The habitability of a stagnant-lid Earth

¹ Technische Universität Berlin, Department of Astronomy and Astrophysics, Hardenbergstr. 36, 10623 Berlin, Germany
e-mail: nic.tosi@gmail.com
² Institute for Planetary Research, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
³ Westfälische Wilhelms-Universität Münster, Institute of Planetology, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

Received: 3 March 2017
Accepted: 24 June 2017

Abstract

Context. Plate tectonics is considered a fundamental component for the habitability of the Earth. Yet whether it is a recurrent feature of terrestrial bodies orbiting other stars or unique to the Earth is unknown. The stagnant lid may rather be the most common tectonic expression on such bodies.

Aims. To understand whether a stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we model the thermal evolution of the mantle, volcanic outgassing of H₂O and CO₂, and resulting climate of an Earth-like planet lacking plate tectonics.

Methods. We used a 1D model of parameterized convection to simulate the evolution of melt generation and the build-up of an atmosphere of H₂O and CO₂ over 4.5 Gyr. We then employed a 1D radiative-convective atmosphere model to calculate the global mean atmospheric temperature and the boundaries of the habitable zone (HZ).

Results. The evolution of the interior is characterized by the initial production of a large amount of partial melt accompanied by a rapid outgassing of H₂O and CO₂. The maximal partial pressure of H₂O is limited to a few tens of bars by the high solubility of water in basaltic melts. The low solubility of CO₂ instead causes most of the carbon to be outgassed, with partial pressures that vary from 1 bar or less if reducing conditions are assumed for the mantle to 100–200 bar for oxidizing conditions. At 1 au, the obtained temperatures generally allow for liquid water on the surface nearly over the entire evolution. While the outer edge of the HZ is mostly influenced by the amount of outgassed CO₂, the inner edge presents a more complex behaviour that is dependent on the partial pressures of both gases.

Conclusions. At 1 au, the stagnant-lid planet considered would be regarded as habitable. The width of the HZ at the end of the evolution, albeit influenced by the amount of outgassed CO₂, can vary in a non-monotonic way depending on the extent of the outgassed H₂O reservoir. Our results suggest that stagnant-lid planets can be habitable over geological timescales and that joint modelling of interior evolution, volcanic outgassing, and accompanying climate is necessary to robustly characterize planetary habitability.