Deep Two-phase, Hemispherical Magma Oceans on Lava Planets

Abstract

Astronomers have discovered a handful of exoplanets with rocky bulk compositions but orbiting that orbit so close to their host star that the surface of the planet must be at least partially molten. It is expected that the dayside of such “lava planets” harbors a rock-vapor atmosphere that flows quickly toward the airless nightside—this partial atmosphere is critical to the interpretation of lava planet observations, but transports negligible heat toward the nightside. As a result, the surface temperature of the magma ocean may range from 3000 K near the substellar point down to 1500 K near the day–night terminator. We use simple models incorporating the thermodynamics and geochemistry of partial melt to predict the physical and chemical properties of the magma ocean as a function of the distance from the substellar point. Our principal findings are that: (1) the dayside magma ocean is much deeper than previously thought, probably extending down to the core–mantle boundary below the substellar point of an Earth-sized planet; (2) much of the dayside is only partially molten, leading to gradients in the surface chemistry of the magma ocean; and (3) the temperature at the base of the silicate mantle is as important as the surface temperature. In the most extreme cases, lava planet interiors could be cold enough such that thermal stratification below the substellar point is gravitationally stable. These findings have important implications for the dynamics of the magma ocean, as well as the composition and dynamics of the atmosphere.

Rocky Planet or Water World? Observability of Low-density Lava World Atmospheres

“…Volatiles can be stored in different parts of the planetary interior, including the upper mantle, the deep mantle, and the iron core (Li et al 2019b;Dorn & Lichtenberg 2021;Kovačević et al 2022;Schlichting & Young 2022;Vazan et al 2022). In the mantle, lava worlds are expected to have deep dayside magma oceans, reaching as deep the core-mantle boundary in some cases (Boukaré et al 2022). Therefore, the availability of volatiles to evaporate/degas at the surface depends on the efficiency of mixing in the magma ocean; for example, Boukaré et al (2022) find that vigorous convection occurs if the core temperature is sufficiently high.…”

“…In the mantle, lava worlds are expected to have deep dayside magma oceans, reaching as deep the core-mantle boundary in some cases (Boukaré et al 2022). Therefore, the availability of volatiles to evaporate/degas at the surface depends on the efficiency of mixing in the magma ocean; for example, Boukaré et al (2022) find that vigorous convection occurs if the core temperature is sufficiently high. Furthermore, buoyancy effects may cause volatile-rich material to rise to the surface, although this depends on its material properties as a function of pressure (e.g., Karki & Ghosh 2021).…”

“…We note that for a dry magma ocean, the surface composition may not be representative of the bulk mantle (e.g., Kite et al 2016;Boukaré et al 2022). For example, rapid vaporization of the most volatile species can result in a chemically distilled surface composition that is not representative of the bulk interior (Kite et al 2016;Nguyen et al 2020;Zilinskas et al 2022).…”

Fizzy Super-Earths: Impacts of Magma Composition on the Bulk Density and Structure of Lava Worlds

“…Therefore, we adopt the solidus temperature at which significant changes in physical properties occur. Specifically, we model the solidus using a method similar to that of Boukaré et al (2022). Boukaré et al (2022) rely on solidus and liquidus curves from Zhang & Herzberg (1994) and Fiquet et al (2010).…”

LavAtmos: An open‐source chemical equilibrium vaporization code for lava worlds

“…These planets are exposed to extreme stellar irradiation that leads to surface temperatures hot enough to prevent the planet from cooling and creating a crust (Boukaré et al., 2022; Henning et al., 2018), exposing the silicate mantle directly to the atmosphere. Furthermore, since the atmosphere is a direct product of the outgassing from the magma ocean and is in equilibrium with it, the atmospheric compositions of these planets are directly influenced by the interior composition (Dorn & Lichtenberg, 2021; Ito et al., 2015; Kite et al., 2016, 2020; Miguel et al., 2011; Nguyen et al., 2020).…”