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

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 rh = 1.24-3.65 AU. A steep evolution of the outgassing rates with heliocentric distance is observed, typically in rh-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.