Ganymede

Galileo Galilei discovered Ganymede on 7 January 1610, among the four large moons now called the Galilean satellites. With a diameter of about 5,260 kilometres, it is larger than Mercury, although far less massive. Ganymede orbits Jupiter at an average distance of roughly 1.07 million kilometres and completes an orbit in about 7.15 Earth days. Like Earth's Moon, it is tidally locked, so the same hemisphere generally faces its planet.

Ganymede’s surface combines old, dark, heavily cratered terrain with brighter regions marked by ridges and grooves. These contrasting landscapes record a complex geological history. Evidence indicates a layered interior with a metallic, iron-rich core, a rocky mantle and an outer ice shell. The exact internal structure and the thickness of its layers remain subjects of investigation.

Galileo spacecraft measurements in 1996 revealed that Ganymede has its own intrinsic magnetic field, the only such field known for a moon. This field produces a miniature magnetosphere embedded inside Jupiter’s much larger magnetosphere, leading to interactions between the two magnetic environments and their charged particles. The field also contributes to auroral emissions near Ganymede’s poles. The discovery was significant because it showed that a moon can generate and sustain a magnetic environment of its own.

Changes in Ganymede’s magnetic environment measured by Galileo suggested the presence of a subsurface electrically conducting layer, consistent with salty liquid water beneath the ice. Subsequent observations of the moon’s auroras by the Hubble Space Telescope added evidence for an ocean. NASA describes the ocean as a strong inference, while its depth, internal layering and contact with the rocky interior are not yet fully determined. The ocean’s possible habitability is a scientific question, not evidence that life exists there.

NASA’s Galileo spacecraft made close observations of Ganymede during its Jupiter mission, and NASA’s Juno spacecraft has also studied the moon during a close flyby. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE) mission is designed to investigate Ganymede’s surface, interior, magnetic field and surrounding plasma, as well as Jupiter’s other icy moons. Detailed measurements of the magnetic environment can help constrain both the moon’s internal structure and the properties of its suspected ocean.