Comets

Abstract

Prior to spacecraft missions to comets, beginning with the Giotto and Vega spacecraft to comet 1P/Halley (the P/ signifies a periodic comet) in March 1986, determinations of the volatile composition of a cometary coma were derived from spectroscopic analyses. Detailed modeling was then used to infer the volatile composition of the cometary nucleus. Since then there have been additional missions, including Deep Impact (2005), Stardust (2006), EPOXI (2010), and most recently, Rosetta, that flew along with comet 67P/Churyumov-Gerasimenko for over 2 years, that have greatly advanced our knowledge of the physics, chemistry, and geology of comets. Only a few comets have been studied this way, but the results provide a framework, or “ground truth”, for remote observations from Earth and spaceborne observatories, both past and future. This chapter focuses on the principal atomic and molecular processes that lead to the observed spectrum as well as the needs for basic atomic and molecular data in the interpretation of these spectra. The largely collisionless and low-density coma, with no gravity or magnetic field, is a unique spectroscopic laboratory, and in recent decades, observations of comets have expanded over the entire electromagnetic spectrum, from X-ray to the submillimeter and millimeter. Although Rosetta has elucidated many key questions, issues concerning the basic molecular composition and the elemental abundances of both the volatile and refractory components of the cometary nucleus, as well as the comet-to-comet variation (particularly between new and evolved periodic comets) of these quantities still remain to be solved. A recent (although slightly out-of-date) compendium covering all aspects of cometary science is the Comets II book 1 , while a review of coma chemistry is given by Mumma and Charnley 2 .