From an orbiting or flyby spacecraft, only two kinds of measurements provide information about the interior of a planetary body: gravity and magnetic field measurements. With the exception of radar soundings of ice, all other remote sensing measurements from spacecraft provide information about the atmosphere or the surface to a depth not exceeding perhaps a few centimeters. The spacecraft trajectory is affected by gravity, which in turn reflects how mass is distributed in the body. However, the detection of a magnetic field of internal origin provides evidence that a dynamo is active in the interior. Planetary dynamos are driven by thermal convection of electrically conducting fluids in the core and lower mantle of the body. In the case of the Earth, the fluid is an iron-rich, molten magma. Rotation of a planet is believed to be an essential ingredient of the process, organizing the convection patterns into cells leading to the existence of a self-regenerating magnetic field.