in making the Earth habitable. Comets should also have delivered organic materials to the surface of the early Earth and these, depending on their nature, may have played a role in the origin of life. Thus, the study of the composition of comets has the potential to provide important insights into the formation and evolution of our Solar System (and by extension, other planetary systems), and the creation of an inhabited Earth.
The study of the chemistry, mineralogy, and isotopic distributions of oxygen in cometary materials can provide unique information that addresses these issues. Our current knowledge of the nature of oxygen in comets is based on several different lines of evidence, including remote telescopic and spacecraft observations of comets, direct laboratory analyses of extraterrestrial samples, and in situ measurements of a small number of individual comets. Information derived from these different approaches suggests that the chemical, mineralogical, and isotopic state of oxygen in these primitive bodies is extremely variable, i.e., comets appear to be made up of a wide range of very different components that are considerably out of equilibrium with each other. This is consistent with the idea that cometary materials should have largely escaped extensive parent body processing.
The study of samples recently returned from Comet 81P/Wild 2 by the Stardust spacecraft has immensely improved our understanding of cometary materials. Of particular interest is the observation that these samples contain intimate mixtures of both volatile and refractory oxygen-bearing materials. This suggests that while comets formed in the outer reaches of the protosolar disk, they were constructed of materials that had originally formed and evolved in a wide variety of locations that spanned essentially the entire radial extent of the protosolar nebula.