Atmospheric Speciation of Rocky Planets from Magma Ocean Outgassing

Tim Lichtenberg (1), Dan J. Bower (2), Mark Hammond (3), Ryan Boukrouche (1), Patrick Sanan (4), Shang-Min Tsai (1), Raymond T. Pierrehumbert (1)

(1) Atmospheric, Oceanic and Planetary Physics, University of Oxford, UK, (2) Center for Space and Habitability, University of Bern, CH, (3) Department of the Geophysical Sciences, University of Chicago, USA, (4) Institute of Geophysics, ETH Zurich, CH

Contact: tim.lichtenberg@physics.ox.ac.uk | Exoplanet Demographics 2020 Slack


Initial conditions

We compare the thermal evolution and outgassing of young, Earth-sized planets each with surface pressure of 260 bar after outgassing. For water this roughly corresponds to 1 Earth ocean. For this initial study the orbital distance is 1 au from a Sun-like star. Simulations are initialized at 100 Myr after star formation and the planet is assumed to be fully molten at simulation start.

Summary

The earliest atmospheres of rocky planets originate from extensive volatile release during magma ocean epochs that occur during assembly of the planet. These establish the initial distribution of the major volatile elements between different chemical reservoirs that subsequently evolve via geological cycles. Current theoretical techniques are limited in exploring the anticipated range of compositional and thermal scenarios of early planetary evolution, even though these are of prime importance to aid astronomical inferences on the environmental context and geological history of extrasolar planets.
Here, we present a coupled numerical framework that links an evolutionary, vertically-resolved model of the planetary silicate mantle with a radiative-convective model of the atmosphere. Using this method we investigate the early evolution of idealized Earth-sized rocky planets with end-member, clear-sky atmospheres dominated by either \(\textrm{CO}\), \(\textrm{N}_{2}\), \(\textrm{O}_{2}\), \(\textrm{H}_{2}\textrm{O}\), \(\textrm{CO}_{2}\), \(\textrm{CH}_{4}\textrm{}\), or \(\textrm{H}_{2}\). We find central metrics of early planetary evolution, such as energy gradient, sequence of mantle solidification, surface pressure, or vertical stratification of the atmosphere, to be intimately controlled by the dominant volatile and outgassing history of the planet. Thermal sequences fall into three general classes with increasing cooling timescale: \(\textrm{CO}\), \(\textrm{N}_{2}\), and \(\textrm{O}_{2}\) with minimal effect, \(\textrm{H}_{2}\textrm{O}\), \(\textrm{CO}_{2}\), and \(\textrm{CH}_{4}\textrm{}\) with intermediate influence, and \(\textrm{H}_{2}\) with several orders of magnitude increase in solidification time and atmosphere vertical stratification.
Our numerical experiments exemplify the capabilities of the presented modeling framework and link the interior and atmospheric evolution of rocky exoplanets with multi-wavelength astronomical observations.

Computational framework


Figure 1.

Volatile solubility in the magma ocean


Figure 3.

Atmosphere thermal structures


Figure 2.

Planetary heat loss to space


Figure 4.

Coupled magma ocean–atmosphere evolution


Figure 5.


Vertical resolution of the planet


Figure 6.

Atmospheric spectra


Figure 7.

Flux contributions from internal atmospheric levels


Figure 8.

Connection between interior processes and spectral appearance


Figure 9.


Figure 10.