FUV Observations of the Inner Coma of 46P/Wirtanen

Far ultraviolet observations of comets yield information about the energetic processes that dissociate14 the sublimated gases from their primitive surfaces. Understanding which emission processes are dom15inant, their eects on the observed cometary spectrum, and how to properly invert the spectrum back16 to composition of the presumably pristine surface ices of a comet nuclei are all critical components for17 proper interpretation and analysis of comets. The close approach of comet 46P/Wirtanen in 2018-201918 provided a unique opportunity to study the inner most parts of a cometary coma with the Hubble19 Space Telescope Cosmic Origins Spectrograph, rarely accessible with remote observations, at lengthscales (100's of km) and wavelengths (900-1430 A~)20 previously probed only by the European Space21 Agency's Rosetta spacecraft. Our observations show a complex picture for the inner coma; atomic22 production rates for H and O that show water is the dominant source of both, an abundance of atomic23 sulfur that is dicult to explain with the lifetimes of common sulfur parent molecules, and a density24 distribution that is poorly t with both Haser and vectorial models.