> Europa, Io, Ganymede, and Callisto are four of Jupiter's moons, and knowncollectively by which name?And the answer: Galilean. There are more than 75 moons surrounding Jupiter, and the largest four are knownas the Galilean moons. They were first documented by Galileo Galilei in the year1610, and were the first objects found to orbit another planet. They're amongthe largest objects in the solar system (with the exception of the sun and theeight planets).Photo credit: Astronomy Stack Exchange[https://astronomy.stackexchange.com/questions/10597/determine-the-moons-of-jupiter-through-a-telescope].As the largest and most massive planet in the solar system, Jupiter has a verystrong gravitational field. As such, Jupiter holds on to many moons — someranging from planetary size to mere boulders floating in space. All four of theGalilean moons are some of the largest in the solar system. In fact, ifJupiter's bright glare wasn't present, the moons could be seen with the nakedeye. 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, aplanet itself!). Ganymede is much like a planet in other attributes as well,including the rock and ice which decorate its surface. Scientists believe itcould even have a liquid iron core and magnetic field. Even more surprising:multiple passes by the Galileo spacecraft in the 1990s and observations from theHubble telescope reveal that deep below the surface of Ganymede, there areoceans 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. Thismeans that its gravitational pull is not affected by the other moons, nor aretheirs 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 sotightly that it only takes about a day and a half to complete a full rotation ofJupiter. Io is the most volcanic object in the entire solar system. Its 400active volcanoes erupt daily, rearranging the surface of the moon. Much of theerupted material is sulfuric, which make the surface of Io yellow, orange andred. The fourth and final of Jupiter's large moons is Europa. Slightly smaller thanour moon, Europa has been known for decades as an incredibly reflective moon,largely believed due to the presence of ice. However, when the Voyagerspacecraft [https://www.askqotd.com/the-voyager-golden-records/] passed Europa,its surface was completely lacking in craters, marks or dents of any kind. Evenmore curious: thin cracks, dark streaks and complex ridges dotted the surface ofthe moon. Europa's smooth surface — an incredibly unique feature for moons — isthought to be the result of the internal material of Europa welling up andforming a new surface, much like lava does on Earth. However, on Europa, thatmaterial is water. It is now believed that Europa has an entire ocean of watersealed under a solid crust of ice. Learn more about Jupiter's amazing moons below.