Transcript: Jupiter's Io is the closest to the planet of the four Galilean satellites. Its diameter is about 3,600 kilometers, and it is the most volcanically active world in the entire solar system. In 1996 a gravitational acceleration experiment on the Galileo space probe was able to measure a large iron core within Io that goes to nearly half the radius of the moon. Io has a larger density than any satellite in the solar system, about 3,500 kilograms per cubic meter, similar density to Mars.