Magnetodynamo lifetimes for rocky, Earth-mass exoplanets with contrasting mantle convection regimes

Abstract

We used a thermal model of an iron core to calculate magnetodynamo evolution in Earth-mass rocky planets to determine the sensitivity of dynamo lifetime and intensity to planets with different mantle tectonic regimes, surface temperatures, and core properties. The heat flow at the core-mantle boundary (CMB) is derived from numerical models of mantle convection with a viscous/pseudoplastic rheology that captures the phenomenology of plate-like tectonics. Our thermal evolution models predict a long-lived (∼8 Gyr) field for Earth and similar dynamo evolution for Earth-mass exoplanets with plate tectonics. Both elevated surface temperature and pressure-dependent mantle viscosity reduce the CMB heat flow but produce only slightly longer-lived dynamos (∼8-9.5 Gyr). Single-plate ("stagnant lid") planets with relatively low CMB heat flow produce long-lived (∼10.5 Gyr) dynamos. These weaker dynamos can cease for several billions of years and subsequently reactivate due to the additional entropy production associated with inner core growth, a possible explanation for the absence of a magnetic field on present-day Venus. We also show that dynamo operation is sensitive to the initial temperature, size, and solidus of a planet's core. These dependencies would severely challenge any attempt to distinguish exoplanets with plate tectonics and stagnant lids based on the presence or absence of a magnetic field. Key Points Long-lived (∼8-10.5 Gyr) dynamos are calculated for rocky Earth-mass exoplanetsCharacterization of exoplanets requires well-determined core propertiesVenus may lack a magnetic field due to a CMB heat flow lower than Earth's ©2013. American Geophysical Union. All Rights Reserved.