Geochemical Journal, Vol. 53 (No. 1), pp. 69-81, 2019
David V. Bekaert,1* Murthy S. Gudipati,2* Bryana Henderson2 and Bernard Marty1
1Centre de Recherches Pétrographiques et Géochimiques, CNRS & Université de Lorraine, Nancy, France
2Science Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.
(Received May 28, 2018; Accepted November 6, 2018)
Laboratory investigations of noble gas trapping in amorphous water ice have been used to predict the noble gas composition of comets and infer on the origin of volatile elements within planetary bodies. However, the recent measurement of the noble gas composition of ice sublimating from comet 67P/Churyumov-Gerasimenko by the Rosetta mission calls for novel experiments regarding the mechanisms of noble gas trapping and evolution in cometary ice analogues. Here, we investigated the ionization dynamics of Xe atoms interacting with water ice using the recently developed Resonant Two-Step Laser Ablation Mass Spectrometry (2S-LAI-MS). Xenon-water mixed ice was ablated with an infrared beam set on the maximum absorption wavelength for water (λ = 2948 nm) at which xenon atoms are kept neutral. Subsequent multiple ionization of xenon and oxygen resulted in periodic Coulomb explosions of Xen+ components (n ∈ [1;6]) and ionized water degradation products (OH+, H+, O+, O2+, O3+). Such explosions could only be detected when Xe and water were mixed together in ice, and not when separated in two overlaid layers. This paper discusses the potential mechanisms accounting for the generation of Coulomb explosions in these experiments and its relevance to cometary ice at closer distances to perihelion. We conclude that multiple ionization of xenon and oxygen in our experiment may be due to electron impact processes resembling cometary electron and ion bombardment, whereby energetic particles of hundreds of eV to a few keV are accelerated towards the comet's nucleus. Electron and ion bombardment could induce significant chemical modifications to, and potentially outgassing from, the cometary surface.
Key words: xenon, comets, coulomb explosion, electron impact, multiple ionization