Low NH₃/H₂O ratio in comet C/2020 F3 (NEOWISE) at 0.7 au from the Sun

¹ Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
e-mail: maria.drozdovskaya@unibe.ch; maria.drozdovskaya.space@gmail.com
² LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, 5 place Jules Janssen, 92195 Meudon, France
e-mail: Dominique.Bockelee@obspm.fr
³ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁴ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
⁵ Astrochemistry Laboratory, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
⁶ Department of Physics, Catholic University of America, Washington, DC 20064, USA
⁷ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK

Received: 14 March 2023
Accepted: 19 July 2023

Abstract

Context. A lower-than-solar elemental nitrogen content has been demonstrated for several comets, including 1P/Halley and 67P/Churyumov-Gerasimenko (67P/C-G) with independent in situ measurements of volatile and refractory budgets. The recently discovered semi-refractory ammonium salts in 67P/C-G are thought to be the missing nitrogen reservoir in comets.

Aims. The thermal desorption of ammonium salts from cometary dust particles leads to their decomposition into ammonia (NH₃) and a corresponding acid. The NH₃/H₂O ratio is expected to increase with decreasing heliocentric distance with evidence for this in near-infrared observations. NH₃ has been claimed to be more extended than expected for a nuclear source. Here, the aim is to constrain the NH₃/H₂O ratio in comet C/2020 F3 (NEOWISE) during its July 2020 passage.

Methods. OH emission from comet C/2020 F3 (NEOWISE) was monitored for 2 months with the Nançay Radio Telescope (NRT) and observed from the Green Bank Telescope (GBT) on 24 July and 11 August 2020. Contemporaneously with the 24 July 2020 OH observations, the NH₃ hyperfine lines were targeted with GBT. From the data, the OH and NH₃ production rates were derived directly, and the H₂O production rate was derived indirectly from the OH.

Results. The concurrent GBT and NRT observations allowed the OH quenching radius to be determined at (5.96 ± 0.10) × 10⁴ km on 24 July 2020, which is important for accurately deriving Q(OH). C/2020 F3 (NEOWISE) was a highly active comet with Q(H₂O) ≈ 2 × 10³⁰ mol s⁻¹ one day before perihelion. The 3σ upper limit for Q_NH3 / Q_H2O is < 0.29% at 0.7 au from the Sun.

Conclusions. The obtained NH₃/H₂O ratio is a factor of a few lower than measurements for other comets at such heliocentric distances. The abundance of NH₃ may vary strongly with time depending on the amount of water-poor dust in the coma. Lifted dust can be heated, fragmented, and super-heated; whereby, ammonium salts, if present, can rapidly thermally disintegrate and modify the NH₃/H₂O ratio.