Evidence for distributed gas sources of hydrogen halides in the coma of comet 67P/Churyumov–Gerasimenko

Johan de Keyser (1) , Frederik Dhooghe (1) , Kathrin Altwegg (2, 3) , Hans Balsiger (2) , Jean-Jacques Berthelier (4) , Christelle Briois (5) , Ursina Calmonte (2) , Gael Cessateur (1) , Michael R. Combi (6) , Eddy Equeter (1) , Bjorn Fiethe (7) , Stephen Fuselier (8, 9) , Sebastien Gasc (2) , Andrew Gibbons (1, 10) , Tamas Gombosi (6) , Herbert Gunell (1) , Myrtha Hassig (2, 8) , Léna Le Roy (3) , Romain Maggiolo (1) , Urs Mall (11) , Bernard Marty (12) , Eddy Neefs (1) , Henri Reme (13) , Martin Rubin (2) , Thierry Semon (2) , Chia-Yu Tzou (2) , Peter Wurz (2, 3)

1 BIRA-IASB - Belgian Institute for Space Aeronomy / Institut d'Aéronomie Spatiale de Belgique
2 Physikalisches Institut [Bern]
3 CSH - Center for Space and Habitability
4 PLANETO - LATMOS
5 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
6 CLaSP - Department of Climate and Space Sciences and Engineering
7 IDA - Institute of Computer and Network Engineering [Braunschweig]
8 SwRI - Southwest Research Institute [San Antonio]
9 UTSA - The University of Texas at San Antonio
10 Service de Chimie Quantique et Photophysique
11 MPS - Max-Planck-Institut für Sonnensystemforschung
12 CRPG - Centre de Recherches Pétrographiques et Géochimiques
13 IRAP - Institut de recherche en astrophysique et planétologie

Rosetta has detected the presence of the hydrogen halides HF, HCl, and HBr in the coma of comet 67P/Churyumov-Gerasimenko. These species are known to freeze out on icy grains in molecular clouds. Analysis of the abundances of HF and HCl as a function of cometocentric distance suggests that these hydrogen halides are released both from the nucleus surface and off dust particles in the inner coma. We present three lines of evidence. First, the abundances of HF and HCl relative to the overall neutral gas in the coma appear to increase with distance, indicating that a net source must be present; since there is no hint at any possible parent species with sufficient abundances that could explain the observed levels of HF or HCl, dust particles are the likely origin. Second, the amplitude of the daily modulation of the halide-to-water density due to the rotation and geometry of 67P's nucleus and the corresponding surface illumination is observed to progressively diminish with distance and comet dust activity; this can be understood from the range of dust particle speeds well below the neutral gas expansion speed, which tends to smooth the coma density profiles. Third, strong halogen abundance changes detected locally in the coma cannot be easily explained from composition changes at the surface, while they can be understood from differences in local gas production from the dust particles.

Keywords: 67P/Churyumov-Gerasimenko; comets

Domains: Astrophysics [astro-ph]

Johan de Keyser, Frederik Dhooghe, Kathrin Altwegg, Hans Balsiger, Jean-Jacques Berthelier, et al.. Evidence for distributed gas sources of hydrogen halides in the coma of comet 67P/Churyumov–Gerasimenko. Monthly Notices of the Royal Astronomical Society, 2017, 469 (Suppl_2), pp.S695-S711. ⟨10.1093/mnras/stx2725⟩. ⟨insu-02765310⟩