Lund Observatory Seminar

Galactic chemical evolution expressed in rocky exoplanet geodynamics

presented by prof. Stephen Mojzsis from the Research Centre for Astronomy and Earth Sciences (CSFK), Budapest, Hungary

Abstract:

Unlike the Hertzsprung–Russell diagram for stars, there remains no formal classification for solid exoplanets composed of varying proportions of fluids, rock+metals and ice. Still, as with stars, planetary mass and composition – expressed in geochemical and cosmochemical terms – modulate bulk physical characteristics.

Two physical attributes control terrestrial-type (‘rocky’) planet interior dynamics: viscosity (η) and intrinsic heat production (A). Viscosity can differ by orders of magnitude between different common mantle silicate minerals (e.g. olivine, pyroxene), so that even small proportional changes yield large differences to η. A key parameter to consider in this context is (Mg:Si:Fe), because this value largely determines which minerals will be present in silicate mantles. Bulk Silicate Earth’s (Mg:Si:Fe) is close to solar values, and we assume that this also holds for terrestrial-type exoplanets in that they follow the compositions of their host stars.

Transition between mechanically weak (olivine-dominated at (Mg/Si)≤1, low η) vs. strong (pyroxene-dominated at (Mg/Si)>1, high η) mantle convective regimes occurs over a narrow transitional range of (Mg/Si) values because small volume fractions of a weak phase are sufficient to form an interconnected network that in turn governs the strain response of mantle rocks to deforming stresses acting upon them.

Heat production is proportionally greater in younger planets owing to more radioactivity and latent accretionary/gravitational heating. Thus, system age is relevant viz. how heat loss is accommodated by interior dynamics and expressed via outgassing to secondary atmospheres. Here we show how combining geodynamics with astrophysical observations provides insights to terrestrial exoplanet η and A vs. age.

Younger (≤2 Gyr) stars tend to have low (Mg/Si)≤1. If these stars mirror silicate exo-mantles, such young low (Mg/Si) pyroxene-rich mantles ought to tend towards both high η and A, with episodic sluggish/rapid convection and thus slow cooling and low oxygen fugacities to degas H₂ and CH₄ under near-surface partial melting conditions [12. Contrariwise, older (>5 Gyr) olivine-rich (high Mg/Si) oxidized (like Earth) exo-mantles should tend towards both low η and A, effectively cool, and degas N₂, CO₂, H₂O ± SO₂.

By implication, a fundamental age-composition dichotomy is anticipated to exist between young (hot, reduced, Fe-rich) and old (cold, oxidized, Fe-poor) rocky exoplanets that can already be evaluated by mass-radius density-age data especially of ultra-short period planets around Sun-like stars, using ionized atmospheres to explore geology.

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