Abstract
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.
Highlights
A comet is often considered to be a cold body made of ices and refractory material, which releases neutral gas by sublimation of the ices embedded among the refractory particles, leading to the formation of the neutral comet atmosphere or gas coma as well as a dust coma
Evidence is presented that indicates that the hydrogen halides HF and HCl sublimate from the surface and from dust particles in the inner coma, which constitute a distributed gas source (Cottin & Fray 2008) in the coma of comet 67P/Churyumov– Gerasimenko
As chlorine is released from the water ice matrix, it recombines with hydrogen and sublimates in the form of the hydrogen halide (Olanrewaju et al 2011; Kama et al 2015); the same recombinative desorption process is thought to occur for the other halogens
Summary
We follow the definitions introduced by Cottin & Fray (2008) regarding distributed sources. We build a simple model of a distributed source in order to provide the background against which the DFMS observations of the hydrogen halides can be interpreted
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