The Return of the Rosetta Target: Keck Near-infrared Observations of Comet 67P/Churyumov–Gerasimenko in 2021

Abstract

High-resolution near-infrared ground-based spectroscopic observations of comet 67P/Churyumov–Gerasimenko near its maximum activity in 2021 were conducted from the W. M. Keck Observatory, using the facility spectrograph NIRSPEC. 67P is the best-studied comet to date because of the unprecedented detail and insights provided by the Rosetta mission during 2014–2016. Because 67P is the only comet where the detailed abundances of many coma volatiles were measured in situ, determining its composition from the ground provides a unique opportunity to interpret Rosetta results within the context of the large database of ground-based compositional measurements of comets. However, previous apparitions, including in 2015, have been unfavorable for in-depth ground-based studies of parent volatiles in 67P. The 2021 apparition of 67P was thus the first-ever opportunity for such observations. We report gas spatial distributions, rotational temperatures, production rates, and relative abundances (or stringent upper limits) among seven volatile species: C2H2, C2H6, HCN, NH3, CH3OH, H2CO, and H2O. The measured abundances of trace species relative to water reveal near average or below average values compared to previous comets studied at infrared wavelengths. Both gas rotational temperatures and the spatial distributions of H2O, C2H6, and HCN measured with Keck-NIRSPEC in 2021 are consistent with the outgassing patterns revealed by Rosetta in 2015 at very similar heliocentric distance (post-perihelion). These results can be integrated with both Rosetta mission findings and ground-based cometary studies of the overall comet population, for which we encourage a wide-scale collaboration across measurement techniques.

The Ice Chemistry in Comets and Planet-forming Disks: Statistical Comparison of CH3OH, H2CO, and NH3 Abundance Ratios

“…Methanol measurements from Rosetta in 67P show on average much lower relative abundances (≈0.2% and 0.9% for the pre-and post-perihelion measurements, respectively) compared to our database median value (2.31%). Nevertheless, these values are in agreement with those of 0.58% ± 0.10%, 0.86% ± 0.14%, and 0.65% ± 0.12% reported by Bonev et al (2023), who observed 67P with NIRSPEC-Keck in 2021. The (sub)millimeter measure of 1.2% ± 0.1% reported in Biver et al (2023) is slightly higher than the Rosina one, but the authors also report a water production rate derived from Odin and Nançay observations that is around half that reported by Läuter et al (2020).…”

“…Finally, it is interesting to notice that data for 67P (Läuter et al 2020;Biver et al 2023;Bonev et al 2023) may lead to a slightly different conclusion if used alone. In fact, 67P shows a lower range for the [CH 3 OH]/[H 2 CO] ratio, with values ranging from 0.5 to 13.4.…”