In recent decades, NASA has sent large spacecraft—Galileo and Cassini, respectively—to fly around Jupiter and Venus to explore the dozens of moons that exist in those planetary systems.

The spacecraft investigated all manner of intriguing moons, from little radiation-saturated hellholes to a world covered in volcanoes. But the most consistently interesting discovery made by these probes was that Jupiter and Saturn are surrounded by small and large moons covered in ice, possessing large water oceans below, or both. This was exciting because where there is water in its liquid state, there is the possibility of life.