This photograph—taken by the Voyager 1 spacecraft at a distance of 123,360 kilometers—makes it clear that Io’s surface is for the most part covered with lava (flows extend up to 100 kilometers). No craters that form on Io’s surface remain visible for long, since they quickly fill with or are covered by lava. 
    The temperature of Io’s 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 Io’s core, liquifying sulfur and its compounds, which then erupt onto the moon’s surface.