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Abstract
Context.The European Space Agency (ESA) Rosetta mission was the most comprehensive study of a comet ever performed. In particular, the Rosetta orbiter, which carried many instruments for monitoring the evolution of the dusty gas emitted by the cometary nucleus, returned an enormous volume of observational data collected from the close vicinity of the nucleus of comet 67P/Churyumov-Gerasimenko.Aims.Such data are expected to yield unique information on the physical processes of gas and dust emission, using current physical model fits to the data. We present such a model (the RZC model) and our procedure of adjustment of this model to the data.Methods.The RZC model consists of two components: (1) a numerical three-dimensional time-dependent code solving the Eulerian/Navier-Stokes equations governing the gas outflow, and a direct simulation Monte Carlo (DSMC) gaskinetic code with the same objective; and (2) an iterative procedure to adjust the assumed model parameters to best-fit the observational data at all times.Results.We demonstrate that our model is able to reproduce the overall features of the local neutral number density and composition measurements of Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) Comet Pressure Sensor (COPS) and Double Focusing Mass Spectrometer (DFMS) instruments in the period August 1–November 30, 2014. The results of numerical simulations show that illumination conditions on the nucleus are the main driver for the gas activity of the comet. We present the distribution of surface inhomogeneity best-fitted to the ROSINA COPS and DFMS in situ measurements.
Highlights
Several works have undertaken the derivation of the distribution of the surface activity of comet 67P/Churyumov-Gerasimenko from the in situ measurements made near to the nucleus by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) consisting of two mass spectrometers and a pressure sensor (Balsiger et al 2007)
We have fitted the data acquired in August–November 2014 by the ROSINA instrument, which are described in detail in Le Roy et al (2015)
For adjustment we used time instances corresponding to the Comet Pressure Sensor (COPS) measurements and the Double Focusing Mass Spectrometer (DFMS) data were linearly interpolated
Summary
Several works have undertaken the derivation of the distribution of the surface activity of comet 67P/Churyumov-Gerasimenko (hereafter 67P) from the in situ measurements made near to the nucleus by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) consisting of two mass spectrometers and a pressure sensor (Balsiger et al 2007) This requires (1) the use of a coma gas model relating the gas flow variables at any point to a set of adjustable parameters meant to represent the nucleus surface gas production (i.e. defining the so-called flow “surface boundary conditions”), and (2) an iterative procedure to derive the best-fit values of these parameters. Considering the fact that inside each surface grid element there may be a complex topography and temperature inhomogeneity, the distribution of ice may assume any pattern, and the CO and CO2 production may have stochastic variations, it was considered impossible to perform a gas-dynamically exact modelling of the near-surface nonequilibrium outflow region. To describe translational-rotational energy exchanges, the Larsen-Borgnakke model was used
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