This is an image of Io.
Click on image for full size
NASA
Io
Io was first discovered by Galileo in 1610, making it one of the
Galilean Satellites. Of the 60 moons it is the 5th closest to Jupiter, with a standoff distance of 421,600 km. It is the 4th largest moon of Jupiter, with a diameter about 2/3 the distance across the United States, of 3630 km (2256 miles).
Io is named after one of Jupiter's many lovers, from Roman mythology. It is the only moon known to have active volcanism, which is visible on the surface.
Io is mostly made of sulfur, iron, and rocky material, which means that Io is nothing at all like the other icy satellites of Jupiter, and has had a very different evolution.
The particles comprising Io's atmosphere readily find their way into the magnetosphere, and create a donut-shaped cloud called the torus. This cloud has a very profound effect on Jupiter's magnetosphere.
Because of all the lava, the environment of Io is pretty unfriendly to life as we know it on earth.
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