THE ROLE OF CARBON IN EXTRASOLAR PLANETARY GEODYNAMICS AND HABITABILITY

Abstract

The proportions of oxygen, carbon and major rock-forming elements (e.g. Mg,\nFe, Si) determine a planet's dominant mineralogy. Variation in a planet's\nmineralogy subsequently affects planetary mantle dynamics as well as any deep\nwater or carbon cycle. Through thermodynamic models and high pressure diamond\nanvil cell experiments, we demonstrate the oxidation potential of C is above\nthat of Fe at all pressures and temperatures indicative of 0.1 - 2 Earth-mass\nplanets. This means that for a planet with (Mg+2Si+Fe+2C)/O > 1, excess C in\nthe mantle will be in the form of diamond. We model the general dynamic state\nof planets as a function of interior temperature, carbon composition, and size,\nshowing that above a critical threshold of $\\sim$3 atom% C, limited to no\nmantle convection will be present assuming an Earth-like geotherm. We assert\nthen that in the C-(Mg+2Si+Fe)-O system, only a very small compositional range\nproduce habitable planets. Planets outside of this habitable range will be\ndynamically sluggish or stagnant, thus having limited carbon or water cycles\nleading to surface conditions inhospitable to life as we know it.\n