Jupiter's four main moons are Io, Europa, Ganymede, Callisto.Ganymede is the largest moon in our solar system with a diameter of 3260 miles. These moons were discovered by Galileo Galilei about the time of January 1610. They are called the Galilean Satellites or Galilean Moons. You can see these moons yourself looking through binoculars. If you look towards Jupiter there is 4 tiny dots of light nearby it. Now Io, Europa, Ganymede, and Callisto are not the only moons Jupiter has at least 63 known moons. The moon Io is about 4.5 billion years old, the same age as Jupiter. Io is the most volcanically active planet in our solar system. The surface of this moon has spots of lava lakes and liquid rocks. Io's average surface temperature is about -202 degrees F, but it's volcanoes can reach 3000 degrees F and Io is commonly referred to a celestial body of ice and fire. Europa 1,900 miles in diameter, making it smaller than our moon but bigger than Pluto, this making it the 15th biggest body in the solar system.