Direct Simulation Monte Carlo modelling of the major species in the coma of comet 67P/Churyumov-Gerasimenko

Nicolas Fougere (1) , Kathrin Altwegg (2) , Jean-Jacques Berthelier (3, 4) , Andre Bieler (1) , Dominique Bockelée-Morvan (4) , Ursina Calmonte (2) , Fabrizio Capaccioni (5) , Michael R. Combi (1) , Johan de Keyser (6) , Vincent Debout (4) , Stéphane Erard (4) , Björn Fiethe (7) , Gianrico Filacchione (5) , Uwe Fink (8) , Stephen A. Fuselier (9) , Tamas I. Gombosi (1) , Kenneth C. Hansen (1) , Myrtha Hässig (9) , Zhenguang Huang (1) , Léna Le Roy (2) , Cédric Leyrat (4) , Alessandra Migliorini (5) , Giuseppe Piccioni (5) , Giovanna Rinaldi (5) , Martin Rubin (2) , Yinsi Shou (1) , Valeriy Tenishev (1) , Gabor Toth (1) , Chia-Yu Tzou (2)

1 CLaSP - Department of Climate and Space Sciences and Engineering
2 WP - Space Research and Planetary Sciences [Bern)
3 PLANETO - LATMOS
4 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
5 IAPS - Istituto di Astrofisica e Planetologia Spaziali - INAF
6 BIRA-IASB - Belgian Institute for Space Aeronomy / Institut d'Aéronomie Spatiale de Belgique
7 IDA - Institute of Computer and Network Engineering [Braunschweig]
8 LPL - Lunar and Planetary Laboratory [Tucson]
9 Space Science Division [San Antonio]

We analyze the ROSINA-DFMS data between August 2014 and February 2016 to examine the effect of seasonal variations on the four major species within the coma of 67P/Churyumov-Gerasimenko (H₂O, CO₂, CO, and O₂), resulting from the tilt in the orientation of the comet's spin axis. Using a numerical data inversion, we derive the nonuniform activity distribution at the surface of the nucleus for these species, suggesting that the activity distribution at the surface of the nucleus has not significantly changed and that the differences observed in the coma are solely due to the variations in illumination conditions. A 3D DSMC model is applied where the boundary conditions are computed with a coupling of the surface activity distributions and the local illumination. The model is able to reproduce the evolution of the densities observed by ROSINA including the changes happening at equinox. While O₂ stays correlated with H₂O as it was before equinox, CO₂ and CO, which had a poor correlation with respect to H₂O pre-equinox, also became well correlated with H₂O post-equinox. The integration of the densities from the model along the line of sight results in column densities directly comparable to the VIRTIS-H observations. Also, the evolution of the volatiles’ production rates is derived from the coma model showing a steepening in the production rate curves after equinox. The model/data comparison suggests that the seasonal effects result in the northern hemisphere of 67P's nucleus being more processed with a layered structure while the southern hemisphere constantly exposes new material.

Mots clés: ROSINA; Rosetta; space vehicles; 67P/Churyumov-Gerasimenko; comets; VIRTIS

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

Nicolas Fougere, Kathrin Altwegg, Jean-Jacques Berthelier, Andre Bieler, Dominique Bockelée-Morvan, et al.. Direct Simulation Monte Carlo modelling of the major species in the coma of comet 67P/Churyumov-Gerasimenko. Monthly Notices of the Royal Astronomical Society, 2016, 462 (suppl. 1), pp.S156-S169. ⟨10.1093/mnras/stw2388⟩. ⟨insu-01370624⟩