Long-term monitoring of the outgassing and composition of comet 67P/Churyumov-Gerasimenko with the Rosetta/MIRO instrument

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, University Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: nicolas.biver@obspm.fr
² JPL/California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
³ University of Massachusetts, 619 Lederle Graduate Research Tower, Amherst, MA, USA
⁴ Max Planck Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁵ National Central University, Jhongli, Taoyuan City 32001, Taiwan
⁶ LERMA, Observatoire de Paris, PSL Research University, Sorbonne Université, CNRS, 61 avenue de l’Observatoire, 75014 Paris, France

Received: 21 December 2018
Accepted: 1 March 2019

Abstract

We present the analysis of ≈100 molecular maps of the coma of comet 67P/Churyumov-Gerasimenko that were obtained with the MIRO submillimeter radiotelescope on board the Rosetta spacecraft. From the spectral line mapping of H₂¹⁶O, H₂¹⁸O, H₂¹⁷O, CH₃OH, NH₃, and CO and some fixed nadir pointings, we retrieved the outgassing pattern and total production rates for these species. The analysis covers the period from July 2014, inbound to perihelion, to June 2016, outbound, and heliocentric distances r_h = 1.24–3.65 AU. A steep evolution of the outgassing rates with heliocentric distance is observed, typically in r_h⁻¹⁶, with significant differences between molecules (e.g. steeper variation for H₂O post-perihelion than for methanol). As a consequence, the abundances relative to water in the coma vary. The CH₃OH and CO abundances increase after perihelion, while the NH₃ abundance peaks around perihelion and then decreases. Outgassing patterns have been modeled as 2D Gaussian jets. The width of these jets is maximum around the equinoxes when the bulk of the outgassing is located near the equator. From July 2014 to February 2015, the outgassing is mostly restricted to a narrower jet (full width at half-maximum ≈80°) originating from high northern latitudes, while around perihelion, most of the gaseous production comes from the southernmost regions ( − 80 ± 5° cometocentric latitude) and forms a 100°–130° (full width at half-maximum) wide fan. We find a peak production of water of 0.8 × 10²⁸ molec. s⁻¹, 2.5 times lower than measured by the ROSINA experiment, and place an upper limit to a 50% additional production that could come from the sublimation of icy grains. We estimate the total loss of ices during this perihelion passage to be 4.18 ± 0.18 × 10⁹ kg. We derive a dust-to-gas ratio in the lost material of 0.7–2.3 (including all sources of errors) based on the nucleus mass loss of 10.5 ± 3.4 × 10⁹ kg estimated by the RSI experiment. We also obtain an estimate of the H₂¹⁸O/H₂¹⁷O ratio of 5.6 ± 0.8.