Coma composition of comet 67P/Churyumov-Gerasimenko from radio-wave spectroscopy^{★,★★,★★★}

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Université Paris-Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: nicolas.biver@obspm.fr
² Stockholm Observatory, Stockholm University, SCFAB-AlbaNova, 10691 Stockholm, Sweden
³ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁵ Solar System Exploration Division, Astrochemistry Laboratory Code 691, NASA-GSFC, Greenbelt, MD 20771, USA
⁶ Department of Physics, Catholic University of America, Washington, DC 20064, USA
⁷ Department of Physics, American University, 3501 Nebraska Ave NW, Washington, DC 20016, USA
⁸ Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
⁹ Solar System Exploration Division, Planetary System Laboratory Code 693, NASA-GSFC, Greenbelt, MD 20771, USA

Received: 12 December 2022
Accepted: 24 February 2023

Abstract

We present the results of a molecular survey of comet 67P/Churyumov-Gerasimenko undertaken with the Institut de RadioAstronomie Millimétrique (IRAM) 30-m radio telescope in November–December 2021, when it had its most favourable apparition in decades. Observations at IRAM 30-m during the 12–16 November period covered 8 GHz bandwidth at 3 mm, 16 GHz at 2 mm, and 60 GHz in the 1 mm window domain. These were completed by snapshots at 1 mm on 12–13 December and a short observation of the H₂O line at 557 GHz with the Odin sub-millimetre observatory on 17.0 November 2021, and with 18-cm observations of OH with the Nançay radio telescope. Less sensitive observations obtained at a previous perihelion passage on 18–22 September 2015 with IRAM and 9–12 November 2015 with Odin are also presented. The gas outflow velocity, outgassing pattern, and temperature have been accurately constrained by the observations. They are perfectly consistent with those measured in situ with the Rosetta/MIRO sub-millimetre instrument in 2015. In particular, the asymmetry of the line is well represented by a jet concentrating three-quarters of the outgassing in about π steradians. We derived abundances relative to water for seven molecules and significant upper limits for approximately five others. The retrieved abundances were compared to those measured in situ at the previous perihelion with Rosetta. While those of HCN, CH₃OH, and HNCO are comparable, 67P is found to be depleted in H₂S and relatively normal in CS (H₂S/CS ≈ 3) in strong contradiction with the Rosetta/ROSINA mass spectrometer measurement of the H₂S/CS₂ (≈100) abundance ratio. While the formaldehyde total abundance found with IRAM 30-m when assuming it to be mostly produced by a distributed source (Haser parent scale length ≈8000 km) is similar to the one derived by Rosetta/ROSINA, we find that the formaldehyde coming from the nucleus is one order of magnitude less abundant than measured in situ by Rosetta/ROSINA.