The Galilean Moons
Way back in January of 1610, Galileo Galilei had just finished his telescope capable of magnifying objects a thousand times. On the tenth of January, he took his telescope up to the top floor of his house and peered through it. As he looked at Jupiter, he noticed three fuzzy white orbs located near the planet that were brighter than the stars around them. Within days he had seen a fourth. These four satellites of Jupiter—Io, Europa, Ganymede, and Callisto—provided the key evidence Galileo needed to prove that not everything revolved around the Earth. Today, these four satellites are known to be some of the most interesting bodies in our solar system.
Although individually they are very different, these moons, with the exception of Callisto, have one thing in common. They are all caught in a “gravitational tug-of-war” with one another and Jupiter. When one of the three moons change its position, the other two shift also, keeping the moons in battle. This change in gravitational pull causes the moons' surfaces to flex. This, in turn, creates heat, which is the cause of most of the interesting features of the moons. These planetary tides create raging volcanoes on Io and perhaps may keep an ocean beneath Europa’s icy surface from melting. Most of Ganymede’s special features, however, were not caused by the tides because they didn’t provide enough heat. Callisto is not affected by these tides at all and it is not expected to join the battle for another ten billion years. Although the tides don’t provide for all of the answers, their discovery is key evidence of why these moons are so spectacular (Cowen 90-91).