Redox hysteresis of super-Earth exoplanets from magma ocean circulation

Astrophysical Journal Letters American Astronomical Society 914:1 (2021) L4

Abstract:

Internal redox reactions may irreversibly alter the mantle composition and volatile inventory of terrestrial and super-Earth exoplanets and affect the prospects for atmospheric observations. The global efficacy of these mechanisms, however, hinges on the transfer of reduced iron from the molten silicate mantle to the metal core. Scaling analysis indicates that turbulent diffusion in the internal magma oceans of sub-Neptunes can kinetically entrain liquid iron droplets and quench core formation. This suggests that the chemical equilibration between core, mantle, and atmosphere may be energetically limited by convective overturn in the magma flow. Hence, molten super-Earths possibly retain a compositional memory of their accretion path. Redox control by magma ocean circulation is positively correlated with planetary heat flow, internal gravity, and planet size. The presence and speciation of remanent atmospheres, surface mineralogy, and core mass fraction of primary envelope-stripped exoplanets may thus constrain magma ocean dynamics.

System-level fractionation of carbon from disk and planetesimal processing

Astrophysical Journal Letters American Astronomical Society 913:2 (2021) L20

Authors:

Tim Lichtenberg, Sebastiaan Krijt

Abstract:

Finding and characterizing extrasolar Earth analogs will rely on interpretation of the planetary system's environmental context. The total budget and fractionation between C-H-O species sensitively affect the climatic and geodynamic state of terrestrial worlds, but their main delivery channels are poorly constrained. We connect numerical models of volatile chemistry and pebble coagulation in the circumstellar disk with the internal compositional evolution of planetesimals during the primary accretion phase. Our simulations demonstrate that disk chemistry and degassing from planetesimals operate on comparable timescales and can fractionate the relative abundances of major water and carbon carriers by orders of magnitude. As a result, individual planetary systems with significant planetesimal processing display increased correlation in the volatile budget of planetary building blocks relative to no internal heating. Planetesimal processing in a subset of systems increases the variance of volatile contents across planetary systems. Our simulations thus suggest that exoplanetary atmospheric compositions may provide constraints on when a specific planet formed.

Ariel planetary interiors white paper

Experimental Astronomy Springer (2021)

Authors:

Ravit Helled, Stephanie Werner, Caroline Dorn, Tristan Guillot, Masahiro Ikoma, Yuichi Ito, Mihkel Kama, Tim Lichtenberg, Yamila Miguel, Oliver Shorttle, Paul J Tackley, Diana Valencia, Allona Vazan

Abstract:

The recently adopted Ariel ESA mission will measure the atmospheric composition of a large number of exoplanets. This information will then be used to better constrain planetary bulk compositions. While the connection between the composition of a planetary atmosphere and the bulk interior is still being investigated, the combination of the atmospheric composition with the measured mass and radius of exoplanets will push the field of exoplanet characterisation to the next level, and provide new insights of the nature of planets in our galaxy. In this white paper, we outline the ongoing activities of the interior working group of the Ariel mission, and list the desirable theoretical developments as well as the challenges in linking planetary atmospheres, bulk composition and interior structure.

Characterizing Regimes of Atmospheric Circulation in Terms of Their Global Superrotation

Journal of the Atmospheric Sciences American Meteorological Society 78:4 (2021) 1245-1258

Authors:

Neil T Lewis, Greg J Colyer, Peter L Read

Abstract:

<jats:title>Abstract</jats:title><jats:p>The global superrotation index <jats:italic>S</jats:italic> compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body corotation with the underlying planet. The index <jats:italic>S</jats:italic> is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of <jats:italic>S</jats:italic> for characterizing regimes of atmospheric circulation by running idealized Earthlike general circulation model experiments over a wide range of rotation rates Ω, 8Ω<jats:sub><jats:italic>E</jats:italic></jats:sub> to Ω<jats:sub><jats:italic>E</jats:italic></jats:sub>/512, where Ω<jats:sub><jats:italic>E</jats:italic></jats:sub> is Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute <jats:italic>S</jats:italic> for each simulated circulation, and study the dependence of <jats:italic>S</jats:italic> on Ω. For all rotation rates considered, <jats:italic>S</jats:italic> is on the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, <jats:italic>S</jats:italic> ≪ 1 and <jats:italic>S</jats:italic> ∝ Ω<jats:sup>−2</jats:sup>, while at low rotation rates <jats:italic>S</jats:italic> ≈ 1/2 = constant. By considering the limiting behavior of theoretical models for <jats:italic>S</jats:italic>, we show how the value of <jats:italic>S</jats:italic> and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. Indices of <jats:italic>S</jats:italic> ≪ 1 and <jats:italic>S</jats:italic> ∝ Ω<jats:sup>−2</jats:sup> define a regime dominated by geostrophic thermal wind balance, and <jats:italic>S</jats:italic> ≈ 1/2 = constant defines a regime where the dynamics are characterized by conservation of angular momentum within a planetary-scale Hadley circulation. Indices of <jats:italic>S</jats:italic> ≫ 1 and <jats:italic>S</jats:italic> ∝ Ω<jats:sup>−2</jats:sup> define an additional regime dominated by cyclostrophic balance and strong equatorial superrotation that is not realized in our simulations.</jats:p>

Characterizing Regimes of Atmospheric Circulation in Terms of Their Global Superrotation

Journal of the Atmospheric Sciences American Meteorological Society 78:4 (2021) 1245-1258

Authors:

Neil T Lewis, Greg J Colyer, Peter L Read

Abstract:

AbstractThe global superrotation index S compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body corotation with the underlying planet. The index S is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of S for characterizing regimes of atmospheric circulation by running idealized Earthlike general circulation model experiments over a wide range of rotation rates Ω, 8ΩE to ΩE/512, where ΩE is Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute S for each simulated circulation, and study the dependence of S on Ω. For all rotation rates considered, S is on the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, S ≪ 1 and S ∝ Ω−2, while at low rotation rates S ≈ 1/2 = constant. By considering the limiting behavior of theoretical models for S, we show how the value of S and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. Indices of S ≪ 1 and S ∝ Ω−2 define a regime dominated by geostrophic thermal wind balance, and S ≈ 1/2 = constant defines a regime where the dynamics are characterized by conservation of angular momentum within a planetary-scale Hadley circulation. Indices of S ≫ 1 and S ∝ Ω−2 define an additional regime dominated by cyclostrophic balance and strong equatorial superrotation that is not realized in our simulations.