Out of the 79 confirmed moons surrounding Jupiter, four ancient ones have astronomers really excited because they offer up some of the most distinctive geology in the solar system, including clues of deep, subsurface oceans that could be home to life.
The Galilean moons are named after Galileo Galilei, who first observed them in 1610. Io, Jupiter’s closest Galilean moon, is unusual because its composition is closer in comparison to the terrestrial planets than the icy bodies that surround it. Io is also the most volcanically active body known in the solar system with over 400 active volcanoes. This is due to the internal heating produced by the pulling and stretching effect of Jupiter's gravity and smaller neighboring moons on Io as it orbits Jupiter.
That same tug-of-war heating effect is the reason why some astronomers believe that Jupiter’s second Galilean moon is home to a deep subsurface ocean up to twice the volume of Earth's oceans. Unlike Io, Europa has an icy surface, but below it, astronomers think there may be a salty ocean that could harbor life-bearing chemistry, including organic molecules. When Europa swings by Jupiter on an elliptical orbit, gravitational tidal forces stretch and flex the sides of the moon, creating internal friction that essentially heats the moon from the inside out. Using its Europa Clipper spacecraft in the 2020s, NASA plans to get a closer look at the moon in order to confirm the existence of a potential ocean.
Europa’s neighbor, Ganymede, is also believed to have an ocean hidden under its thick icy shell. The moon’s magnetic field may be affected by an electrically conducting layer from within, which hints at the existence of a liquid saltwater ocean. But Ganymede’s distance from Jupiter means there’s a weaker tidal force at play, so it’s less likely that liquid water is present. The moon is the only one in the solar system known to have a magnetosphere, protecting its surface from harmful radiation and solar wind. As Jupiter’s largest moon, Ganymede would be considered a planet if it were orbiting the Sun.
Out of all four Galilean moons, Callisto is the farthest from Jupiter. The moon is purported to be one of the most heavily cratered objects in the solar system. With some impact basins expanding to about 2,600 kilometers across, these craters could teach us a lot about the formation of our early solar system, as astronomers believe its surface hasn’t changed in over four billion years.
Unmanned spacecraft have been conducting flybys of the Galilean moons for decades and findings from these missions suggest an even greater presence of water. Along with NASA’s Europa Clipper, the European Space Agency’s JUpiter ICy moons Explorer aka JUICE will gain a deeper understanding of the moons’ mysterious properties. But getting to these moons will be no small feat since the craft will need to slingshot around the Sun and then travel about 800 million kilometers before being caught by Jupiter’s massive gravitational pull. If the spacecraft can survive this arduous journey and manage to generate enough energy to travel between the moons, astronomers will gain an unparalleled glimpse into what lies beneath their surfaces, which could even result in the discovery of the first forms of life beyond Earth.