- is a large, rocky, vocanically active moon of Jupiter 
- a very colorful moon because its volcanoes let out something called molten sulfur
- third largest moon 
- 1, 942 miles/3,636 kilometers in diameter, which is close to the size of our moon 
- mean distance from Jupiter is 220,000 miles/422,000 kilometers 
- has a mass of 8.93 * 1022 kg 
- takes 1.77 days to orbit Jupiter 
- doughnut-shaped plasma cloud around Jupiter near Io's orbit, known as the "Io plasma torus"; caused by Jupiter's strong magnetic field 
- independently discovered by Galileo and Marius in 1610 
- most volcanically active body in the solar system
- volcanoes are driven by hot silicate magma. 
- one of the only three moons with an atmosphere in the solar system