Influence of Heat Flow from the Planetary Interior on Surface Habitability

Abstract:

Can heat flow from a rocky planet’s interior help maintain a surface environment amenable to life? On cold rocky Earth-like planets with shallow oceans (≤5 km depth), the additional energy might make the difference between a world supporting potentially detectable (if regional) habitats, and a world that appears uninhabitable. To test this hypothesis, we introduce heat flow from the planetary interior as a forcing to the NASA/GISS ROCKE-3D exoplanet GCM, to explore whether it can enhance the habitability of marginal terrestrial rocky planets (e.g., “snowball” worlds). GCM simulations of modern Earth have not included geothermal heat flux as a climate forcing, as the ~0.1 W/m² average heat flow is considerably less than the 2.9 W/m² heating caused by anthropogenic greenhouse gases [Flanner, GRL 2009]. However, modern Earth measurements in the oceans [Downes et al., GRL 2019] and results from ocean GCMs [Barnes et al., Ocean Model. 2017] suggest that localized higher heat flux can have measurable thermodynamic and dynamic impacts, such as weakening deep ocean stratification and increased poleward ocean heat transports. A similar result with ROCKE-3D would be important, since our snowball Earth simulations without geothermal heat flow produce features like ocean stratification that should have hampered the vertical transport of nutrients, and ultimately the survival of life.

We have created a set of spatially variable geothermal heat fluxes based on modern Earth heat flow patterns [Davies, Geochem. Geophys. Geosys. 2013] to test the sensitivity of the ROCKE-3D GCM to such input, initially in a modern Earth context. Preliminary results show that after ~2000 yrs of an on-going spin-up run, even modest heat fluxes have a measurable impact on sea surface temperatures (up to +1°C locally) and mixed layer depths (up to 100 m locally), with related sea ice area/thickness loss and decrease in surface albedo in polar regions. However, near-equilibrium estimates of heat flux impacts on vertical ocean mixing and surface ocean conditions require longer simulations at the higher spatial resolutions needed to express heat flow at the scale of most ocean ridges. Ongoing simulations are focused on testing the impacts of horizontal resolution and flux scaling on surface environments, and impacts on the ocean carbon cycle.