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These is a drawing of the interior of Io.
Click on image for full size
NASA

Interior of Io

The diagram to the left shows the interior of Io, based on recent measurements by the Galileo spacecraft. It shows a large iron/sulphur core (the Earth's core is iron/nickel), overlain with a silicate rocky material. There is no ice present within Io.

When the Galileo spacecraft flew by Io it took measurements which showed that Io was separated into two layers, as shown in this picture.

The history of Io is one of continuous warming due to tidal forces, which results in extensive volcanic activity on the surface. The tidal forces cause Io to warm up the same way a coat hanger, folded back and forth will cause coat hanger to warm.

As shown in the figure, possible pools of melted, liquid sulphur, encounter the hot pockets of Io's interior and boil, providing Io with geysers on the surface.

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