Io, the third largest moon of Jupiter, is the most volcanically active body in our solar system. When the Voyager 1 spacecraft took the first close-ups of Io in 1979, it revealed that this moon, which is only a little larger than Earth’s moon, was covered with active volcanoes — more than 400!
Jupiter’s four largest moons are called the Galilean moons, because they were all discovered by the famous scientist Galileo Galilei. Two of them, Ganymede and Europa, exert forces on Io’s orbit that make it travel in a lopsided ellipse. According to NASA, Io is also subjected to intense tidal forces due to its widely varying distances from the planet. These forces create “tides” that are similar to those of Earth’s oceans, but way more extreme — and on solid ground, not water! In fact, unlike its fellow Galilean moons, Io appears to have no water.
While Io’s fiery volcanic plumes can rise to almost 200 miles above the surface, the moon also generates a huge amount of electricity as it cuts through Jupiter’s magnetic field. This creates lightning in the planet’s upper atmosphere.
Galileo discovered Io and the other three Galilean moons over two days in January 1610. He didn’t name them after himself; that came later. He originally dubbed them the “Medicean planets” after the rich and powerful Medici family, who supported his work. Galileo didn’t even name the individual moons, but only numbered them I, II, III, and IV. They didn’t get their current names — all taken from figures in Greek mythology — until the mid-1800s.
In Galileo’s time, people believed that the solar system revolved around Earth. His discovery marked the first time anyone had observed a moon orbiting another planet besides Earth, and it helped lead scientists to conclude that planets in our solar system orbit the sun.