What are Jupiter’s Galilean moons?

Like Io, Europa orbits close to Jupiter and has sizable moons orbiting beyond it. The resulting gravitational effects also melt Europa’s subsurface layer, but because this moon is composed mostly of water ice, its melted layer is a huge ocean of liquid water.

Data collected by NASA’s Voyager and Galileo missions suggest that under about 30 kilometers (19 miles) of ice, Europa may be holding an ocean 70-100 kilometers (43-62 miles) deep. Even though Europa is smaller than Earth’s moon, its oceans are thought to contain twice as much liquid water as all of Earth’s oceans and lakes combined.

This could make Europa an especially promising place to look for life, because liquid water is crucial for life as we know it. Europa’s subsurface oceans may have other life-friendly qualities as well, including protection from radiation. Europa’s oceans also appear to be salty, which might mean that they could be in direct contact with a rocky seafloor below. Earth’s oceans get most of their salt from hydrothermal vents in the seafloor, and these vents are among the most likely places where life first emerged on Earth. The basic ingredients for making life seem to be water, organic compounds, and an energy source, and the heat coming out of hydrothermal vents could have provided the energy needed to turn non-living ingredients into life.

For all these reasons, Europa is an important target in the search for life beyond Earth. To better understand this world and its possible habitability, two missions are going to study Europa in the coming years: the European Space Agency’s Jupiter Icy Moons Explorer (Juice), which launched in April 2023, and NASA’s Europa Clipper, which is set to launch in 2024.

Continuing our trajectory out from Jupiter, the next Galilean moon we come to is Ganymede. It is the largest moon in the Solar System, even bigger than the planet Mercury. It’s also the only moon in the Solar System that has its own magnetic field.