Io Moon Facts
jupiter moon io
The colorful Jupiter moon Io, the most volcanically active body in the solar system
  • Io is the fifth moon in distance from the planet Jupiter.
  • It orbits Jupiter at an average distance of 262,000 miles (422,000 km,) similar to the distance the Moon orbits Earth.
  • Io is the third largest of Jupiter's moons and very similar in size to our own moon.
  • It is the most volcanically active object in the solar system.
  • Volcanic plumes rise almost 190 miles (300 km) above the surface.
  • Io has an iron core just like Earth, meaning it could have its own magnetic field.
  • The moon's orbit cuts across Jupiter's powerful magnetic field producing 400,000 volts of electricity across its surface.
  • Jupiter's magnetic field strips off 1 ton of material from Io every second!
  • The moon was discovered in January 1610 by the Italian astronomer Galileo Galilei.
  • Io is often compared to a pizza due to its unusual surface which is made up of sulfur dioxide deposits.

volcanic eruption
A giant plume erupting from Io's northern polar Tvashtar volcano
Io's Surface
Jupiter's huge gravity causes Io's surface to bulge up and down by as much as 100 meters! This also generates tremendous heat inside Io, turning its subsurface into liquid. This molten lava frequently escapes causing its surface to constantly change.
Io's Atmosphere
Io's atmosphere is roughly 1 millionth as dense as Earth's atmosphere and made primarily of sulfur.
Io's Temperature
Temperatures on Io vary from extremely hot to extremely cold. Areas which feature volcanic activity can reach temperatures of over 2000C (4000F). Away from these areas however will see temperatures plunge to -143C (-230F).

Origin of Name
Io is named from Greek mythology after the priestess of Hera who became a lover of Zeus.
Life on Io
Io is an extremely hostile environment with little or no chance for the existence of life.

Io - The Volcanic Moon

surface of io
The spectacular surface of Io with volcanic plume
As Voyager 1 and 2 approached the Jupiter system in the late 1970's NASA scientists fully expected the craft to discover that its moons would be cold, dead and not particularly interesting, instead what they found was quite astonishing. One moon in particular stood out, Io, scientists were amazed to find out that it was in fact geologically active with volcanoes constantly erupting on its surface. The sulfur deposits from the eruptions also made the small moon strikingly colorful.

Scientists initially scratched their heads and wondered how a small moon in one of the coldest parts of the solar system could be generating so much heat. The explanation was in fact very simple, Io's orbit around Jupiter is extremely elliptical, meaning that sometimes it's close to the giant planet and other times it's further away. This causes Jupiter’s strong gravity to push and pull the small moon, producing immense heat inside Io and creating molten lava below its surface.

Scientists had expected to find something similar to our moon but what they had in fact discovered was the most geologically active body in the entire solar system.