Solar XUV and ENA‐driven water loss from early Venus' steam atmosphere

Abstract

We present a study on the influence of the upper atmosphere hydrodynamic escape of hydrogen, driven by the solar soft X‐ray and extreme ultraviolet radiation (XUV), on an expected outgassed steam atmosphere of early Venus. By assuming that the young Sun was either a weak or moderately active young G star, we estimated the water loss from a hydrogen dominated thermosphere due to the absorption of the solar XUV flux and the precipitation of solar wind produced energetic hydrogen atoms (ENAs). The production of ENAs and their interaction with the hydrodynamic extended upper atmosphere, including collision‐related feedback processes, have been calculated by means of Monte Carlo models. ENAs that collide in the upper atmosphere deposit their energy and heat the surrounding atmosphere mainly above the main XUV energy deposition layer. It is shown that precipitating ENAs modify the thermal structure of the upper atmosphere, but the enhancement of the thermal escape rates caused by these energetic hydrogen atoms is negligible. Our results also indicate that the majority of oxygen arising from dissociated H2O molecules is left behind during the first 100 Myr. It is thus suggested that the main part of the remaining oxygen has been absorbed by crustal oxidation.

Venusian Habitable Climate Scenarios: Modeling Venus through time and applications to slowly rotating Venus-Like Exoplanets

“…This would be accomplished by sequestering the O 2 left behind by H 2 O dissociation in the magmatic crust and upper mantle (e.g. Lebrun et al, 2013;Gillmann et al, 2009;Lichtenegger et al, 2016;Lammer et al, 2018). We discuss how large quantities of O 2 can be lost after a significant period of habitability in bullet 6 of Section 8, in lieu of early MO losses.…”

“…The timing of the MO termination is critical in more than one way. If the steam and CO 2 atmosphere cooled sufficiently for MO crystallization to occur by the time of the Late Veneer (also referred to as "Late Accretion") then even if Venus lost most/all of its primordial H 2 O through escape processes (Gillmann et al, 2009;Hamano et al, 2013;Lichtenegger et al, 2016) there may have been a second chance to obtain a surface ocean, albeit a shallow one. Recent work by Greenwood et al (2018) implies that Earth may have received as much as 30% of its H 2 O inventory in post-accretion impact delivery, consistent with research that shows that the entire H 2 O budget cannot come from the late veneer (Morbidelli & Wood, 2015).…”

“…They assume the surface was never cool enough to allow liquid water to condense. The water would again be photodissociated and the hydrogen would have been lost to space (Lichtenegger et al, 2016). The leftover oxygen would have dissolved in the magma ocean.…”

Escape and fractionation of volatiles and noble gases from Mars-sized planetary embryos and growing protoplanets

“…However, more recent models of the winds of young Sun-like stars (Johnstone et al, 2015a) suggests that they were much more tenuous than previously expected. The resulting energy input into planetary atmospheres by production of energetic neutral atoms is thus not very efficient and the stellar EUV luminosity is the dominant driver of atmospheric escape (Lichtenegger et al, 2016).…”

The Role of N2as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats

“…If the young host star's EUV flux is not too high, some residual O 2 may remain in the atmosphere as a product of H 2 O dissociation, while the hydrogen atoms escape hydrodynamically (Zahnle and Kasting, 1986). The remaining oxygen and atmospheric nitrogen could have been incorporated into the planet's hot magmatic crust, where they oxidized the upper mantle (Gillmann et al, 2009;Hamano et al, 2013;Kurosawa, 2015;Lichtenegger et al, 2016;Wordsworth, 2016;Lammer et al, 2018). Under such conditions, the highly oxidized surface material can be mixed with reduced, nitrogen-rich material.…”