Thermal evolution of lava planets

Mahesh Herath ( Université McGill )

The horizontal and vertical extent of magma oceans on lava planets depend on the interior thermal evolution of the planet. It may be possible to learn more about the history of lava planets by linking the characteristics of their magma oceans to their interior thermodynamics and structure. We present a low-order model for simulating the thermal history of tidally locked lava planets. We start the model with a completely molten mantle and evolve it for ten billion years. We adopt a fixed surface temperature of 3000 K for the irradiated day-side, but allow the night-side temperature to evolve along with the underlying layers. We simulate planets of radius 1.0R_♁, 1.5R_♁ and different core mass fractions although the latter does not affect the results. We found that the day-side magma ocean on these planets has a depth that depends on the planetary radius. The night-side, on the other hand, completely solidifies within 800 million years in the absence of substantial tidal heating or day-night heat transport. We show that the night-side of a lava planet can be kept molten if at least 20 per cent of absorbed stellar power is transmitted from the day-side to the night-side. Such day-night transport could be sustained if the magma has a viscosity of water, which is plausible at these temperatures. Alternatively, the night-side could remain molten if the planet is tidally heated at the rate of 8 x 10^-4 W/kg of mush, which is plausible for orbital eccentricities of e > 7 x 10^-3. Night-side cooling is a runaway process: the magma becomes more viscous and the mush solidifies, reducing both day-night heat transport and tidal heating. Measurements of the night-sides of lava planets are therefore a sensitive probe of the thermal history of these planets.