11:00 AM - 11:15 AM
[PPS21-33] The vigor and the regime diagram of thermal convection in the mantle of massive super-Earths
Strong effects of adiabatic compression reduce hot plume activity significantly, while keeping cold plume activity high. The effects on hot plumes become more prominent as r increases, because the lithosphere becomes thicker as r increases and the potential temperature of the isothermal core increases. This results in decreasing difference of potential temperature between hot plumes and surround material, thus in decreasing buoyancy force of hot plumes.
We also studied the convective regime diagram on the plane of Ra and r. The threshold value of r for transition to the stagnant lid regime from small viscosity contrast regime increases as Ra increases in super-Earths in contrast to the diagram of the earlier Boussinesq model [Kameyama and Ogawa, 2000]. At high Ra relevant to massive super-Earths, the threshold value is larger than that expected in the Earth. To understand the reason why the threshold value of r increases as Ra increases, we present the viscosity contrast between the surface of the planet and the bottom of the lithosphere reff. In contrast to the increasing the threshold value of r, the reff is constant even the Rayleigh number increases. Thus, the reff is more relevant to transition to the stagnant lid regime rather than the viscosity contrast r in the whole mantle.
We also found that the Nusselt number Nu, which is the efficiency of heat transport by thermal convection, is considerably reduced compared with the earlier Boussinesq model. At Ra=1010 and r=107, the Nu is only 2.7 and 14% of the value expected from the earlier Boussinesq model. The thickness of the lithosphere is about 30% of the depth of the whole mantle. From systematic numerical simulation, Nu is fitted as a function of Ra and r. The power index on Ra is 0.27. This value is somewhat smaller than that in the earlier Boussinesq model (0.31) [Christensen, 1984].
The thick lithosphere shown in our model implies that plate tectonics is difficult to operate in super-Earths. However, the high threshold value in r for regime change suggests that the lithosphere moves in a way different from plate tectonics. Thermal convection may be in the small viscosity contrast regime in super-Earths and the surface may be fully involved in the convective current.