Life of the Martian dynamo

Abstract

When the core dynamo of Mars initiated and when and why it ceased is not well understood. Attempt is made in this study to constrain the active period of the core dynamo, assuming that it was powered by vigorous thermal convection in the liquid iron core. Two distinct periods of the planet are investigated: the latest stage of accretion when the growing Mars embryo likely experienced high-velocity embryo–embryo collisions, and during the late bombardment at ∼4Ga that created 20 giant basins on Mars. The impact heating of Mars embryo by a large embryo, 1000 or 1500km diameter, results in thermal stratification of the core where temperature increases with radius. The thermally stratified core requires 50–120Myr to cool and regain vigorous convection, powering a strong core dynamo. The almost non-magnetic extended area of the primordial crust in the southern hemisphere supports this likely scenario. The impact heating of Mars by the seven largest of the 20 impacts is studied in detail. The collective battering the core dynamo by the impacts probably kills the dynamo. A strong core dynamo existed for ∼350Myr, until Ares and the following Amazonis impacts introduced substantial perturbations to the core temperature and constrained the core convection to the upper ∼200km for ∼28Myr, during which no strong core dynamo was generated in this part of the core. However, it is possible that the strong dynamo that existed deep in the core prior to Ares impact retained its strength while the condition gradually changed from supercritical to subcritical until Acidalia impact which likely killed the dynamo.