As we’ve seen in our study of the Jovian planets, the actual planets themselves aren’t the only important space-related object that provides useful and insightful information. Every Jovian planet has some sort of celestial object orbiting or surrounding it, especially the moons surrounding Jupiter. Discovered by Galileo Galilei way back in 1610 (on January 10th), the four different Galilean moons (which include Io, Europa, Ganymede, and Callisto) exhibit different tectonic, spatial, and compositional characteristics that make each world unique and interesting to study!
Starting with the innermost-orbiting moon, we discuss Io, the most volcanically active world in our Solar System. Sizing up at around the dimensions of a dwarf planet, Io contains large volcanoes littered throughout the entire moon’s surface. Seldom does the moon have impact craters, as the super-frequent eruptions remove them from the surface almost immediately. Similar to Earth, the volcanoes on Io follow a similar outgassing pattern (however, for Io, it’s with Sulfur Dioxide). And lastly, Io’s constantly active volcanoes indicate to scientists that it experiences tidal heating (tidal forces causing constant flexes and stretches of the moon’s core), allowing it to be so active!
Onto the second innermost-orbiting moon, we discuss Europa, one of the most intriguing celestial bodies in our Solar System. It sits as a stark contrast to Io — not only is its surface covered by water ice, but it additionally seems like liquid water is flowing throughout its interior (since there are a lack of impact craters, some type of geographical movement is occurring). Scientists believe that there’s internal heat hot enough to melt some of the surface ice for the internal flowing as a result of photographical evidence, gravitational measurements, and magnetic fields. All in all, Europa could be the first place we find liquid water outside our own Earth (which would be pretty cool!).
For the third and fourth moons, we discuss both Ganymede and Callisto. As the largest moon in the Solar System, Ganymede’s surface of relatively young ice suggests a constant upwelling of water / slush to the surface (which would mean there’s an ocean there too, like Europa’s). However, because Ganymede is so large, it makes sense that it’s still active (at least, much more so than Europa). Additionally, we can’t forget to mention Callisto, a heavily cratered ball of ice that represents the stereotypical Jovian moon. It has old-looking surface littered with craters and ice-like material. However, as it lacks any sort of tectonic or volcanic activity, it isn’t seemingly as significant as the other three moons (though it may have a subsurface ocean, like the other moons).
All in all, the four different Galilean moons represent an eclectic collection of different quirks and characteristics that make them unique! If you’d like to further research the different moons of Jupiter, more information can be found at the linked website! And finally, a picture of the different moons can be found below.