This animation shows the interior structure of the four Galilean moons of Jupiter: Io, Europa, Ganymede and Callisto. While Io is an active volcanic world with a magma ocean below its crust, the other moons are icy, and are thought to host subsurface liquid oceans that will be explored in detail by ESA’s Jupiter Icy Moons Explorer.

Europa has a dynamic icy crust overlying what is thought to be a liquid water ocean. It also has a rocky mantle and solid core. Juice will search for liquid water below the most active sites observed on the surface. The spacecraft will study material exchange between the interior and the surface, as well as measure the moon’s gravity field to better define its structure and layers.

Ganymede has an icy crust with evidence of a subsurface liquid ocean overlying an icy mantle, a rocky mantle, and core. Juice will characterise the icy crust and the extent of the subsurface ocean. It will also measure Ganymede’s intrinsic magnetic field in great detail. It is the only moon in the Solar System to generate its own magnetic field, which has complex interactions with Jupiter’s own vast magnetic field, and Juice will study the relationship between the two.

Callisto has an icy crust and is suspected to have a subsurface liquid ocean with its internal core likely a mix of ice and rocks. Here Juice’s focus will be the understanding of the interior structure and the detection of the putative subsurface ocean.

Understanding the state of the moons’ interiors, and in particular if they host liquid water, will help answer important questions about the habitability potential of these moons, and of similar exoplanet systems elsewhere in our Universe.