Io and Jupiter


In Greek mythology Io was a priestess of Hera (Zeus’ wife) and a nymph who was seduced by Zeus. He changed her into a heifer to escape detection. Io is also the name of the innermost of the four Galilean moons of the planet Jupiter. The most volcanic body in the Solar System, Io is 3,600 kilometers in diameter, about the size of planet Earth’s moon.

While cruising past Jupiter at the turn of the millennium, the Cassini spacecraft captured this view of Io with Jupiter as a backdrop–offering an impressive demonstration of the ruling planet’s relative size. (An astronomer from another star system would probably describe our solar system as having one main planet and assorted debris.) Although Io appears to be located just in front of the swirling Jovian clouds, Io is about 350,000 km above Jupiter’s cloud tops. That’s roughly the same as the distance between Earth and Moon. The Cassini spacecraft itself was about 10 million km from Jupiter when this picture was taken.

Image Credit: NASA