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Galileo Galilei discovered the four largest satellites of Jupiter in 1610. Io is the one closest to the planet. Io is periodically flexed as it orbits Jupiter every 42.5 hours and feels, as a result of its non-circular orbit, the constantly changing mighty gravitational pull of the planet. The heat arising in Io's interior from this process makes it the most volcanically active body in the solar system. Here Io casts a shadow on the cloud tops on Jupiter.

CREDIT: NASA/JPL/University of Arizona.

Source:
http://photojournal.jpl.nasa.gov/catalog/PIA02860