The Galilean Moons are the major moons of Jupiter. They include Io, Europa, Ganymede, and Callisto. They were discovered in 1610 by Galileo Galilei. Io is the most geologically active body in the Solar System, Europa is the smallest of the four but is believed to have a huge underground ocean, Ganymede is the largest of the four. It is larger than Mercury and almost the size of Mars, and Callisto is the most heavily-cratered object in the Solar System. The Galilean Moons are so large, that if Jupiter’s glare didn’t block them out, you could see them in the night sky with the naked eye.
The Moons[]
Io[]
Main Article: Io
Io is the second smallest of the Galilean Moons, about the same size as the Earth‘s Moon. It is yellow in color and has many volcanoes, which erupt very often and are mostly active. The plumes from these volcanoes can be seen from space. Io’s volcanic activity is probably due to the Tidal Forces Jupiter puts on the natural sattelite.
Europa[]
Main Article: Europa
Europa is the second Galilean Moon from Jupiter and it is also the smallest. It has a crust made of ice and is believed to have a liquid water ocean below that crust. Europa is one of the targets for life beyond Earth. NASA plans to send a space mission there in the near future. Jupiter also puts tidal forces here, and it is believed that the volcanic activity produced by these tidal forces is thought to warm Europa’s ocean.
Ganymede[]
Main Article: Ganymede
Ganymede is the largest of the Galilean Moons and is also the largest of the moons in the Solar System. It is larger than Mercury and would be classified as a Planet if it orbited the Sun rather than Jupiter. Ganymede does not get the full blast of tidal forces that Europa and Io do, but it is likely it got the full blast in the past. Ganymede is also the third Galilean Moon from Jupiter and is in a 4:2:1 resonance with Io and Europa. This means for every 4 orbits of Io and 2 orbits of Europa, Ganymede will orbit once. Ganymede also may have an underground ocean.
Callisto[]
Main Article: Callisto
Callisto is the fourth Galilean Moon from Jupiter. It has never experienced the full blast of Jupiter’s tidal forces. It is the second largest of the Galilean Moons, barely smaller than Mercury. It is the most heavily cratered object in the solar system, with new craters forming all the time. It is mostly black with some white parts from the deep craters, and it’s terrain is rugged. Unlike the rest of the Galilean Moons, Callisto is not locked in an orbital resonance with the rest of the moons.