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. 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.