Europa is the smallest of Jupiter’s four Galilean moons. It is about 3,122 km in diameter, making it slightly smaller than Earth’s Moon and the least massive member of the group.
The Galilean moons—Io, Europa, Ganymede, and Callisto—were observed by Galileo Galilei in January 1610. Their discovery was historically important because it showed that celestial bodies could orbit a planet other than Earth, challenging the traditional Earth-centred view of the cosmos.
Europa is the second moon outward from Jupiter, between volcanic Io and larger Ganymede. Despite its smaller size, it is one of the most scientifically intriguing worlds in the Solar System. A global ocean is thought to lie beneath its icy crust, kept liquid partly by tidal heating caused by Jupiter’s gravity.
The usual size ranking is Ganymede, Callisto, Io, then Europa. A common mistake is choosing Io because it is Jupiter’s innermost Galilean moon; Io is actually the second-smallest.