The Moons of
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.
Io’s volcanic activity is due to the gravitational influence from Jupiter and other moons. As Io orbits, Jupiter tugs at it from the inside and the other large moons pull from the outside, slightly elongating its orbit. The fluctuating gravitational forces stretch and squeeze Io, generating internal friction and heat that powers its dynamic geology.
Every second, Io’s volcanoes spew out about a ton of particles – mostly sulfur and oxygen compounds. Io is the main supply of material that feeds into Jupiter’s magnetosphere, filling it with a gas of charged particles called plasma.
Below Europa’s icy surface may be a liquid ocean – a possible environment for life.
Many giant planets orbiting other stars probably have icy moons as well, and if life can exist on Europa, then maybe it can take hold elsewhere.
As the largest moon in the solar system, Ganymede is also the only one with its own magnetic field.
Like Europa, Ganymede might have a subsurface ocean. Any sea that exists is probably sandwiched between two layers of ice – like Callisto – meaning the ocean wouldn’t be in contact with a rocky seafloor. As a result, the ocean wouldn’t have access to any possible sources of energy from the planet’s interior. Coupled with the fact that the ice layer is probably very thick, Ganymede isn’t as likely to be habitable.
Almost as big as Mercury, Callisto is one of the most cratered surfaces in the Jovian system.
Callisto may have a subsurface ocean just like Europa and Ganymede. Scientists discovered that Callisto has a weak magnetic field that fluctuates with Jupiter’s rotation. One explanation for this is that Jupiter’s rotating magnetic field induces an electrical current inside Callisto, which in turn generates a magnetic field around the moon. One way electrical currents can course through the moon is if there were a subsurface sea filled with charged particles. Callisto’s ocean, if it exists, probably sits between two layers of ice, like Ganymede.