Tides on Jupiter's moon Io, illustration. Diagram showing the gravitational flexion (tidal stretching) of Io by the large gravitational field of Jupiter. The stretching is maximal at the point of closest approach (lower left). The tidal stretching distorts Io's shape from a sphere to an ovoid shape. This produces internal heat inside Io, and accounts for the volcanic activity seen on this Moon. Io rotates on its axis every 1.77 days (in the same direction as Jupiter's rotation), the same time it takes to orbit Jupiter. This is known as tidal locking, and a body that permanently faces its parent body is said to be tidally locked.