Callisto
Callisto is the outermost of the four Galilean moons. It is the third largest moon in the solar system and is slightly smaller than Mercury. Its mass is about one third of Mercury’s (Arnett “Callisto”).
Callisto is probably the most heavily cratered object in the solar system. It also has the oldest landscape in the solar system (about four billion years old). This shows that Callisto must have been a very quiet place throughout its existence with little or no geological activity. One reason for this is because Callisto isn’t involved with the gravitational tides that the other three Galilean moons are (Morgan “Callisto”).
Much is still unknown about these four strange moons of Jupiter. In the years to come, scientists plan to send more advanced spacecrafts up to Jupiter to further explore the mysteries of these moons. With more research, we may find life outside of Earth for the first time on Europa. Or we may finally understand how the tidal heating works. Whatever we find, it will only be one small discovery compared to the infinite number of the things we have yet to discover in the far reaches of space.