Tides on Lava Worlds: Application to Close-in Exoplanets and the Early Earth-Moon System

TL;DR

This paper introduces an analytical model for the tidal response of magma oceans on rocky planets, revealing significant effects on early Earth-Moon dynamics and the spin-orbit evolution of close-in exoplanets, driven by fluid behavior.

Contribution

It provides a new model that accounts for phase transitions in magma oceans, improving predictions of tidal interactions and heat generation on lava worlds.

Findings

Early Lunar recession driven by Earth's molten surface within 10^4-10^5 years.

Molten surfaces alter exoplanet spin-orbit dynamics, avoiding resonances.

Fluid-driven tidal heating influences exoplanet surface temperatures.

Abstract

Understanding the physics of planetary magma oceans has been the subject of growing efforts, in light of the increasing abundance of Solar system samples and extrasolar surveys. A rocky planet harboring such an ocean is likely to interact tidally with its host star, planetary companions, or satellites. To date, however, models of the tidal response and heat generation of magma oceans have been restricted to the framework of weakly viscous solids, ignoring the dynamical fluid behavior of the ocean beyond a critical melt fraction. Here we provide a handy analytical model that accommodates this phase transition, allowing for a physical estimation of the tidal response of lava worlds. We apply the model in two settings: The tidal history of the early Earth-Moon system in the aftermath of the giant impact; and the tidal interplay between short-period exoplanets and their host stars. For the former, we show that the fluid behavior of the Earth's molten surface drives efficient early Lunar recession to ${\sim} 25$ Earth radii within $10^4{-} 10^5$ years, in contrast with earlier predictions. For close-in exoplanets, we report on how their molten surfaces significantly change their spin-orbit dynamics, allowing them to evade spin-orbit resonances and accelerating their track towards tidal synchronization from a Gyr to Myr timescale. Moreover, we re-evaluate the energy budgets of detected close-in exoplanets, highlighting how the surface thermodynamics of these planets are likely controlled by enhanced, fluid-driven tidal heating, rather than vigorous insolation, and how this regime change substantially alters predictions for their surface temperatures.