Here's why:
* Ancient Surface: Callisto's surface is ancient, dating back to the early days of the solar system. This means it has been bombarded by asteroids and comets for billions of years.
* Lack of Geological Activity: Unlike some other moons (like Io or Europa), Callisto has very little geological activity. This means its craters remain largely undisturbed and visible.
* High Crater Density: Callisto's surface is absolutely covered in craters, some of them enormous. The density of craters is incredibly high compared to other celestial bodies.
While other moons, like the Moon, have a significant number of craters, Callisto takes the title of most heavily cratered.
