How Dynamos Die

Mars, the Moon, and several large planetesimals are thought to have had dynamos in the past. These extinct dynamos must have gone through a process of magnetic decay, or "dynamo death", where convection in the core either stopped or was no longer rigorous enough to generate a magnetic field. We discuss the various ways that dynamos can die, from global thermal stratification to complete core solidification. We investigate the time-variable magnetic fields produced by numerical dynamo models that transition from strong-field dynamos at high Rayleigh number to weak-field dynamos and eventually to dynamo death. We compute maps of the planetary surface magnetic field as the dynamos die to test whether they are likely to leave a geocentric axial dipolar remnant magnetic signature in the crust. In application to the Moon and Mars we consider at what time during the dynamo's lifetime the magnetizations were acquired in the crustal rocks that have been preserved. We speculate on whether they were acquired during an early strong-field dipolar stage, during a late weak-field stage, or at some intermediate stage. And how those stages of dynamo death are expected to be manifested in the remanent magnetization record.