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K. J. Zahnle (NASA Ames Research Center)
Xenon is the heaviest gas likely to be found in an atmosphere. Yet there is a great deal of evidence of xenon escape. Isotopes of Xe in air are mass-fractionated with respect to the solar wind and meteorites, and Xe is depleted with respect to krypton. Radiogenic daughters of 129I (half-life 15.7 m.y.) and 244Pu (82 m.y.) are detectable but rarer than they ought to be: e.g. Earth has only ~ 1% of the 129Xe one expects given primordial amounts of 129I. The same considerations apply to martian Xe. Hydrodynamic hydrogen escape can plausibly mass-fractionate noble gases, but it is not known how or why Xe was singled out. Here we ask whether Xe preferentially escaped because it is the only noble gas more easily ionized than hydrogen. Hydrodynamic escape is fueled by solar EUV radiation, leaving the hydrogen partially ionized. In this environment gases more difficult to ionize than hydrogen are neutral, while those more easily ionized are ions. In hydrodynamic escape, heavy atoms are dragged along when collisions push the heavy atoms upward faster than gravity can pull them back. Differences in collision cross-sections between neutrals and between ions and neutrals are small compared to differences in mass. However, ions interact strongly with each other and consequently cross-sections for ion-ion interactions are large. We show that ionized Xe can get strongly coupled to the escape of ionized hydrogen, and thus that hydrodynamic escape can apply more strongly to xenon than to the other noble gases.