The balance of carbon dioxide and water on tectonically dead worlds

Abstract:

Conventional habitable zone (HZ) theory for exoplanets assumes efficient carbonate-silicate equilibration via plate tectonics. This in turn places natural assumptions on the atmospheric conditions and constituent concentrations we might observe in traditional H2O-N2-CO2 terrestrial atmospheres along the HZ, such as the expectation that the carbonate-silicate thermostat will cut off any possibility of high atmospheric partial pressures of carbon dioxide near the inner HZ. However, recent geophysical theory suggests many exoplanets may be in tectonically different regimes from Earth. To increase our understanding of the range of possible atmospheric and surface compositions for potentially habitable exoplanets, here we explore an alternative end-member for the carbon cycle in which both volcanic fluxes and erosion rates are negligible. We use a coupled model approach combining a 1-D radiative convective model for the atmosphere and an aqueous chemistry model, which jointly partition carbon dioxide and water between gaseous and aqueous phases in the planetary atmosphere and ocean, respectively. We explore the role of a saturated calcium carbonate layer in ocean acidification and carbon uptake, which serves as our simplified representation of carbonate-silicate feedback for the case of a tectonically inactive world. By varying the total carbon and water budgets, we investigate habitability and observable atmospheric properties in a range of planetary settings, from arid dune planets to water worlds.