This photographΓÇötaken by the Voyager 1 spacecraft at a distance of 123,360 kilometersΓÇömakes it clear that IoΓÇÖs surface is for the most part covered with lava (flows extend up to 100 kilometers). No craters that form on IoΓÇÖs surface remain visible for long, since they quickly fill with or are covered by lava.
The temperature of IoΓÇÖs lava has been estimated at around 400 degrees Kelvin (sulfur melts at 385 degrees Kelvin). No one knows exactly why Io is so hot. According to one popular theory, gravitational forces continually pull and push the moon out of its orbit. This squeezing and pumping produces a great deal of heat in IoΓÇÖs core, liquifying sulfur and its compounds, which then erupt onto the moonΓÇÖs surface.
