Comparison of 3D kinetic and hydrodynamic models to ROSINA-COPS measurements of the neutral coma of 67P/Churyumov-Gerasimenko

¹ University of Michigan, Climate and Space Sciences and Engineering Department, Ann Arbor, MI, USA
e-mail: abieler@umich.edu
² University of Bern, Space Research and Planetary Sciences, 3012 Bern, Switzerland
³ University of Bern, Center for Space and Habitability, 3012 Bern, Switzerland
⁴ Southwest Research Institute, Space Science and Engineering, San Antonio, TX, USA
⁵ Belgian Institute for Space Aeronomy (BIRA-IASB), Space Physics Division, 1180 Brussels, Belgium
⁶ LATMOS Laboratoire Atmosphères, Milieux, Observations Spatiales, 75252 Paris, France
⁷ Max-Planck Institute for Solar System Research, 37077 Göttingen, Germany
⁸ Institut de Recherche en Astrophysique et Planétologie IRAP, 31400 Toulouse, France
⁹ Technical University of Braunschweig, 38106 Braunschweig, Germany

Received: 24 March 2015
Accepted: 10 July 2015

Abstract

67P/Churyumov-Gerasimenko (67P) is a Jupiter-family comet and the object of investigation of the European Space Agency mission Rosetta. This report presents the first full 3D simulation results of 67P’s neutral gas coma. In this study we include results from a direct simulation Monte Carlo method, a hydrodynamic code, and a purely geometric calculation which computes the total illuminated surface area on the nucleus. All models include the triangulated 3D shape model of 67P as well as realistic illumination and shadowing conditions. The basic concept is the assumption that these illumination conditions on the nucleus are the main driver for the gas activity of the comet. As a consequence, the total production rate of 67P varies as a function of solar insolation. The best agreement between the model and the data is achieved when gas fluxes on the night side are in the range of 7% to 10% of the maximum flux, accounting for contributions from the most volatile components. To validate the output of our numerical simulations we compare the results of all three models to in situ gas number density measurements from the ROSINA COPS instrument. We are able to reproduce the overall features of these local neutral number density measurements of ROSINA COPS for the time period between early August 2014 and January 1 2015 with all three models. Some details in the measurements are not reproduced and warrant further investigation and refinement of the models. However, the overall assumption that illumination conditions on the nucleus are at least an important driver of the gas activity is validated by the models. According to our simulation results we find the total production rate of 67P to be constant between August and November 2014 with a value of about 1 × 10²⁶ molecules s^-1.