Strange messenger: A new history of hydrogen on Earth as told by xenon

Atmospheric xenon is strongly mass fractionated, the result of a process that apparently continued through the Archean. Previous models that explain Xe fractionation by hydrodynamic hydrogen escape cannot gracefully explain how Xe escaped when Ar and Kr did not, nor do they allow Xe to escape in the Archean. Here we show that Xe is the only noble gas that can escape as an ion in a photo-ionized hydrogen planetary wind. This works because Xe ionizes more easily than hydrogen, and as an ion it couples strongly to other ions. Ions can escape in the absence of a geomagnetic field or along polar magnetic field lines that open into interplanetary space. To quantify the hypothesis we construct new 1-D models of hydrodynamic diffusion-limited hydrogen escape from highly-irradiated CO2-H2-H atmospheres. We find that Xe can escape if the hydrogen (or methane) mixing ratio exceeded 1% and solar EUV (extreme ultraviolet) irradiation exceeded ten times the modern quiet Sun. The long duration of Xe escape implies that, if a constant process, Earth lost the hydrogen from at least one ocean of water, roughly evenly split between the Hadean and the Archean. However, to account not only for Xe's fractionation but also for its depletion with respect to Kr and primordial 244Pu, requires higher rates of Xe escape that must be limited to small apertures or short periods of time, which suggests that Xe escape was restricted to polar windows by a geomagnetic field, or dominated by outbursts of high solar activity, or limited to transient episodes of abundant hydrogen, or a combination of these. Xenon escape stopped when (i) the hydrogen (or methane) mixing ratio became too small, or (ii) EUV radiation from the aging Sun became too weak, or (iii) charge exchange between Xe ions and O2 rendered Xe neutral. All three are plausible.