Rocky Io is the closest Galilean moon to Jupiter.
The craters that mark its surface are volcanic craters, not the result of meteorite bombardment.
The tallest volcanoes are about 5.5 miles (8.8 km) high.
Instead of molten rock, sulfur compounds spew out of Io’s volcanoes, covering evidence of any impact craters and giving the moon a new crust, including hot lakes of sulfur, some few tenths of an inch (centimeter) thick every century or so.
Spacecraft observations have noted volcanic matter being ejected some 180 miles (300 km) skyward every few seconds.
Io’s volcanic activity comes from a gravitational tug-of-war between Jupiter on the one side and the moons Europa and Ganymede on the other.
Jupiter’s nearby moon lo has many constantly active volcanoes.
Instead of releasing lava, as volcanoes do on Earth, they spew sulfur compounds. lo receives vast amounts of radiation from Jupiter, making it one of the most deadly environments in the solar system.
In 1979, the Voyager spacecraft observed volcanic matter on lo being ejected some 180 miles (288 km) skyward.
lo orbits Jupiter within the planet’s magnetic field, which causes a channel of electricity, like a high-tension wire.
The currents exceed 3 million amps, and great auroras result where the channel meets Jupiter’s atmosphere.