Earth's Thermal and Magnetic Paradoxes
Recent discoveries relating to the thermal and magnetic history of the Earth have revealed that a clement surface and a dynamo generated magnetic field have persisted over almost all of Earth history. However, the preferred mantle radioactivity and heat loss efficiency predict wide spread mantle melting, or a “thermal catastrophe”, only 2−3 Ga. Similarly, paleomagnetic observations that indicate a geodynamo as old as 3.4 Gyr are at odds with the “new core paradox”, which claims insufficient energy to drive the ancient geodynamo prior to inner core nucleation ~ 1 Ga in light of recent revisions to the thermal conductivity of the core. In Driscoll & Bercovici (2014, PEPI) we demonstrate that to avoid both the mantle and core paradoxes restricts the present core heat flow to the range 13 − 17 TW, radiogenic core heat production to 2-4 TW, and inner core age to 0.5 − 1.0 Ga. This solution accommodates the geochemically preferred mantle radiogenic heat production of 13 TW, while maintaining a mostly solid mantle and core generated magnetic field over the geological history of the planet.
Earth's climate is stabilized over geologic time scales by the tectonic-carbon cycle, where CO2, a major greenhouse gas, is exchanged between the atmosphere, ocean floor, and mantle. Walker et al. (1981) demonstrated that a negative feedback between CO2 atmospheric pressure and weathering rate should stabilize atmospheric CO2 and climate. Following this idea many global carbon cycle models have been constructed (e.g. Burner et al., 1983) in an attempt to couple fluctuations in ground temperature and CO2 in the Phanerozoic. Such a stabilizing feedback should be possible on other terrestrial planets, but to date is unique to the Earth. Why?
In Driscoll & Bercovici (2013, ICARUS) we have developed a model for the initiation of the carbon cycle in the Hadean. The model considers the stability of liquid water at the surface as a function of orbital distance and CO2 pressure, the initial degassing of the ocean from the mantle, cycling of CO2 from atmosphere to mantle via silicate weathering, and the influence of tectonic uplift and exhumation rates on carbon weathering. Steady states are found for a range of degassing rates, plate speeds, and atmospheric volatile escape rates.
Future work will expand the surface-atmosphere model to include the thermal cooling of Earth's interior, and application to Venus where no steady state carbon cycle is reached due to no liquid water and a stagnant lid. The long term goal of this project is to develop a general planet history that determines whether the planet is Earth-like (plate tectonics, magnetic field, liquid water) or Venus-like (stagnant lid, no magnetic field, and runaway greenhouse) given a few simple parameters, such as planet mass and composition, orbital properties, and initial conditions. Such a generalized theory will be needed in order to interpret new exoplanet observations that provide only sparse information about the planet.
Below: Box model diagram of carbon-tectonic-atmosphere system.