In the study of heliophysics, planetary dynamos are important in understanding how various planetary bodies produce their magnetospheres which then so intricately interact with the solar wind. The dynamo mechanisms in planets are also very similar to those in the Sun, as well as in stars and other astrophysical bodies. Investigating planetary magnetic fields, therefore, provides data points in understanding magnetohydrodynamic processes in a broad range of astrophysical settings.

The investigation of planetary dynamos was predominantly focused on Earth's magnetic field until the mid-to-late twentieth century when planetary missions began to provide data on magnetic fields of other planets. Through our exploration of the solar system, we have discovered the diversity of planetary magnetic fields and realized the importance of magnetic fields in acting as probes of planetary interior structure, composition, and thermal evolution. As examples, magnetic field data were fundamental in discovering the global oceans of Europa, Ganymede, and Callisto, they demonstrated that Mercury and Ganymede each have a liquid iron outer core, and provided a main line of support for a helium-insolubility layer in Saturn.

Several aspects of planetary dynamos have been covered in previous chapters in the Heliophysics series. For a review of theoretical magnetohydrodynamics, applicable to planets as well as other astrophysical bodies, see Ch. 3 in Vol. I (Rempel, 2009a). In addition, an overview of planetary magnetic field properties can be found in Ch. 13 of that volume (Bagenal, 2009) and further details on the geomagnetic field and planetary dynamos can be found in Ch. 7 of Vol. III (Christensen, 2010).

This chapter serves two purposes. First, it provides an update on our understanding of planetary magnetic fields and dynamos from new mission data and dynamo models since the previous volumes of this series were written. Wherever possible, we refer the reader to specific chapters in previous volumes (see Table 1.2) rather than repeat too much information. However, we review the most important concepts and findings needed here so that this chapter is also self-contained. Second, this chapter delves into the frontier (or fringe, depending on your perspective) of planetary dynamo studies by reviewing our understanding of dynamos in small bodies and extrasolar planets.

Dynamo fundamentals

Dynamo action refers to the conversion of mechanical energy into electromagnetic energy through induction.