Abstract
We present two-dimensional numerical models of thermal convection of a compressible fluid in the mantles of super-Earths calculated under the truncated anelastic liquid approximation to discuss how adiabatic compression affects the thermal convection, depending on planetary mass. The convection is driven by basal heating, the viscosity depends on temperature, and the thermal expansivity and the reference density depend on the depth. We varied all of the magnitude of adiabatic heating, the Rayleigh number, the depth profile of the thermal expansivity, and that of the reference density in accordance with the planetary mass. The effects on thermal convection become substantial, when the planetary mass normalized by the Earth’s mass Mp exceeds a threshold Mc, about 4. Hot plumes ascending from the core–mantle boundary become thinner with increasing Mp; they become almost invisible except around the core–mantle boundary, when Mp > Mc. The lithosphere that develops along the surface boundary due to the temperature dependence of viscosity becomes thicker with increasing Mp and is about twice as thick as that at Mp = 1 when Mp = 9.4. The convective velocity is almost independent of Mp. These results are in a striking contrast with earlier predictions that are made based on the models where the effects of adiabatic compression are neglected; it is important to take account of the effects of adiabatic compression properly in the exploration of mantle dynamics such as plate tectonics and hot spot volcanisms in massive super-Earths. Further researches are necessary to clarify the dependence of Mc on the surface temperature and the material properties of the convecting mantle.
Highlights
Motivated by recent detection of a large number of super-Earths, i.e., extrasolar planets with a mass of up to ten times the Earth’s (e.g., Borucki et al 2011), many researchers have explored dynamics of the mantle of super-Earths, because it is a key to understanding their tectonics, thermal history, and surface environments (e.g., Valencia et al 2007; Valencia and O’Connell 2009; van Heck and Tackley 2011; Foley et al 2012; Lenardic and Crowley 2012; Stein et al 2013; Tackley et al 2013; Stamenković and Breuer 2014)
Miyagoshi et al (2014, 2015) found that the lithosphere becomes much thicker and hot ascending plumes become thinner than expected from Boussinesq models in a planet of ten times the Earth’s mass. These results suggest that it is necessary to revisit the issue of plate tectonics and hot spot volcanism in super-Earths, taking account of the effects of adiabatic heating (e.g., Valencia et al 2007; Valencia and O’Connell 2009; van Heck and Tackley 2011; Foley et al 2012)
At Mp= 1, hot plumes ascending from the core–mantle boundary are as prominent as the cold plumes that descend from the top cold thermal boundary layer (TBL), and the convective velocity around the cold plumes is comparable to that around the hot plumes
Summary
Motivated by recent detection of a large number of super-Earths, i.e., extrasolar planets with a mass of up to ten times the Earth’s (e.g., Borucki et al 2011), many researchers have explored dynamics of the mantle of super-Earths, because it is a key to understanding their tectonics, thermal history, and surface environments (e.g., Valencia et al 2007; Valencia and O’Connell 2009; van Heck and Tackley 2011; Foley et al 2012; Lenardic and Crowley 2012; Stein et al 2013; Tackley et al 2013; Stamenković and Breuer 2014). These results suggest that it is necessary to revisit the issue of plate tectonics and hot spot volcanism in super-Earths, taking account of the effects of adiabatic heating (e.g., Valencia et al 2007; Valencia and O’Connell 2009; van Heck and Tackley 2011; Foley et al 2012).
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