Abstract
Adiabatic compression is a key factor that exerts control over thermal convection in the compressible solid mantle of super-Earths. To discuss the effects of adiabatic compression, we present a numerical model of transient convection in the cooling mantle of a super-Earth that is ten times larger in size than the Earth. The calculations started with the shallow mantle that was hotter than expected by the extrapolation from the deep mantle conditions. This type of initial thermal state of the mantle is expected to naturally occur in real super-Earths due to heating by giant impacts at the time of their formation. With our initial setup conditions, the convection temporarily occurs as a layered convection for the first several to ten billion years of the calculation and then changes its style into a whole layer convection. The long duration of the transient stage suggests that mantle convection currently occurs as a temporal layered convection in many of the super-Earths. A temporal layered convection, if it occurs, can exert control over the tectonic activities of super-Earths. Future studies should clarify how internal heating and complicated rheological properties of mantle materials including their pressure dependence affect the duration of the temporal layered convection.Graphical abstract.
Highlights
Dynamics of the mantle in super-Earths is a key issue in studying their thermal history and surface environments
In the course of these numerical studies of mantle convection in super-Earths, we noticed that the nature of the thermal convection substantially changes with time, as the convection approaches the statistical steady state, and that the period of the transitional stage becomes as long as ten billion years based on the parameter values of massive super-Earths; the length of the transitional stage is comparable to the age of the universe
We present an example of transient mantle convection we encountered in our numerical studies of a massive super-Earth to discuss how the nature of thermal convection can change in the course of its approach to the statistical steady state and on what timescale the change occurs (note that Miyagoshi et al (2015) reports only the convections in their statistical steady states)
Summary
Dynamics of the mantle in super-Earths is a key issue in studying their thermal history and surface environments. Stimulated by the detection of a large number of super-Earths, which are extrasolar terrestrial planets with a mass of up to ten times the Earth’s (e.g., Borucki et al 2011), many theoretical and numerical studies have been conducted to explain the structure and dynamics of their interior In many of these studies, mantle convection is modeled by the parameterized convection method (e.g., Valencia and O’connell 2009; Stamenković et al 2011) or by a steady convection of a Boussinesq fluid where the effect of adiabatic compression is neglected (e.g., van Heck and Tackley 2011; Foley et al 2012; Stein et al 2013). Given the long time span needed for the effects of the initial conditions to disappear, caution is necessary to apply these numerical results to super-Earths
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