On the habitability of a stagnant-lid Earth

Tosi N, Stracke B, Godolt M, Ruedas, T, Grenfell L, Höning D, Nikolaou A, Plesa A-C, Breuer D, Spohn T

Zusammenfassung

Whether plate tectonics is a recurrent feature of terrestrial bodies orbiting other stars or is unique to the Earth is unknown. The stagnant-lid may rather be the most common tectonic mode through which terrestrial bodies operate. Here we model the thermal history of the mantle, the outgassing evolution of H2O and CO2, and the resulting climate of a hypothetical planet with the same mass, radius, and composition as the Earth, but lacking plate tectonics. We employ a 1-D model of parameterized stagnant-lid convection to simulate the evolution of melt generation, crust production, and volatile extraction over a timespan of 4.5 Gyr, focusing on the effects of three key mantle parameters: the initial temperature, which controls the overall volume of partial melt produced; the initial water content, which affects the mantle rheology and solidus temperature; and the oxygen fugacity, which is employed in a model of redox melting to determine the amount of carbon stored in partial melts. We assume that the planet lost its primo rdial atmosphere but possesses an Earth-like ocean, and use the H2O and CO2 outgassed by melts extracted from the interior to build up a secondary atmosphere over time. We calculate the atmospheric pressure based on the solubility of H2O and CO2 in basaltic magmas at the evolving pressure conditions of the surface. We then employ a 1-D radiative-convective, cloud-free stationary atmospheric model to calculate the resulting atmospheric temperature, pressure and water content, and the corresponding boundaries of the habitable zone. The evolution of the interior is characterized by an initial production of a large amount of partial melt accompanied by the formation of crust that rapidly grows until its thickness matches that of the stagnant lid so that the conv ecting sublithospheric mantle prevents further crustal growth. On the one hand, the high solubility of water in surface magmas limits the maximal partial pressure of atmospheric H2O to a few tens of bars, even for initial water concentrations in excess of thousands of ppm. On the other hand, the low solubility of CO2 causes most of the carbon to be outgassed. As a consequence, the partial pressure of atmospheric CO2 is mainly controlled by the redox state of the mantle, with values that range from a few up to tens of bars for oxygen fugacities between the iron-wüstite buffer and one log-unit above it. Our results suggest rather warm (habitable) surface temperatures over long timescales.