Europa is one of the four large moons of Jupiter, and it is about 3,120 kilometers (1,940 miles) across. That is its mean diameter, a little less than the width of the Moon. Europa is covered in a shell of ice, and scientists think a salty ocean lies underneath.

Size

NASA gives Europa's mean radius as 1,560.8 kilometers. The mean radius is the radius of a perfect sphere with the same volume as Europa. Doubling it gives a mean diameter of about 3,120 kilometers. NASA's Jet Propulsion Laboratory lists the same radius, known to within 0.3 kilometers.

Europa is the smallest of the four moons that Galileo Galilei discovered around Jupiter in 1610, now called the Galilean moons. The other three, Io, Ganymede and Callisto, are all wider.

Size comparisons

Europa is about nine-tenths as wide as the Moon, which is 3,475 kilometers (2,159 miles) across.
Io, the next large moon closer to Jupiter, is about 3,640 kilometers across, about 17% wider than Europa.
Europa is about a quarter as wide as Earth, and Earth has about 68 times its volume.
If Jupiter were shrunk to the size of a basketball, Europa would be a bead about 5 millimeters wide, circling about 1.2 meters (3.8 feet) from the ball's center.
About 74 marathons laid end to end would reach across Europa.

An ocean under the ice

Scientists think Europa's ice shell is 15–25 kilometers (10–15 miles) thick. Beneath it lies an ocean 60–150 kilometers (40–100 miles) deep. Europa is only a quarter as wide as Earth, but its ocean may hold about twice as much water as all of Earth's oceans.

The best evidence for the ocean came from NASA's Galileo spacecraft, which explored Jupiter and its moons from 1995 to 2003. Galileo found that Europa disturbs Jupiter's magnetic field in a way that points to a deep layer of salty water. A future mission still has to confirm it.

A young, cracked surface

Europa has very few craters, so its surface is young. Based on the craters counted, it is at most 40 to 90 million years old, depending on which count is used. Reddish-brown cracks crisscross the ice, and some run for thousands of kilometers. Because the surface is ice, Europa reflects 5.5 times as much sunlight as the Moon.

The cracks are probably made by tides. Io, Europa and Ganymede pull on each other in a regular rhythm: for every orbit Ganymede makes, Europa makes two and Io makes four. These pulls keep Europa's orbit slightly stretched, so Jupiter's pull on it keeps changing. The changing pull stretches and relaxes the moon, which likely cracks the ice.

Visiting Europa

NASA's Europa Clipper spacecraft launched on October 14, 2024, and is due to reach Jupiter in April 2030. It will orbit Jupiter and make 49 close flybys of Europa. Its main goal is to find out whether there are places below the ice that could support life.

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