Io Moon Facts

The planet Jupiter has 67 moons and Io is the fifth of these moons and the fourth largest in our solar system.

Io Moon Profile

Discovered By:Galileo Galilei January 8, 1610
Diameter:3,643.2 km
Mass:8.93 x 10^22 kg (1.2 Moons)
Orbit Distance:421,800 km
Orbit Period:1.77 days
Surface Temperature:-163 °C

Io also has the distinction of being the most volcanically active body in our solar system.

The volcanos on Io erupt with Sulphur as high as 190 miles/300 km. Io is named after a nymph that the mythological Greek god Zeus was in love with.

Who discovered the moon Io

Io, along with Callisto, Ganymede, and Europa was discovered by Galileo Galilea, the Italian scientist, in 1610. This Jovian moons that orbited a planet were the first to be discovered other than our own moon, in the solar system.

The telescopes of the time were very and although Galileo discovered Io, he could really tell the difference between Europa and Io until the next night when the sky was clearer.

The discovery of these moons is very important as it led to the scientific fact that planets orbit around the sun. Previously. They thought the Earth was the center of our solar system.

Space Missions

We have had quite a few spacecraft missions that have flown by Jupiter and its many moons. Each mission sends back images and information that adds to our knowledge of the planet and the moons.

A majority of our data comes from both the Voyager and Galileo space missions. The Galileo space probe went as low as 162 mi/261 km to view the surfaces of the moons.

Based on the many images and the information that we have, NASA decided to describe Io as “a giant pizza covered with melted cheese and splotches of tomato and ripe olives.”

This is a great description as the Io core is made up of iron sulfide or iron, however, it has an outer layer that is brown silicate. This gives it the rather unique orange, red, black, yellow, and white splotchy appearance.

The colors are due to the silicates such as sulfur dioxide and sulfur that are layered on the surface to create yellow to yellow-green areas.

Io Plasma Torus

Around Jupiter there is a donut-shaped plasma cloud and it’s in this area that Io exists. This area is called the “Io plasma torus,” and is caused by Jupiter’s extremely strong magnetic field.

When Io rotates around the planet the torus removes Io’s ions turning it into a kind of electrical reaction so that it produces radiation that is 1000 times stronger than would be required to kill a human being.

Io gets around 3,500 rem of radiation each day. At 100 rem, a human being would die in only a couple of weeks.

Volcanoes on Io

Other than Earth, Io is the only known celestial body to have active volcanoes. The activity of the over 400 volcanoes on Io is so great that it is considered to be the most volcanically and geologically active object in our solar system.

Plumes from some of the Io volcanoes can hit 190 mi/300 km high above the surface.

As Io orbits around Jupiter it is squeezed and stretched, causing the tidal heating and a bulge that changes the surface.

It is the irregular orbit that contributes to the tidal activity, making this moon so volcanically active.

Facts about Io Moon

  • Io’s surface has over 100 mountains, with some of the peaks taller than our Mount Everest.
  • Io’s core is molten iron sulfide or iron that is surrounded by silicate rock.
  • Io is closer to one of the terrestrial planets in composition than other moons.
  • Io is denser than Earth’s moon.
  • The study and discovery of Io also helped Kepler in establishing his three scientific laws of planets around the sun and the measurement of the speed of light.
  • Due to the constant eruption of its volcanoes, Io does have a very thin atmosphere made of sulfur dioxide.
  • Future robotic missions to Io may be planned to study the volcanoes.
  • Due to the very high radiation on Io, there aren’t any planned human missions.


  • What is the description that NASA uses for Io?
    a giant pizza covered with melted cheese and splotches of tomato and ripe olives
  • What activity occurs on Io that establishes it for a record in our solar system?
    highest geological/volcanic activity
  • What is Io’s position out of Jupiter’s 67 moons?
  • Why was the discovery of Io important?
    it led to the scientific fact that planets orbit around the sun
  • What sets Io apart in its composition when compared to other moons?
    it’s closer to a terrestrial planet
  • Could there be manned space missions planned for Io?
    no,radiation too high