The effect of galactic chemical evolution on terrestrial exoplanet composition and tectonics

C. O'Neill*, Julian Lowman, Jonathon Wasiliev

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

2 Citations (Scopus)

Abstract

The increasing exoplanet catalogue of terrestrial to super-Earth sized rocky planets has necessitated approaches to address their surface conditions, degassing, tectonics, and habitability. One of the key unknowns is the spectrum of compositional endmembers, particularly in geophysically critical elements such as Fe (relative to Si), and heat producing elements (HPEs), including U, Th and K. The fractional distribution of these elements determines core size, gravity, and internal temperatures, which govern the geodynamics of rocky planets. Whilst the relative proportions of these elements can vary enormously in solar-system bodies, on long timescales, galactic chemical evolution (GCE) predicates the availability of these elements for building rocky planets. Existing models demonstrate a systematic dilution of heat producing elements over time, and an increase in Fe/Si due to increased Type Ia supernova activity since galaxy formation. Here we test the consequences of these trends for terrestrial planet behaviour. We assume an Earth-type planet – i.e., planets that accreted similarly to Earth, and inherited elements in a similar partitioning ratio as Earth to the Sun. We varied their Fe/Si ratios, and thus core size, and use a mineral physics package to calculate internal structures, physical properties, and gravity. Planets forming early in the Milky Way's history tend to have low Fe/Si ratios, and thus small cores, although elevated HPE budgets. The configuration of convection in planets that have large mantle fractions relative to the Earth tends to focus stress in the lithosphere and promote tectonic activity. Together with lower gravity, and thus weaker surface faults, plate tectonic behaviour is enhanced in small-core planets. In contrast, planets forming currently have higher Fe/Si, core size, higher gravity, and a lower propensity to plate tectonics. Our results imply a high-propensity for plate tectonics on Earth-sized planets early in galactic history, with the tendency for tectonics diminishing as the galaxy evolved.

Original languageEnglish
Article number114025
Pages (from-to)1-11
Number of pages11
JournalIcarus
Volume352
DOIs
Publication statusPublished - Dec 2020

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