Planetary Science Seminar spring-2016

Powering a Magnetic Field on Earth but not Venus

Earth's global magnetic field arises from vigorous convection within the liquid outer core. Paleomagnetic evidence suggests that the geodynamo has operated for at least 3.4 billion years. Available power sources in standard models include secular cooling and compositional convection driven by the solidifying inner core's expulsion of light elements. But recent experiments and theory imply that the thermal conductivity of iron is two or three times larger than typically assumed in these models. This presents a problem: a large increase in the conductive heat flux implies that the inner core is less than one billion years old, yet sustaining the geodynamo with thermal convection alone during earlier epochs is difficult. I will demonstrate that the precipitation of magnesium-bearing minerals from the core could serve as an alternate power source. A small amount of magnesium partitions into core alloy in the high-temperature aftermath of giant impacts. Transport of magnesium as oxide or silicate across the core/mantle boundary is an order of magnitude more efficient per unit mass as a source of buoyancy than inner-core growth. Even if Venus suffered giant impacts like Earth, however, stagnant-lid convection in the mantle would limit core cooling enough to kill a dynamo.