Revisiting Rosetta’s Targets: Comparative Ultraviolet Spectroscopy of 46P/Wirtanen and 67P/Churyumov-Gerasimenko with HST

As soon as the European Space Agency Rosetta mission decided to target comet 46P/Wirtanen, a broad campaign was organized to characterize its activity and bulk composition of its coma. In the UV, observations were obtained with the Faint Object Spectrograph on the Hubble Space Telescope (HST) in 1996 and 1997. These provided information on the OH, CS, and C2 production rates and on the nucleus’ reflectance, which was used to guide the development of the Alice ultraviolet spectrograph (UVS) that would fly onboard Rosetta. Eventually, those wavelengths would not be sampled by Alice due to a design decision to focus on atomic emissions (and thus a bandpass that excluded the near UV (2000-3200 Å) diatomic features (Stern et al. 1997). The delay in launch of the Rosetta launch date necessitated a change in target to something more dynamically favorable, which was notably larger and had less active area relative to its size; 67P/Churyumov-Gerasimenko.

No spectral UV observations of the broad coma of 67P were obtained prior to the arrival of the Rosetta with the exception of a single set of IUE observations (Feldman et al. 2004). The limited snapshot characterization of the OH, CS, and C2 emissions by that study were not sufficient to inform rigorous analysis of the far UV (800-2000 Å) spectra, where atomic S was particularly abundant and the parent molecule difficult to constrain (Feaga et al. 2015, Feldman et al. 2017).

We obtained HST Space Telescope Imaging Spectrograph (STIS) observations of 46P and 67P during their most recent apparitions in 2019 and 2021, respectively, to compare the NUV molecular emissions of Rosetta’s targets, investigate the atomic sulfur parent molecule CS2, and place them in context with Alice UVS observations of 67P’s near-nucleus coma to understand just how different these two targets are, both in terms of activity and chemistry.

Acknowledgements: Based on observations with the NASA/ESA/CSA Hubble Space Telescope obtained from the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under. NASA contract NAS5-26555. We’d like to acknowledge Space Telescope Science Institute funding that supported this research: observations of 46P were supported by HST-GO-15625, observations of 67P were supported by HST-GO-16049. Funds that support these programs were provided through grants from the STSCI under NASA contract NAS-26555.