Geodynamics

```Check out two critical geophysics papers on why time
and probability are critical for geophysics.

Stamenković et al. (2016)
Stamenković & Seager (2016)```

Solid Earth & Planet Interiors
Right below our feet is a large ball of energy slowly cooling into outer space. This cooling of our planet significantly impacts our climate, plate tectonics, and life. But what determines the evolution of the Earth & planetary interiors? What are the properties of mantle and core minerals? And do all planet interiors evolve similarly? I use first principles to model and understand the evolution of the Earth and planet interiors—accounting for time evolution and intrinsic or current uncertainties.

Tectonics & Planet Surfaces
The lithosphere is barrier and connector between the deep interior of a rocky planet and its surface environment. On Earth, plate tectonics is responsible for recycling the lithosphere back into the mantle and for significantly driving volcanism and planetary cooling. However, other planets like Mars or Venus show no signs of plate tectonics. How does plate tectonics operate? How did it evolve on the Earth and on what kind of other planets can we expect plate tectonics, or other forms of mantle convection? I use thermodynamics to understand the evolution of plate tectonics and other forms of surface recycling.

Highlights in a Nutshell

• Time Evolution is critical.  Steady state models do not suffice to sufficiently well describe planetary interiors, especially for planets as or more massive than the Earth (see Stamenković et al., 2012, 2016).
• Geodynamic Evolution is of probabilistic nature. We should account for uncertainties as much as we can, carry them forward when computing a planet’s evolution, and look for robust results. Especially for exoplanets, we need to use a probabilistic approach when studying exogeodynamics (see Stamenković & Seager, 2016; Stamenković et al.,  2016).
• Never evolve planets back in time. It’s just wrong ;-)…there are many paths forward in time but only one backwards…and we do not know the initial conditions.
• Don’t forget the pressure-dependence of mantle viscosity—it’s a game changer (Stamenković et al., 2011, 2012).

More Highlights

• Melting temperature, thermal conductivity, and viscosity for mantle rock might be much larger than previously assumed – with strong implications for the Earth’s evolution and for the structure and evolution of rocky exoplanets: I compute state of the art viscosity, thermal expansivity, and thermal conductivity (phonon, radiative, and electronic) for the Earth’s mantle and for super-Earths from first principles. Total thermal conductivity increases strongly with depth. Our viscosity model satisfies the current viscosity constraints for the Earth. Computed melting temperatures are indirectly supported by melting experiments for MgO (McWilliams et al. 2012, Science).
• Earth’s thermal history needs a revision: Especially the interior conditions of the early Earth might have been much hotter than previously assumed – with strong implications for volcanism, the water cycle, and plate tectonics: Assuming full mantle convection (versus layered convection), I can only reproduce the inner core radius constraint for large activation volumes. Pressure-independent models do not satisfy the viscosity constraints by many orders of magnitude. They can only then satisfy the inner core radius constraint if there is either layered mantle or stagnant lid convection throughout most of the Earth’s history. New results on the validity of previous thermal evolution models (Stamenkovic and Breuer 2014, Stamenkovic 2014) additionally support the findings that early Earth heat flow has been overestimated (see as well Tectonics).
• Initial lower mantle temperature conditions control the thermal evolution of massive planets. Steady state models do not suffice: Initially molten super-Earths have a hot super-adiabatic temperature profile. Assuming interior temperatures from literature, the lowermost mantle would be stagnant. Hence, super-Earths might be much hotter than we think. Steady-state calculation are not sufficient to show this behavior, as cooling takes many billion years (even when forming molten). Ineffective lower mantle cooling impacts dynamo action & plate tectonics. Shorter volcanic activity on super-Earths. Problems for climate regulation. Problems for differentiation of rocky super-Earths.
• Bottom-up approach for the drivers of plate tectonics revising our common “beliefs” about plate failure and subduction and the thermal evolution of rocky planets: showing that temporal variation of basal shear stresses and asthenospheric channels drive the initiation of plate tectonics, and that classic self-determined boundary layer analysis cannot be used to model the evolution of the Earth. In more detail, we use 1-D thermal history models and 3-D numerical experiments to study the impact of dynamic thermal disequilibrium and large temporal variations of normal and shear stresses on the initiation of plate tectonics. Previous models that explored plate tectonics initiation from a steady state, single plate mode of convection concluded that normal stresses govern the initiation of plate tectonics, which based on our 1-D model leads to plate yielding being more likely with increasing interior heat and planet mass for a depth-dependent Byerlee yield stress. Using 3-D spherical shell mantle convection models in an episodic regime allows us to explore larger temporal stress variations than can be addressed by considering plate failure from a steady state stagnant lid configuration. The episodic models show that an increase in convective mantle shear stress at the lithospheric base initiates plate failure, which leads with our 1-D model to plate yielding being less likely with increasing interior heat and planet mass. In this out-of-equilibrium and strongly time-dependent stress scenario, the onset of lithospheric overturn events cannot be explained by boundary layer thickening and normal stresses alone. Our results indicate that in order to understand the initiation of plate tectonics, one should consider the temporal variation of stresses and dynamic disequilibrium.
• The connections between plate tectonics, planet composition, structure, and initial conditions: Showing how the initiation and maintenance of plate tectonics depend on planet composition (i.e., concentration of radioactive elements, iron, and carbon), initial conditions, and core and planet size. With crucial implications for Earth, Mars, and exoplanets. In more detail, to understand the evolution and the habitability of any rocky exoplanet demands detailed knowledge about its geophysical state and history— such as predicting the tectonic mode of a planet. Yet no astronomical observation can directly confirm or rule out the occurrence of plate tectonics on a given exoplanet. Moreover, the field of plate tectonics is still young— questioning whether we should study plate tectonics on exoplanets at this point in time. In this work, we determine the limitations and the emerging possibilities of exogeophysics, the science of connecting geophysics to exoplanets, on the example of plate tectonics. Assuming current uncertainties in model and planet parameters, we develop a qualitatively probabilistic and conservative framework to estimate on what kind of planets and where in the Galaxy plate tectonics might occur. This we achieve by modeling how plate yielding, the most critical condition needed for plate mobility and subduction, is affected by directly observable (planet mass, size) or indirectly, to some degree, assessable planet properties ( structure and composition). Our framework not only highlights the importance of a planet’ s chemistry for the existence of plate tectonics and the path toward practical exogeophysics but also demonstrates how exoplanet science can actually help to better understand geophysics and the fundamentals of plate tectonics on Earth itself.