James Clerk Maxwell Prize Address: Exploring the Solar System with a Magnetometer *

The space age had hardly begun when, in 1958, Sputnik 3 carried the first magnetometer into Earth's upper atmosphere. Magnetometers on subsequent spacecraft missions characterized Earth's magnetosphere and whetted interest in planetary environments. By 1989 spacecraft had characterized the magnetic fields of all planets and begun to explore their magnetospheres. Spacecraft measurements of the plasma and field environments of the largest moons of Jupiter and Saturn extended the range of magnetohydrodynamic conditions available for analysis. Comparative magnetospheric studies became possible. The dynamics of a magnetosphere varies with external conditions such as the pressure imposed by the flowing plasma of the solar wind (for a planet) or the rotating plasma of a planetary magnetosphere (for a moon) and depends on whether the magnetic pressure or the dynamic pressure of the external plasma dominates. The spatial scale of the central body, the magnitude of the magnetic field at its surface, its rotation period and the presence of plasma sources within the magnetospheric cavity affect the structure of the magnetosphere and modify the dynamics.  Near several moons of Jupiter and Saturn, magnetometers on the Galileo and Cassini spacecraft sensed perturbation fields arising from currents induced beneath their surfaces by the time-varying magnetospheric magnetic field in which they are embedded. The inductive responses implied that conducting layers (liquid oceans or magma layers) are present within those moons. The inductive responses of Jupiter's moons will be further investigated by magnetometers on two major spacecraft missions in the early 2030s.  ESA's JUICE mission will pass by multiple moons and ultimately be directed into orbit around Ganymede. NASA's Europa Clipper will encounter Ganymede as its periapsis is lowered to near the orbit of Europa, and make nearly 50 close flybys of Europa. Both missions propose to establish the habitability of the moons.