A deep dynamo generating Mercury’s magnetic field

The latest numerical models of the geodynamo can account for the behaviour of Earth's magnetic field pretty well: Mercury has proved a harder nut to crack. Like Earth, it has a dipolar magnetic field, probably generated by a dynamo from convective motions in the planet's liquid iron core. But Mercury's field is a hundred times weaker than Earth's, and this poses a problem for the dynamo theory. A new explanation for the discrepancy has been proposed, accounting for both the observed field strength and the magnetic field geometry observed during the Mariner 10 flybys. The new model assumes that the dynamo operates only deep down in the core, where it generates a strong field. The outer regions of the core are stably layered, so do not convect heat, but they are electrically conducting and the dynamo-generated field is therefore strongly damped. Data from NASA's Messenger probe, en route to Mercury, and ESA's planned Bepi Colombo mission should provide a thorough test for the model.