Io is the fourth largest moon in the Solar System.
Jupiter’s Ganymede is the largest; our moon is the fifth largest.
Io was named after a priestess of Hera, Io. Hera was one of Zeus’ lovers.
Jupiter’s four Galilean moons are named after a differnet Zeus lover.
Io has over 400 active volcanoes making it the most geologically active object in the solar system. Excluding Earth. Earth has between 1,500-3,000.
Galileo discovered Io January 8, 1610. This discovery, with the other 3 Galilean satellites, furthered the adoption of the Copernican Model of the Solar System, the development of Kepler’s laws of motion, and the first measurement of the speed of light.
Io also has over 100 mountains; some peak higher than Mount Everest.
Io’s volcanism is responsible for many of that satellite’s unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur.