Four of Jupiter’s largest moons – the Galilean moons – are about the size Earth’s moon or larger. With ice-covered oceans, sulfur-spewing volcanoes, magnetospheres and surfaces covered with traces of geological activity, these moons are fascinating worlds in their own right. Three of these moons may even have internal liquid-water oceans.
Other giant planets orbiting other stars may have similar kinds of moons, and because those moons may harbor life – or at least boast environments friendly to life – they’re important places to explore.
Although Juno’s mission is to explore Jupiter itself, it will also study how the moons influence the planet and its magnetosphere. For example, the gravity of the Galilean moons alters Jupiter’s shape in subtle ways. The planet’s magnetic field also sweeps up particles that are ejected from the moons’ surfaces, filling the magnetosphere.
Dotted with erupting volcanoes, Io is the most geologically active moon in the solar system.
Below Europa’s icy surface may be a liquid ocean – a possible environment for life.
As the largest moon in the solar system, Ganymede is also the only one with its own magnetic field.
Almost as big as Mercury, Callisto is one of the most cratered surfaces in the Jovian system.