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Why is Io volcanically active?
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Jupiter
Photograph of Jupiter taken by Voyager 1 on February 1, 1979, at a range of 32.7 million km (20.3 million miles). Prominent are the planet's pastel-shaded cloud bands and Great Red Spot (lower centre).
Io is the most volcanically active body in the solar system due to a process known as tidal heating.12
While radioactive decay is insufficient to generate such intense activity in a body of Io's size, the moon is subject to powerful gravitational forces from Jupiter and its neighboring Galilean satellites. Specifically, a gravitational resonance between Io and the moon Europa forces Io into a slightly eccentric orbit.12
As Io travels along this path, it experiences continual gravitational "tugging" that causes the satellite to flex. This constant internal friction generates immense heat, which melts rock beneath the crust and powers the hundreds of volcanoes observed on its surface.12