Jupiter’s Moon Io

A bit larger than our Moon, Io is the innermost of Jupiter’s four large moons. As it circles Jupiter, Io is gravitationally perturbed by its three sister moons, first being pushed slightly closer to Jupiter and then being pulled farther away. As Io gets closer to Jupiter, the tidal force exerted on it by Jupiter strengthens and tends to squeeze it into a slightly elliptical shape. As it moves farther away from Jupiter the tidal force on it becomes weaker, and it relaxes back into a more spherical shape. This constant kneading creates friction, which heats up Io’s interior and causes active volcanoes to pockmark its entire surface. No other body in the solar system today is as violently volcanically active as Io.