Io is the Galilean moon that is closest to Jupiter. It is 421,600 km away from Jupiter. Io is slightly larger than Earth’s moon. Scientists believe that Io’s composition is very much like that of Earth. Io has an iron core with a molten layer of silicate rock covering the core. Io has active volcanos and tectonic plates on its surface. It is believed that the tectonic movement is caused by the pull of Jupiter’s gravity combined with the gravity of the other Galilean moons. The tectonic movement builds mountains and volcanoes on the surface of Io. Io’s the atmosphere is made up of sulfur dioxide. The volcanic eruptions produce this sulfur dioxide gaseous layer. Io appears to be red and orange in color. The red and orange coloring is due to the sulfur condensing.
Europa is second Galilean moon and is slightly smaller than Earth’s moon. It is 670,900 km away from Jupiter. Europa, like Io, has a composition very much like that of Earth. Europa has an iron core covered with silicate rock. Additionally, Europa is covered with a layer of ice with a liquid ocean beneath that covers the entire surface. Scientists believe this liquid may be water, H2O. Recent Hubble space telescope photos show that Europa has a thin atmosphere of oxygen gas, probably from the ice evaporating and splitting into Hydrogen and Oxygen gasses around the surface.
Ganymede is the third and the largest of the Galilean moons. Ganymede is 1,070,000 km away from Jupiter. The composition of Ganymede is very similar to Europa. Ganymede has an iron core surrounded by a silicate mantle as well as a layer of ice surrounding it. Scientists believe that there is also a liquid layer beneath the ice, just as there is on Europa. The atmosphere surrounding Ganymede is made up of a thin layer of oxygen.
Callisto is the second largest and outermost moon of Jupiter. Callisto is 1,883,000 km away from Jupiter. This moon is a little smaller than the planet Mercury. Scientists do not know for sure what the composition of Callisto is, although its core is believed to be somewhat rocky. It is known that the make-up of Callisto is 40% ice and 50% rock and iron, but what is not known is if the rock and iron are mixed or in separate layers. Callisto also has a thin and unstable atmosphere of mostly carbon dioxide gas.