Rocky worlds: What do a planet's orbital parameters tell us about its mantle state?

Rocky (exo)planets can be classified based on their mantle viscous state. The mantle viscosity influences the efficiency of convection and heat loss of the planet, altering the outgassing rate. Low viscous planets are hypothesized to have strong volcanic activity reshaping the surface and changing the atmosphere. This can be seen in other almost-similar rocky bodies such as Io, one of Jupiter’s Galilean moons, and would be expected of young rocky exoplanets. Whereas, the intermediate viscous planets have less vigorous resurfacing. They experience occasional complete mantle-overturn to slow-moving plate tectonics driven by mantle convection. As a result, their atmospheres vary little or smoothly across time. High viscous planets can be seen as inert, with little or no mantle convection. Moreover, hotspot volcanism might still occasionally contribute to outgassing, producing a less dominant atmosphere. Through this relationship, a planet’s atmosphere could reveal information about the evolution of a planet's interior and surface. However, we rely on primary observables to characterize (exo)planets. So, is there a correlation between a planet's orbital position and mantle viscosity? The answer to this question would aid in the characterization of rocky exoplanets, which is the focus of this work. 

 

To study the relationship between a planet’s mantle viscous state, interior composition, and structure. We use Perple-X to generate mineral physics properties, and Burnman to build a 1-dimensional depth profile of the planet. From this, 2-dimensional annulus compressible convection models are developed using ASPECT. And, exploring the stagnant lid, the episodic lid, and the tectonic convection regimes. We consider the anelastic liquid approximation (ALA), and the truncated anelastic liquid approximation (TALA) formulations. An isoviscous profile results in a hot mantle but can be used for first-order approximations of mantle dynamics without a crust. However, the presence of crust requires for temperature-dependent stratified viscosity profile for the deeper mantle to allow for the cooling of the mantle. The stratification structure of the mantle is determined by the temperature sensitivity of the mineral phases present in the depth profile of the mantle. The iron mass fraction of the planet, which is highly dependent on the orbital position dictates the thermal state of the given planet. Moreover, a high mass iron fraction in the mantle results in a highly viscous planet. Which cools much faster but with a higher average temperature in the earlier phases of evolution. And vice versa to a similar mantle state with less iron mass fraction in the mantle.