It is generally accepted that large scale planetary magnetic fields are caused by the motions of electrically conducting fluids in planetary deep interiors. What remains unclear is how the morphology of a planetary magnetic field can be affected by the geophysical, geochemical and thermal state of the planetary deep interior. The recent results from the Kepler space probe indicate that there are a large number of terrestrial exoplanets in our galaxy, if our own solar system is any indication this implies that a great variety of terrestrial planets exist.

One previously overlooked factor is the effect an electrically conducting lower mantle could have on the dynamo. Recent experimental and theoretical work has shown that some materials commonly thought to be present in terrestrial planet mantles (e.g. CaSiO3, Al2SO3, and FeO) will conduct electricity at the pressures present in planets as small as Earth. A solid, electrically conducting lower mantle layer should have a significant effect on any dynamo present in the planet.

In this talk I will gently introduce planetary dynamo theory, then focus on the effect a metallized mantle will have on the observable magnetic field of terrestrial exoplanets. Finally I will discuss the prospects for detecting these magnetic fields from Earth.