Europa, Io, Ganymede, and Callisto are four of Jupiter's moons, and known collectively by which name?
And the answer: Galilean.
There are more than 75 moons surrounding Jupiter, and the largest four are known as the Galilean moons. They were first documented by Galileo Galilei in the year 1610, and were the first objects found to orbit another planet. They're among the largest objects in the solar system (with the exception of the sun and the eight planets).
As the largest and most massive planet in the solar system, Jupiter has a very strong gravitational field. As such, Jupiter holds on to many moons — some ranging from planetary size to mere boulders floating in space. All four of the Galilean moons are some of the largest in the solar system. In fact, if Jupiter's bright glare wasn't present, the moons could be seen with the naked eye.
Ganymede is the largest of Jupiter's moons. At over 3,000 miles in diameter, Ganymede is the largest moon of any planet (and is even larger than Mercury, a planet itself!). Ganymede is much like a planet in other attributes as well, including the rock and ice which decorate its surface. Scientists believe it could even have a liquid iron core and magnetic field. Even more surprising: multiple passes by the Galileo spacecraft in the 1990s and observations from the Hubble telescope reveal that deep below the surface of Ganymede, there are oceans of salty water.
The second largest moon, Callisto, is much like Ganymede in physical attributes, but orbits Jupiter much farther away — almost a 1.5 million miles away. This means that its gravitational pull is not affected by the other moons, nor are theirs affected by its own.
The third moon, Io, on the other hand, rotates around Jupiter much more closely. In fact, it is so small (about the same size as our own moon) and rotates so tightly that it only takes about a day and a half to complete a full rotation of Jupiter. Io is the most volcanic object in the entire solar system. Its 400 active volcanoes erupt daily, rearranging the surface of the moon. Much of the erupted material is sulfuric, which make the surface of Io yellow, orange and red.
The fourth and final of Jupiter's large moons is Europa. Slightly smaller than our moon, Europa has been known for decades as an incredibly reflective moon, largely believed due to the presence of ice. However, when the Voyager spacecraft passed Europa, its surface was completely lacking in craters, marks or dents of any kind. Even more curious: thin cracks, dark streaks and complex ridges dotted the surface of the moon. Europa's smooth surface — an incredibly unique feature for moons — is thought to be the result of the internal material of Europa welling up and forming a new surface, much like lava does on Earth. However, on Europa, that material is water. It is now believed that Europa has an entire ocean of water sealed under a solid crust of ice.
Learn more about Jupiter's amazing moons below.