The Dehydration of Water Worlds via Atmospheric Losses

Abstract

We present a three-species multi-fluid MHD model (H + , H 2 O + and e − ), endowed with the requisite atmospheric chemistry, that is capable of accurately quantifying the magnitude of water ion losses from exoplanets. We apply this model to a water world with Earth-like parameters orbiting a Sun-like star for three cases: (i) current normal solar wind conditions, (ii) ancient normal solar wind conditions, and (iii) one extreme "Carrington-type" space weather event. We demonstrate that the ion escape rate for (ii), with a value of 6.0×10 26 s −1 , is about an order of magnitude higher than the corresponding value of 6.7×10 25 s −1 for (i). Studies of ion losses induced by space weather events, where the ion escape rates can reach ∼ 10 28 s −1 , are crucial for understanding how an active, early solar-type star (e.g., with frequent coronal mass ejections) could have accelerated the depletion of the exoplanet's atmosphere. We briefly explore the ramifications arising from the loss of water ions, especially for planets orbiting M-dwarfs where such effects are likely to be significant.

Planetary Magnetism as a Parameter in Exoplanet Habitability

“…Water is lost through a number of non-thermal processes such as dissociation and dissociative recombination (Geppert & Larsson 2008), ion pickup (Lammer et al 2006), charge exchange (Dong et al 2017), sputtering (Terada et al 2009), and atmospheric escape (Zahnle & Catling 2017). Future research should be conducted to more carefully investigate which water-loss processes dominate in different planetary scenarios.…”

QUEST: A New Frontiers Uranus orbiter mission concept study

“…Unraveling these processes, particularly in contrast to the magnetic interactions ongoing at the other giant planets, will provide further evidence to support current models of magnetospheric dynamics. Additionally, the study of the solar wind-Uranus interaction may also facilitate our understanding of the stellar wind-planet interaction of exoplanets with high magnetic obliquities [39,40] thereby expanding our magnetospheric comparative planetology capabilities beyond our own solar system. Additionally, Voyager 2 arrived when Uranus was near northern summer solstice [44] and the proposed arrival date of 2045 for the QUEST mission would be near the autumnal equinox.…”

Role of Planetary Obliquity in Regulating Atmospheric Escape: G-dwarf versus M-dwarf Earth-like Exoplanets

“…Additional details concerning the reactions are outlined in Table 1. For a brief discussion of the utility of multifluid MHD models in our Solar system, we refer the reader to Section 2 of Dong et al (2017a).…”