Water ortho-to-para ratio in the coma of comet 67P/Churyumov-Gerasimenko

Abstract

Context. Abundance ratios of the nuclear-spin isomers of H2O and NH3 have been measured in about two dozen comets, with a mean value corresponding to a nuclear-spin temperature of ~30 K. The real meaning of these unequilibrated nuclear-spin abundance ratios is still debated. However, an equilibrated water ortho-to-para ratio of 3 is also commonly observed. Aims. The H channel of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-H) on board Rosetta provided high-resolution 2.5–2.9 μm spectra of H2O vapour in the coma of comet 67P/Churyumov-Gerasimenko (67P), which are suitable for the determination of the ortho-to-para ratio (OPR) of water in this comet. Methods. A large dataset of VIRTIS-H spectra obtained in limb-sounding viewing geometry was analysed, covering heliocentric distances from 1.24 to 2.73 au and altitudes from a few hundred metres to > 100 km. The OPR, together with the H2O rotational temperature and column density, were derived for each spectra using a database of fluorescence synthetic spectra that include both fundamental and hot vibrational water bands. The weak lines of the v1, v1 + v3 − v1 and v2 + v3 − v2 bands in the 2.774–2.910 μm range were used to calculate by how much the strong v3 band centred at 2.67 μm is attenuated due to optical depth effects, expressed by the attenuation factor fatten. Results. Most ortho-to-para ratio determinations are strongly affected by opacity effects, as demonstrated by the observed anti-correlation between the OPR and the column density, and the correlation between the OPR and attenuation factor fatten. Based on both radiative transfer calculations and OPR values obtained in low-opacity conditions, we derive an OPR of 2.94 ± 0.06 for comet 67P. Measured water rotational temperatures show a decrease in gas kinetic temperature with increasing altitude caused by adiabatic cooling. Heliocentric variations are also observed, with warmer temperatures near perihelion. Conclusions. The water ortho-to-para ratio measured in the coma of 67P is consistent with laboratory experiments showing that water vapour that has thermally desorbed from water ice has a statistical value of 3, regardless of the past formation process of water ice.