Long-lived magnetic field in earth-like terrestrial planets

Abstract

We do not know precisely whether powering a planetary magnetic field is a common or rare feature of Earth-like planets, and for how many billion years it should likely be operating. If planetary dynamos should be driven by internal heat flow essentially (i.e. thermally driven dynamo, TDD), the answer relies on the energy budget of the planetary interior. On Earth, the Moon-forming impact provided a lot of energy ∼4.5 billion years ago and the magnetic field remains strong until present. Despite extensive work on this subject, the controversial nature of the outer-core's thermal conductivity (k_Cond) makes the energy budget of the core open to discussions. Here we present new experimental constraints on the evolution of k_Cond with pressure from 22 to 150 GPa and temperature from 1050 to 2700 K. k_Cond is obtained by numerical modeling of the mechanism of propagation of a short heat pulse through a thin foil of iron loaded, compressed and heated in the laser-heated diamond anvil cell. With good coverage of large P-T domains, the accuracy of our measurements enables an accurate modeling of k_Cond as a function of the molar volume of Fe and temperature. We refine a value of 37(5) W/m/K at P-T conditions found at the Earth core-mantle boundary (CMB). If the geodynamo should be TDD essentially, this implies CMB cooling by 380–450 K over the entire Earth history and an inner core 1.9(4) billion years old. By comparing the mantle efficiency to extract heat at the CMB with the power requirement to sustain a TDD, we show that the geodynamo is unlikely to stop until the Earth's core is solidified. This effect comes from a decreasing power requirement to generate the dynamo with decreasing the temperature at the CMB. Now applying our P-T dependent k_Cond model to terrestrial planets with various external radii and fractions of silicate and metal, we show that the core size is a critical parameter. For a core smaller than a critical size, mantle convection induces a sufficient CMB heat flow to enable a TDD. This can explain absence of dynamo on Mars and possibly on Venus also if its core would be relatively large. Calculations show that exoplanets 1.5 times heavier than the Earth are unlikely to present an alive TDD, especially if they present large bulk densities. In contrast, the small exoplanets reported to date could host a TDD.