Magma oceanography of the dense, ultrashort-period sub-Earth GJ 367 b

The dawn of high-resolution observations with the James Webb Space Telescope will enable spatially resolved observations of ultrashort-period rocky exoplanets. Some of these planets orbit so closely to their star that they lack an atmosphere [1], which gives direct access to their surfaces and opens a window to infer their geodynamics [2]. The physical parameters of the ultrashort-period sub-Earth GJ 367 b have been observationally constrained to a planetary radius of about 0.72 to 0.75 Earth-radii and a mass between 0.48 and 0.55 Earth-masses, implying a density of 6200 to 8500 kg/m³ [3, 4], which puts this planet in a Mercury-like interior regime with a thin mantle overlying a fractionally large core.

The dayside temperature ranges between 1500 to 1800 K, thus suggesting the presence of a permanent magma ocean or dayside magma pond on the surface, induced by stellar irradiation. The large uncertainty on the age of the stellar system, between 30 Myr [4] and about 8 Gyr [3], however, introduce severe uncertainties related to the compositional and thermal evolution of the planetary mantle. In this study we perform global 2D spherical annulus StagYY simulations [5, 6] of solid-state mantle convection and surface melting with the goal to constrain the geometric and compositional properties of the planet. Constraining the spatial dimensions of thermodynamic properties of partially molten, atmosphere-less planets like GJ 367 b offers unique opportunities to constrain the compositional fractionation during magma ocean epochs and provides avenues to constrain the delivery and loss cycle of atmophile elements on strongly irradiated exoplanets.

References:

[1] L. Kreidberg and 18 co-authors. Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b. Nature, 573:87–90, 2019.

[2] T. G. Meier, D. J. Bower, T. Lichtenberg, P. J. Tackley, and B.-O. Demory. Hemispheric Tectonics on LHS 3844b. Astrophys. J. Lett., 908:L48, 2021.

[3] K.W.F. Lam and 78 co-authors. GJ 367 b: A dense, ultrashort-period sub-earth planet transiting a nearby red dwarf star. Science, 374:1271–1275, 2021.

[4] W. Brandner, P. Calissendorff, N. Frankel, and F. Cantalloube. High-contrast, high-angular resolution view of the GJ367 exoplanet system. Mon. Notices Royal Astron. Soc., 513:661–669, 2022.

[5] J. W. Hernlund and P. J. Tackley. Modeling mantle convection in the spherical annulus. Phys. Earth Planet. Int., 171:48–54, 2008.

[6] P. J. Tackley. Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Phys. Earth Planet. Int., 171:7–18, 2008.