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

Accepted for publication in Astronomy & AstrophysicsWe present the analysis of ≈100 molecular maps of the coma of comet 67P/Churyumov-Gerasimenko obtained with the MIRO submillimeter radio telescope on board the Rosetta spacecraft. From the mapping of spectral lines of H 16 2 O, H 18 2 O, H 17 2 O, CH 3 OH, NH 3 and CO, and some fixed nadir pointings, we retrieved the outgassing pattern and total production rate 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 to 3.65 AU. A steep evolution of the outgassing rates with heliocentric distance is observed, typically in r −6 h , with significant differences between molecules (e.g. steeper variation for H 2 O post perihelion than for methanol). As a consequence the abundances relative to water in the coma vary. The CH 3 OH and CO abundances increase after perihelion while the NH 3 abundance peaks around perihelion and decreases after. Outgassing patterns have been modeled as 2-D 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 Feb. 2015 the outgassing is mostly restricted to a narrower jet (Full Width at Half Maximum, FWHM ≈80 •) originating from high northern latitudes, while around perihelion most of the gaseous production comes from the southernmost regions (−80±5 • cometocentric latitude) forming a 100-130 • (FWHM) wide fan. We find a peak production of water of 0.8 × 10 28 molec. s −1 , 2.5 times smaller than measured by the ROSINA experiment, and put 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 4.18 ± 0.18 × 10 9 kg. We derive a dust-togas ratio in the lost material of 0.7 to 2.3 (including all sources of errors), based on the nucleus mass loss of 10.5 ± 3.4 × 10 9 kg estimated by the RSI experiment. We also obtain an estimate of the H 18 2 O/H 17 2 O ratio of 5.6 ± 0.8