The Comet Cipher: Understanding the Ultraviolet Emissions of Cometary Comae

Abstract

The study of cometary activity as a strategy to probe the icy planetesimals from early solar system formation is a relatively new endeavor. The effort began in earnest after observational confirmation in the late 1960's that the vast amounts of hydrogen and hydroxyl (OH) molecules in cometary comae are evidence of an icy object sublimating water into space. In the decades since, a key goal of solar system science has been to observe more comets with a wider array of techniques in order to constrain the initial molecular building blocks of our solar system, characterize composition patterns between comet types, and identify any peculiarities that may make our solar system more suitable for life than any other. However, cometary activity is an uncooperative partner in this investigation; at times the activity can be sporadic and weak, at other times explosive, and at far fewer times predictable. Understanding what processes can influence observations of cometary activity, specifically in the ultraviolet (UV) wavelengths of light, is the motivation for this Dissertation. Four projects are described in this work that probe how the UV emissions from two comets, 67P/Churyumov-Gerasimenko and 46P/Wirtanen, reflect the outgassing molecules from their surface and the processes that govern their emissions. Following an introduction about comet formation, dynamics, and observations, three chapters are dedicated to observations from the Alice UV spectrograph to investigate emission processes near (within 100s of km) the nucleus of 67P. The processes can range from outbursts of material from the cometary surface to outbursts of energetic plasma ejected from the Sun, revealing new science about the relevant scales for photon and collisional excitation mechanisms. The lessons learned from near-nucleus coma observations at 67P are then applied to 46P with a remote observing campaign using the Hubble Space Telescope to search for similar traces of the emission mechanism. Ultimately this Dissertation reflects on the implications of the work on the larger astrophysical community and lays out future directions for UV comet research.