Ignan Earths: Habitability of Terrestrial Planets With Extreme Internal Heating

Abstract

Is it possible for a rocky planet to have too much internal heating to maintain a habitable surface environment? In the Solar System, the best example of a world with high internal heating is Jupiter's moon Io, which has a heat flux of approximately 2 W $m^{-2}$ compared to the Earth's $\sim$ 90 mW $m^{-2}$. The ultimate upper limit to internal heating rates is the Tidal Venus Limit, where the geothermal heat flux exceeds the Runaway Greenhouse Limit of $\sim$300 W $m^{-2}$ for an Earth-mass planet. Between Io and a Tidal Venus there is a wide range of internal heating rates whose effects on planetary habitability remain unexplored. We investigate the habitability of these worlds, referred to as Ignan Earth's. We demonstrate how the mantle will remain largely solid despite high internal heating, allowing for the formation of a convectively buoyant and stable crust. In addition, we model the long-term climate of Ignan Earth's by simulating the carbonate-silicate cycle in a vertical tectonic regime (known as heat-pipe tectonics, expected to dominate on such worlds) at varying amounts of internal heating. We find that Earth-mass planets with internal heating fluxes below $\sim$15 W $m^{-2}$ produce average surface temperatures that Earth has experienced in its past (below 30$°$C), and worlds with significantly higher heat fluxes still result in surface temperatures far below that of 100$°$C, indicating a wide range of internal heating rates may be conducive with habitability.