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This is an image of the surface of Io, looking down on a volcano and the lava plain surrounding it.
Click on image for full size
NASA

Surface of Io

The surface of Io is completely volcanic, with lava flows of sulfur from the interior, as shown in this image. White patches in the image are areas of frost made of sulfur dioxide instead of water.

The heat for this volcanism comes from the special position of Io relative to Jupiter and the other moons. Io is a small moon, very close to Jupiter, and well inside the other 3 main moons. Io feels the gravitational pull first of Jupiter, then of the outside moons when they sweep by. The alternating pull of gravity has a push-me, pull-me effect on Io, which deforms the moon, and warms it just the way a coat hanger warms when you bend it first one way, then the other way.

Due to this warming, Io has many volcanoes.

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