FUV Observations of the Inner Coma of 46P/Wirtanen

Abstract

Far-ultraviolet observations of comets yield information about the energetic processes that dissociate the sublimated gases from their primitive surfaces. Understanding which emission processes are dominant, their effects on the observed cometary spectrum, and how to properly invert the spectrum back to the composition of the presumably pristine surface ices of a comet nuclei are all critical components for proper interpretation and analysis of comets. The close approach of comet 46P/Wirtanen in 2018–2019 provided a unique opportunity to study the innermost parts of a cometary coma with the Hubble Space Telescope Cosmic Origins Spectrograph, rarely accessible with remote observations, at length scales (hundreds of kilometers) and wavelengths (900–1430 Å) previously probed only by the European Space Agency’s Rosetta spacecraft. Our observations show a complex picture for the inner coma; atomic production rates for H and O that show water is the dominant source of both, an abundance of atomic sulfur that is difficult to explain with the lifetimes of common sulfur parent molecules, and a density distribution that is poorly fit with both Haser and vectorial models.

Narrowband Observations of Comet 46P/Wirtanen During its Exceptional Apparition of 2018/19 II: Photometry, Jet Morphology, and Modeling Results

“…This raises the intriguing possibility that H 2 O may not be the primary driver of Wirtanen's activity, or at least that another volatile parent species contributes non-negligibly. However, Wirtanen was found to be poor in CO (Biver et al 2019a;Saki et al 2020), while derived atomic production rates in the far UV suggest that CO 2 and O 2 were "not abundant" in comparison to H 2 O (Noonan et al 2021). Thus, there is no obvious volatile species that might explain the behavior.…”

The Evolution of Activity and Chemical Composition in Rosetta’s Comet Targets across Multiple Apparitions: Complications for CS2 as the CS Parent in Comet Nuclei

“…We focus instead on the detection of OH and CS emissions in the G230L spectra. As with the COS observations discussed in Noonan et al (2021), the STIS observations were conducted with three pointing schemes relative to the optocenter of the comet in hopes of observing different physical processes in the coma. Two sets of spectra were obtained using the 52″ × 0 5 STIS slit at angular offsets of 0″, 2 5, and 8″ toward the Sun, with the offsets performed as close to the Sun/comet vector as HST's slit orientation would allow.…”

“…As mentioned in Section 1, there are contemporaneous observations taken with COS in the same observing campaign (GO-15625; PI: D. Bodewits). The 46P COS data have been discussed in a separate publication (Noonan et al 2021) and are briefly revisited here, while the 67P COS data are presented here using the same reduction methods. Only the S I λ1479 multiplet was detected and analyzed from the 67P COS data set.…”

“…In this paper we discuss observations of 46P and 67P obtained 2019 January 9-20 and 2021 October 21 and December 11, respectively, with HST/STIS between wavelengths of 2300 and 3200 Å. We also reanalyze COS far-UV data from Noonan et al (2021) to compare to COS data of 67P collected on 2021 October 18. In Section 2 we review the observing strategy, as well as the STIS instrument and data reduction.…”

The Transition from the Kuiper Belt to the Jupiter-Family (Comets)

“…The search for sulfur in the comae of JFCs has taken on new importance, as the presence or absence of sulfur-bearing materials in JFCs would provide significant insights regarding the compositional variations in the protoplanetesimal disk. The recent strong detection of atomic sulfur at 1425 Å in the JFC 46P/Wirtanen rivals that of atomic hydrogen (Noonan et al 2021), and is consistent with emission directly from the nucleus or from grains very near the surface, similar to atomic sulfur and other sulfur-bearing species in 67P/Churyumov-Gerasimenko (Calmonte et al 2016). Additional searches are underway for other sulfur-bearing species in JFCs to test models of cometary ices and their subsequent processing history (Presler-Marshall et al 2020;Saki et al 2020;Altwegg et al 2022).…”