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Io (moon) facts for kids

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True-color image taken by the Galileo probe
Galileo spacecraft true-color image of Io. The dark spot just left of the center is the erupting volcano Prometheus. The whitish plains on either side of it are coated with volcanically deposited sulfur dioxide frost. The yellower regions contain a higher proportion of sulfur
Discovered by Galileo Galilei
Discovery date January 7, 1610
Jupiter I
Adjectives Ionian
Orbital characteristics
Periapsis 420,000 km (0.002 807 AU)
Apoapsis 423,400 km (0.002 830 AU)
Mean orbit radius
421,700 km (0.002 819 AU)
Eccentricity 0.0041
1.769 137 786 d (152 853.504 7 s, 42 h)
17.334 km/s
Inclination 2.21° (to the ecliptic)
0.05° (to Jupiter's equator)
Satellite of Jupiter
Physical characteristics
Dimensions 3,660.0 × 3,637.4 × 3,630.6 km
Mean radius
1,821.3 km (0.286 Earths)
41,910,000 km2 (0.082 Earths)
Volume 2.53×1010 km3 (0.023 Earths)
Mass 8.9319×1022 kg (0.015 Earths)
Mean density
3.528 g/cm3
1.796 m/s2 (0.183 g)
2.558 km/s
Equatorial rotation velocity
271 km/h
Albedo 0.63 ± 0.02
Surface temp. min mean max
Surface 130 K 200 K
5.02 (opposition)
Surface pressure
Composition by volume 90% sulfur dioxide

Io (Jupiter I) is the innermost of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon, has the highest density of all the moons, and has the least amount of water of any known astronomical object in the Solar System. It was discovered in 1610 and was named after the mythological character Io, a priestess of Hera who became one of Zeus' partners.

Jupiter has 66 moons, but only four are large ones comparable to our Moon. Io is one of these four, and the third largest. Io is 3642.6 km, or 0.28 Earths, wide. It only has a mass of about 1.4% of the mass of Earth. It takes 42 hours (1¾ Earth days) for Io to rotate on its axis. It also takes 42 hours for Io to complete one orbit around Jupiter. Since these numbers are the same, it means that the same side of Io always faces Jupiter. Earth's moon works the same way, which is why we only see one side of the moon from earth. If you stood on the surface of Io, you would weigh less than you do on Earth. A person who weighs 200 pounds (90kg) on Earth would weigh about 36 pounds (16kg) on Io. So the gravity, of course, pulls less on you.


Galileo Galilei, the discoverer of Io

Io played a significant role in the development of astronomy in the 17th and 18th centuries. It was discovered in January 1610 by Galileo Galilei, along with the other Galilean satellites. This discovery 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.

From Earth, Io remained just a point of light until the late 19th and early 20th centuries, when it became possible to see its large-scale surface features, such as the dark red polar and bright equatorial regions. In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters.

Galileo spacecraft leaves the Orbiter
Galileo spacecraft leaves the Orbiter

The Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between Io and Jupiter's magnetosphere and the existence of a belt of high-energy radiation centered on Io's orbit. Further observations have been made by Cassini–Huygens in 2000 and New Horizons in 2007, as well as from Earth-based telescopes and the Hubble Space Telescope.

The first spacecraft to pass by Io were the twin Pioneer 10 and 11 probes on 3 December 1973 and 2 December 1974. Radio tracking provided an improved estimate of Io's mass, which, along with the best available information of Io's size and was composed primarily of silicate rock rather than water ice. The Pioneers also revealed the presence of a thin atmosphere at Io and intense radiation belts near the orbit of Io. The camera on board Pioneer 11 took the only good image of Io obtained by either spacecraft, showing its north polar region. Close-up images were planned during Pioneer 10s encounter with Io, but those observations were lost because of the high-radiation environment.


Io from Galileo and Voyager missions
Io's surface map

Based on their experience with the ancient surfaces of the Moon, Mars, and Mercury, scientists expected to see numerous impact craters in Voyager 1s first images of Io. The density of impact craters across Io's surface would have given clues to Io's age. However, they were surprised to discover that the surface was almost completely lacking in impact craters, but was instead covered in smooth plains dotted with tall mountains, pits of various shapes and sizes, and volcanic lava flows.

Io diagram
Io diagram

Compared to most worlds observed to that point, Io's surface was covered in a variety of colorful materials (leading Io to be compared to a rotten orange or to pizza) from various sulfurous compounds. The lack of impact craters indicated that Io's surface is geologically young, like the terrestrial surface; volcanic materials continuously bury craters as they are produced. This result was spectacularly confirmed as at least nine active volcanoes were observed by Voyager 1.


PIA01667-Io's Pele Hemisphere After Pillan Changes
Enhanced-color Galileo 1997

With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto.

Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (300 mi) above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Io's silicate crust. Some of these peaks are taller than Mount Everest.

Unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron-sulfide core. Most of Io's surface is composed of extensive plains coated with sulfur and sulfur-dioxide frost.

Vulcanic Explosion on Io
Enormous volcanic explosion can be seen silhouetted against dark space over Io

Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due to compounds of sulfur. When sulfur warms up and cools down, it changes colour. Numerous extensive lava flows, several more than 500 km (300 mi) in length, also mark the surface. The materials produced by this volcanism make up Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere.

Io Facts

Jupiter and Io as seen by Voyager 1
Jupiter and Io as seen by Voyager 1
  • Io is the most volcanically active body in the Solar System.
  • Io is one of the few moons to have an atmosphere although it is very thin.
  • At any given time, there are approximately 9 volcanoes erupting on Io.
  • Io is so close to Jupiter that Jupiter causes "tides" in its surface. These "tides" are what makes it so volcanically active.
  • Io is one of only five of Jupiter's moons that can eclipse the Sun as seen from Jupiter.
  • Io is the fourth-largest moon in the solar system.

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