Just in time for late spring, early summer Jupiter, the largest planet in our solar system, brightens in the night sky. With a good pair of binoculars or a small telescope one can see not only this beautiful planet but also four of its numerous moons.
The four easily-visible moons of Jupiter are called Galilean moons because they were first discovered by Galileo Galilei in 1610. The names of these four moons all come from Greek mythology.
The largest of the Galilean moons is Ganymede. It is 8 percent larger than the planet Mercury and the ninth-largest object in our solar system. The surface of Ganymede is crisscrossed with ridges and grooves in weird patterns. It takes seven earth days for Ganymede to orbit the massive planet Jupiter.
The name Ganymede is the name of the cup-bearer to the Greek gods. Here is a sculpture by Bertel Thorvaldsen entitled “Ganymede and the Eagle.”
The next largest Galilean moon is Callisto. The surface of Callisto is the most heavily cratered of any surface in the solar system. It does not show any signs of volcanic activity or earthquakes. As with earth’s moon, Callisto is tidally locked to Jupiter meaning that the same side of this moon always faces Jupiter.
Next up is the moon Europa. Unlike Callisto, that is covered with craters, Europa has the smoothest surface of any body in the solar system. It is covered with a solid sheet of ice several miles thick.
The most fascinating of the four Galilean moons is Io. Unlike Callisto that has no volcanic activity, Io is the most volcanically active body in the solar system with over 400 active volcanoes. Scientists have measured Io’s volcanoes spewing material 180 miles out into space at speeds of 2000 miles per hour. Io’s lava is 1000 degrees hotter than the lava of earth’s volcanoes.
Because it is positioned between Jupiter and the other Galilean moons, Io is subject to tidal heating as gravity pushes and pulls on this moon. Some of Io’s mountains are taller than Mount Everest and there are no visible craters on Io’s surface.
Over 60 other moons of Jupiter have been found. The mass of the planet is more than twice that of all the other planets in our solar system combined.
Even though Jupiter is so far from the sun there is still a lot of seismic and geologic activity occurring both on the surface of Jupiter itself and on its many moons. This presents a significant problem for those who believe that our solar system is billions of years old. If our solar system is indeed billions of years old one would expect everything out by Jupiter to be old cold and dead.
Follow this link to watch a short video about this fascinating planet: