Using Cometary Activity to Trace the Physical and Chemical Evolution of Cometary Nuclei

Historically, minor bodies are classified as comets based on observations of “activity,” which refers to the appearance of dust and gas around the nucleus. While cometary activity has been observed for centuries, at ever-increasing heliocentric distances, our understanding of the mechanisms for producing cometary activity and its heliocentric dependence has only recently been developed to the level of sophistication needed to make detailed comparison with the observations. A thorough understanding of cometary activity is closely coupled with knowledge about the formation of comets, thermal models of cometary nuclei, and chemistry in the coma. This chapter summarizes the specific chemical and physical changes that a comet nucleus undergoes, concentrating on the active phases. The specific drivers of activity are discussed, as well as the means of measuring the activity in comets. Finally, some historical and modern examples of specific types of cometary activity are discussed and are used to make inferences about both primordial differences between comet dynamical classes and evolutionary, or aging, effects.

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