The Thermodynamics of Life on a Planetary Scale

The thermodynamically-constrained fluxes of gases to and from a biosphere has profound, planet-wide consequences. These fluxes can directly control the redox state of the surface environment, the atmospheric composition, and the concentration of nutrients and metals in the oceans. Through these direct effects, they also create strong forcings on the climate, the redox state of the interior of the planet, and the detectability of the biosphere by remote observations. In this presentation, we will model chemosynthetic-based biospheres, and apply them to questions related to the habitability of Archean Earth. This will be accomplished by coupling together two models: one model codifies a thermodynamic energy balance concept for habitability (Hoehler, 2009); the other model predicts a planet's atmospheric composition and climate while balancing the redox state of the surface environment (Domagal-Goldman et al., 2014). The result of this coupling is the calculation of a self-consistent planetary surface environment, that treats the ecosystem as one system component that is coupled to atmospheric, oceanic, and subsurface system components. The model will predict the standing biomass size, net biological productivity, and resulting gas fluxes to and from a biosphere with a given set of volcanic and hydrothermal inputs to the planetary surface. This will also enable predictions of the secondary effects life has on its environment, including controls on climate and the long-term evolution of the redox state of the planet's surface and near-surface. Finally, this will aid us in predicting the strength of biosignatures for remote observations and ultimately help us interpret spectra from extrasolar planets. In this presentation, we will review the energy balance concept for habitability, describe how that has been codified in our models, and give preliminary results from this coupling.