Elemental behaviour of nitrogen and heavy noble gases in interstellar ice analogues and the implications for cometary bodies

Abstract

<p>Laboratory investigations of noble gases trapped in cometary ice analogues, exposed to photo-irradiation [1], have the potential to provide important information on the evolution of cometary matter under interstellar and solar system conditions. Historical studies on the capacity of amorphous water ices to trap and retain volatiles above their sublimation temperature [1, 2] failed to predict the noble gas elemental and isotopic composition of comets, as measured in ice sublimating from comet 67P/Churyumov-Gerasimenko [3]. However, these studies did not investigate the potential effect of irradiation on elemental fractionation, nor the isotopic fractionation that may occur during trapping and release of volatiles from cometary ice.</p><p>Here, we have developped an experimental setup to form cometary ice analogues from mixtures of water, nitrogen and noble gases. We aim at understanding the elemental and isotopic behaviour of nitrogen and noble gases trapped in amorphous water ice, when subject to conditions akin to those experienced by outer solar system bodies. In this experiment, we therefore form amorphous water ice at temperatures and pressures down to 25 K and 10<sup>-8</sup> mbar, respectively). Upon heating, water structure changes from amorphous to crystalline ice at around 140K, with the liberation all gas that was initially trapped within the ice. This corresponds to the gas phase in the coma of comets, where both trapped gas and sublimating water are present. In the experiment, gases released from the ice are analysed using a quadrupole mass spectrometer.</p><p>We investigate the evolution of N<sub>2</sub>/Ar/Kr/Xe in the ice from their condensation temperature to their release into the gaseous phase, and the potential effects of this trapping mechanism on the isotope composition of noble gases carried in water ice. We found that reproducing the Ar/Kr/Xe ratios of the comet from a solar composition might be possible when the ice forms at “high” temperature around 70K. Additionnally, high <sup>14</sup>N/<sup>36</sup>Ar measured in comet 67P/C-G [3] may also be replicated when the ice forms at high temperatures. A low temperature formation of around 30K for the comet, as previouslty suggested [4], thus requires a different (or additional) source than solar-type gas for both noble gases and nitrogen.</p><p>Taken together, these experimental investigations will ultimately help us to better understand the genesis and evolution of volatile elements within outer solar system icy bodies, and their role as precursors to the terrestrial atmopshere.</p><p>[1] Bar-Nun, A. and Kleinfeld, I., 1989. Icarus, 80 [2] Notesco et al., 2003. Icarus, 162 [3] Rubin et al., 2018. Science advances, 4 [4] Rubin et al., 2015. Science, 348.</p>