1. Galilean Moons. By: Mary Claire Paddock

2. Galilean moons. The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter.

3. Io Io is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometers (2,263 mi), the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus. With over 400 active volcanoes, Io is the most geologically active object in the Solar System.

4. Europa Europa is the sixth moon of the planet Jupiter, and the smallest of its four Galilean satellites. Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and streaks, while craters are relatively infrequent.