On the Dynamics of a Coupled Climate Mantle System and the Implications for Planetary Habitability and Evolution

Abstract:

Plate tectonics is thought to be a crucial ingredient of planetary habitability because it facillitates the operation of the long term carbon cycle, with its inherent stabilizing climate feedbacks, throughout geologic history. However, recent work in understanding the geodynamical causes of plate tectonics highlights the importance of a cool climate in allowing a plate tectonic, rather than a stagnant lid, mode of mantle convection to operate. Thus the climate and the mode of mantle convection (i.e. plate tectonic or stagnat lid) may be fully coupled. I present new work on the dynamics of such a fully coupled atmosphere-mantle system to assess the stability of the plate tectonic-cool climate state for varying planet size, total planetary carbon budget, and mantle temperature (which maps to planet age). I use a model that couples recently developed scaling laws for mantle convection with grain-damage, a grain size feedback mechanism for generating plate tectonic style mantle convection, with a simple carbon cycle box model and a 1-D radiative-convective atmosphere model. Steady-state models demonstrate that the negative climate feedbacks resulting from the long term carbon cycle are preserved, and possibly even enhanced, when the vigor of plate tectonics depends on surface temperature. Thus planets with temperate climates (e.g. those within the habitable zone) are more likely to have plate tectonics and to maintain their habitable climates over geologic time. A key question is then what the initial surface conditions of a planet are, and how they influence its subsequent evolution. In particular, if planets undergo a magma ocean phase, significant degassing from the solidifying mantle could lead to CO₂ rich proto-atmospheres, which would inhibit plate-tectonics, and the recycling of carbon into the mantle via subduction. I focus on this case, with a CO₂ rich proto-atmosphere, as it acts against the development of plate tectonics and a habitable climate. Using full evolution models of the coupled mantle-carbon cycle system I show the conditions necessary for a stable plate-tectonic-habitable climate state to develop, and the timescales for the evolution from a post-magma ocean, CO₂ rich atmosphere, to a temperate climate. Finally, the implications of this work for planetary habitability are discussed.