Of all the planets in the Solar system, Jupiter has the 2nd most moons, 3 behind Saturn. This means that Jupiter has 79 known natural satellites. Of these, 63 are less than 10 kilometres in diameter and have only been discovered since 1975. The four largest moons, visible from Earth with binoculars on a clear night are known as the “Galilean moons.” The names of these Galilean Moons are Io, Europa, Ganymede, and Callisto, which we will be discussing a little bit today.
The discovery of these moons by Galileo Galilea in January of 1610 is a crucial turning point for astronomy because this was the first time a moon was discovered orbiting a planet other than Earth. This heralded in the understanding that planets in our solar system orbit the Sun, instead of our solar system revolving around Earth.
The first moon of Jupiter is called Io, which is the most volcanically active world in the Solar System, with the number of volcanos numbering in the hundreds. Due to its size and orbit, Io is caught in a tug-of-war between Jupiter’s massive gravity and the smaller but precisely timed pulls from two neighboring moons that orbit farther from Jupiter, Europa and Ganymede.  Because of the constant volcanism and intense radiation, Io is an unlikely candidate for life to exist. But it is this volcanism that is responsible for shaping the geography of Io. Some volcanos on the surface have been known to produce plumes of sulfur and sulfur dioxide that climb as high as 500 km or 300 miles above the surface.
Fun Fact: Io is the fourth-largest moon in the solar system, has the highest density of all solar system moons, and has the lowest amount of water of any known astronomical object in the Solar System
Europe is a really promising place to look for life. This is because although Europa is only one-fourth the diameter of Earth, its ocean may contain twice as much water as all of Earth’s oceans combined. This is a reset of scientists observing water ice in telescopes and the Voyager spacecraft providing hints that there might be liquid water on Europa. Europa is about 90 percent the size of Earth’s Moon, but is 5.5x brighter than the Earth’s moon would be at tat position. Europa orbits Jupiter every 3.5 days and is locked by gravity to Jupiter, so the same hemisphere of the moon always faces the planet. Jupiter takes about 4,333 Earth days (or about 12 Earth years) to orbit the Sun (a Jovian year).
Ganymede is the largest moon in our solar system and the only moon with its own magnetic field. This magnetic field has been known to cause auroras, which are ribbons of glowing, electrified gas, in regions circling the moon’s north and south poles. It is thought that this magnetic field is caused by the convection of iron in the core of the planet.
On its surface, there have been large, bright regions of ridges and grooves that slice across older, darker terrains. Scientists have also found strong evidence of an underground ocean on Ganymede. Astronomers using the Hubble Space Telescope found evidence of thin oxygen atmosphere on Ganymede in 1996. The atmosphere is far too thin to support life as we know it. 
Due to its size, Ganymede is 26% larger than the planet Mercury by volume, although it is only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System
With 99% of the diameter of Mercury, but only a third of its masss, the moon Callisrto is Jupiter’s second largest moon. It is tidally lockdown to Jupiter and is composed of approximately equal amounts of rock and ices, with a density that is the lowest density and surface gravity of Jupiter’s major moons. Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation.
Check out the Planetary Bodies Category for previous and upcoming articles on the solar system planets.
 = https://solarsystem.nasa.gov/moons/jupiter-moons/io/overview/#introduction_otp
 = https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/
 = https://solarsystem.nasa.gov/moons/jupiter-moons/ganymede/in-depth/