Time-sensitive Keck/NIRSPEC observations of the Rosetta target 67P/Churyumov-Gerasimenko during its favorable 2021 apparition

67P/Churyumov-Gerasimenko is the best-studied comet to date because of the unprecedented detail and insights provided by the Rosetta mission during 2014-2016. The comet has been placed in the carbon-chain-depleted taxonomic class based on ground-based observations of fragment coma species. However, previous apparitions, including in 2015, have been unfavorable for in-depth ground-based observations focused on native volatiles. The 2021 apparition of 67P provided excellent observing geometry, was the first perihelion passage since Rosetta, and was the most favorable one until at least the mid-2030s. With this motivation, a high-resolution, near-infrared spectroscopic observing campaign during October – December 2021 utilized the complementary strengths of the NASA IRTF (Gibb et al. presentation in this meeting) and the W. M. Keck Observatory.

We present results from long-slit spectroscopy with the NIRSPEC spectrograph at the Keck 2 telescope, which offered maximum sensitivity for ground-based long-slit infrared measurements. We report gas rotational temperatures, production rates or stringent upper limits, and compositional relative abundances for C2H2, C2H6, HCN, NH3, CH3OH, H2CO, and H2O. The spatial distributions of species with brighter emissions are also included. These recent ground-based studies of 67P are essential to better place the unique Rosetta results into the context of global coma measurements in comets studied as a population with remote sensing techniques.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate and grateful to have the opportunity to conduct observations from this mountain. Various co-authors gratefully acknowledge support by the NSF Astronomy and Astrophysics Research Grants program, a NASA Earth and Space Science Fellowship, and grants from the NASA Emerging Worlds, Solar System Workings, and Solar System Observations Programs. The ongoing synergy between Rosetta and ground-based observations benefited from discussions at ISSI-Bern during meetings of Team 361, “From Qualitative to Quantitative: Exploring the Early Solar System by Connecting Comet Composition and Protoplanetary Disk Models”.