Ganymede is the biggest moon orbiting Jupiter and I find it more interesting than Jupiter itself. It is bigger than planet Mercury! I assume it is the white dot left on the photo above.
Ganymede is composed of approximately equal amounts of silicate rock and water. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined, it is said. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the moon. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. And it orbits Jupiter since 4 billion years. Fascinating stuff.