Of Jupiter’s four large Galilean satellites, Io is the closest to the planet. It’s about the same size as Europa and the Earth’s Moon. It may be the strangest satellite humans have ever studied. Voyager photos of Io revealed erupting volcanoes. This wasn’t too surprising, because Jupiter’s intense tidal forces heat the interior of the satellite, just like on Europa. But subsequent observations show that volcanoes are always erupting on Io — in fact, it’s the most volcanically active world in the Solar System! Material shot up by the many volcanoes rises high above the surface of the little satellite and then falls back in a lazy arc due to Io’s weak gravity. Volcanic activity adds about 10 centimeters of new material to the surface each year.
Volcano Pele on Io erupting. Click here for original source URL
Io moving across the disk of Jupiter. Click here for original source URL
Jupiter's Io. Click here for original source URL.
Io’s intensely hot volcanism has apparently evaporated and blown away all of the satellite’s water and other volatile materials. The volcanoes have also created a large torus of plasma around Jupiter. As the more volatile compounds were lost, heavier sulfur and metallic compounds remained. Sulfur compounds became the dominant surface materials, in a process unique among all the bodies in the solar system. Sulfur minerals are the source of Io’s vibrant colors: yellow, orange, red, white and black. The color of sulfur changes, depending on its temperature. Pure liquid sulfur is black or dark brown. The darkest spots on Io are volcanic craters of molten or near-molten sulfur. Instruments on Voyager measured the temperatures of the dark spots as 600 to 700 K (621 to 800 °F), consistent with near-molten sulfur deposits. Galileo measured the temperatures of some Ionian volcanoes to be as high as 2,000 K (3,140 °F), which is hotter than any present terrestrial eruptions. Images taken over time, as Galileo continued its mission, show a changing landscape, marred by volcanic eruptions, lava lakes, and curtains of fiery magma. A visit to Io would reveal a truly strange world with sulfuric volcanoes erupting beneath Jupiter, which would dominate the sky at close range.
Possible internal structure of Io. Click here for original source URL.
Io’s core is also very different from the other large satellites. In 1996, scientists analyzed the gravitational accelerations acting on the Galileo probe as it flew very close to Io. They found that the satellite has a large iron core, probably in the range of 36% to 52% of Io's radius. This piece of evidence agrees with the theory that tidal heating has melted the entire interior, which then differentiated. It also explains why Io has a higher density than any other known satellite (3530 kg m-3, which is almost as high as Mars' mean density)