Abstract
Context.Cometary outgassing is induced by the sublimation of ices and the ejection of dust originating from the nucleus. Therefore measuring the composition and dynamics of the cometary gas provides information concerning the interior composition of the body. Nevertheless, the bulk composition differs from the coma composition, and numerical models are required to simulate the main physical processes induced by the illumination of the icy body.Aims.The objectives of this study are to bring new constraints on the interior composition of the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P) by comparing the results of a thermophysical model applied to the nucleus of 67P and the coma measurements made by the Reflectron-type Time-Of-Flight (RTOF) mass spectrometer. This last is one of the three instruments of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), used during the Rosetta mission.Methods.Using a thermophysical model of the comet nucleus, we studied the evolution of the stratigraphy (position of the sublimation and crystallisation fronts), the temperature of the surface and subsurface, and the dynamics and spatial distribution of the volatiles (H2O, CO2and CO). We compared them with the in situ measurements from ROSINA/RTOF and an inverse coma model.Results.We observed the evolution of the surface and near surface temperature, and the deepening of sublimation fronts. The thickness of the dust layer covering the surface strongly influences the H2O outgassing but not the more volatiles species. The CO outgassing is highly sensitive to the initial CO/H2O ratio, as well as to the presence of trapped CO in the amorphous ice.Conclusions.The study of the influence of the initial parameters on the computed volatile fluxes and the comparison with ROSINA/RTOF measurements provide a range of values for an initial dust mantle thickness and a range of values for the volatile ratio. These imply the presence of trapped CO. Nevertheless, further studies are required to reproduce the strong change of behaviour observed in RTOF measurements between September 2014 and February 2015.
Highlights
The Rosetta mission studied the coma of comet 67P/ChuryumovGerasimenko (67P) from August 2014 to September 2016
We focus on the role of three distinct parameters that the Rosetta mission did not directly determine but did help to constrain: the ratios of the volatiles (CO2/H2O and CO/H2), the thickness of the dust mantle, and the presence of trapped CO in the ice
We studied the influence of imposing a dust mantle from the start with different thicknesses: 0 m, 0.07 m, 0.1 m, and 0.2 m covering the northern hemisphere
Summary
The Rosetta mission studied the coma of comet 67P/ChuryumovGerasimenko (67P) from August 2014 to September 2016. A comparison between the illumination conditions during the mission and the change in relative abundance of CO2/H2O and CO/H2O ratios indicated a dichotomy between the northern hemisphere and the southern hemisphere. A dust mantle covering some parts of the surface of 67P surface was reported in Capaccioni et al (2015), Sierks et al (2015) and El-Maarry et al (2015). The nucleus’s northern hemisphere experiences a long and soft summer while the southern hemisphere experiences a warmer but shorter summer, resulting in an intense sublimation of ices, erosion, and ejection of dust particles in the coma, which are partly redeposited in the northern hemisphere and form an insulating layer covering the surface
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