The role of planetary interior in the long-term evolution of atmospheric CO2 on Earth-like exoplanets

Abstract

Context: The long-term carbonate-silicate cycle plays an important role in the evolution of Earth's climate and, therefore, may also be an important mechanism in the evolution of the climates of Earth-like exoplanets. Aims: We investigate the effects of radiogenic mantle heating, core size, and planetary mass on the evolution of the atmospheric partial $CO_2$ pressure, and the ability of a long-term carbon cycle driven by plate tectonics to control the atmospheric $CO_2$ pressure. Methods: We developed a box-model which connects carbon cycling to parametrized mantle convection. The carbon cycle was coupled to the thermal evolution via the plate speed, which depends on the global Rayleigh number. Results: We find decreasing atmospheric $CO_2$ pressure with time, up to an order of magnitude over 10 Gyr. Higher abundances of radioactive isotopes result in higher $CO_2$ pressures. We find a decreasing Rayleigh number and plate speed toward planets with larger core mass fractions $f_c$, which leads to lower atmospheric $CO_2$ pressure. More massive planets may favor the development of more $CO_2$ rich atmospheres due to hotter interiors. Conclusions: The dependence of plate tectonics on mantle cooling has a significant effect on the long-term evolution of the atmospheric $CO_2$ pressure. Carbon cycling mediated by plate tectonics is efficient in regulating planetary climates for a wide range of mantle radioactive isotope abundances, planet masses and core sizes. More efficient carbon cycling on planets with a high mantle abundance of thorium or uranium highlights the importance of mapping the abundances of these elements in host stars of potentially habitable exoplanets. Inefficient carbon recycling on planets with a large core mass fraction ($f_c\gtrsim 0.8$) emphasizes the importance of precise mass-radius measurements of Earth-sized exoplanets.