Jupiter has 79 moons that orbit this giant, and scientists often suggest that had Jupiter grown just a bit more, it would have become a sun with a miniature solar system. Io is the fifth moon of the 79 and is one of the moons that is of high interest to the scientific community.

Io highest resolution Introduction

Io’s volcanic activity is the highest in the entire solar system, with volcanoes erupting fountains of lava 190 mi/300 km high.

The volcanoes are due to the gravitational pull on one side by Jupiter and then the perfectly balanced pull on the others side of two others moons, Ganymede and Europa. This constant dance of gravity puts Io in a position of incredible tidal forces that cause the surface to bulge in and out as much as 330 ft/100 m.

To give an idea of comparison, Earth’s tidal forces on the water of our planet are only 60 ft/18 m.

Io is in constant motion, with a surface covered in plains of liquid rock and lakes filled with lava. Astronomers have been studying Io and have mapped around 150 existing volcanoes with new hotspots appearing.

One side of Io will always be facing Jupiter as it is “tidally locked” in the same way that our moon is to Earth. This moon of Jupiter is just a bit larger than our moon and is the third largest of all of Jupiter’s moons.

Io’s orbit is a comfortable 262,000 mi/422,000 km from its parent planet. It slices across Jupiter’s powerful magnetic lines of force and this action has given it the name of a kind of electric generator.

Io can develop as high as 400,000 volts and create a current rate of 3 million amperes. The electric current flows around the least powerful of the magnetic field lines created by Jupiter and this can in turn cause lightning in the upper atmosphere of Jupiter.

Io Statistics:

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


Galileo Galilei was having a difficult time on January 7, 1610 figuring out if the placement of Io and Europa was one moon or two. On January 8th – he officially made the discovery of Io, placing a distinction between Europa and Io.

He continued to discover three additional moons around Jupiter, Europa, Ganymede and Callisto, and this became the first time that a moon was discovered to orbit another planet in the solar system outside of our Earth.

The importance of Galileo’s discovery must be emphasized. Up until this time, astronomers assumed that all planets, and even the sun, orbited Earth.

Galileo’s understanding of the true state of the solar system included the sun as the center, all planets orbiting around the sun, and moons orbiting around their parent planets. This was a complete shakedown in the scientific world.

Galileo named this moon Jupiter I, but by the mid-1800s it was renamed as Io. Many of the moons are named after mythological creatures and gods. In Greek mythology, Io was one of Hera’s priestesses.

Hera was the wife of Zeus and the daughter of the king of Argos, Inachus. In Roman mythology, they use the name Jupiter instead of Zeus. The story tells that Zeus fell in love with Io, but to keep from being caught by Hera, he turned Io into a cow.

Formation, Structure and Surface:

The inside of Io has an incredible amount of heat that builds up due to the constant tidal forces that are generated. This keeps the subsurface in a liquid form and as the pressure builds up, it escapes through the constant volcanic activity. Due to the volcanic lava flow, the surface of Io is constantly renewing itself.

As any asteroids crash into Io, creating impact craters, the lava fills it in so that they become molten lava lakes. The lava spreads out onto the floodplains with a new surface.

Scientists aren’t completely sure what composition materials make up Io, but some theories include that it is mostly made up of molten sulfur, its associated compounds, and silicate rock.

This composition might explain why the surface has such varied colors and temperatures; some of which are too hot for sulfur alone.

Temperatures on Io range on the surface from an average of -202 degrees F/- 130 degrees C to 3,000 degrees F/1,649 degrees C. This radical temperature variation is why scientists often call Io the “Celestial body of fire and ice.” The intense cold is part of creation of what they refer to as the sulfur dioxide snowfields.

As curious as the outside of Io, so is the inside. Io’s interior is made up of iron or iron sulfide at its core and an outer layer made of brown silicate.

Io Earth Moonsize comparison Formation

The combination of the materials and volcanic activity give Io and surface that has many splotchy colors, including yellow, orange, black, white, and red.

Scientists are observing Io’s long-term volcanic activity and hotspots by using two of the largest telescopes in the world. They have been able to track around 48 hotspots and see that the lava lakes have “waves” that provide information on the cycle of magma on the surface of Io.

Continued studies of Io have presented the researches with a lot of questions. The biggest question is that in volcanoes seem to appear in locations that scientists don’t expect them to be.

When they analyze the areas that have the highest heat, scientists expect that those will also be the zones with the most volcanic activity. However, Io has surprised them with higher volcanic activity in areas that are further east.

One study that has been disproven suggested that there were molten and solid rock underneath the surface that was combined with an ocean that rubbed against each other and offset the volcanic activity.

This was disproven when scientists used the aurora of Io to show that the only way the moon would have an aurora is if there wasn’t any molten ocean.

Atmosphere and Magnetosphere:

The thin atmosphere of Io is mostly made up of sulfur dioxide and, unlike some of the other moons, it doesn’t seem to have water of any sort. The Galileo spacecraft has sent data that showed that Io has an iron core at its center which gives it its own magnetic field.

The gravitational forces placed on Io from Jupiter are so intense that they not only contribute to the volcanic activity which spew Sulphur dioxide into the atmosphere, but also turn Io into a kind of electric generator.

The magnetic forces of Jupiter’s rotation steal about one ton of the material of Io every second. This material is then ionized and forms a “plasma torus,” which is a doughnut-shaped cloud of radiation. Not all of the ions are held and those that escape enter the upper atmosphere of Jupiter and create auroras.

Hemisphere After Pillan Atmosphere

The combination effort of the Gemini North Telescope in Hawaii and the Texas Echelon Cross Echelle Spectrograph (TEXES) resulted in the release of a report in 2018 that showed that Io has a collapsible atmosphere.

This occurs during the time of the day when Io is in Jupiter’s shadow and the envelope of sulfur dioxide gas freezes up. Once Io returns to the sunlight, the frozen sulfur dioxide converts back into a gaseous form. This situation was one that scientists had suspected but until this research, they had never had it confirmed.

Could Life Exist?

Io’s constant state of volcanic activity combined with the intensity of the radiation make it an unlikely candidate for life to begin, adapt, and thrive.

Interesting Information:

  • Io became the first moon outside of our Earth’s moon to be discovered.
  • The description of Io by NASA calls the moon “A giant pizza covered with melted cheese and splotches of tomato and ripe olives.”
  • Even though we have had a number of spacecraft do flybys of Io, all of the information that we have collected about Io comes from the Voyager 1 and 2 and Galileo spacecraft missions.
  • Io’s radiation is due to the “plasma torus” and is 1000 times stronger that what is required to kill people.
  • So far, scientists have counted over 400 active volcanoes on Io, but since new hotspots are always happening, this number can quickly change.
  • Io has an irregular orbit around Jupiter that increases the tidal activity and is the cause of such high volcanic activity.
  • Due to the constant compression of the crust of Io, it has uplifted the surface and there are over 100 mountains. Some of the mountains are taller than Mount Everest at a height of 4 mi/6 km and 98 mi/157 km long.
  • Io stands out in the solar system moons as most other moons are made up of mostly rock and water ice. Io is made up of silicate rock that surrounds either an iron sulfide or molten iron core. This condition places Io closer to that of a terrestrial planet composition.


We have sent quite a few spacecraft to explore Jupiter and its moons. The first spacecraft was the Pioneer 10 that arrived in 1973; this was followed in 1974 by Pioneer 11. Later Voyager 1 and 2 did flybys that sent back incredible images. The Galileo spacecraft produced even more detailed images as it passed the surfaces of the moons as low as 162 mi/261 km.

Since 2003, three additional spacecraft have been sent for close observations of Io: Ulysses, Cassini, and New Horizons. Although Juno was sent out, its focus is on Jupiter and not the moons.

The Hubble Space Telescope has taken images of Jupiter’s northern pole using ultraviolet light filters. One of the important things that scientists noted was the auroral structures which are believed to be caused by Io due to the magnetic field lines of Jupiter that connect to Io.

Tvashtarvideo Exploration

Scientists create computer models to try to figure out how Io formed. They believe that Io was created in an area around Jupiter where there was a lot of water ice.

They think that at one point, in the beginning of its creating, there may have been water on Io and, along with Io’s heat, there might have been a possibility of life developing at that time. However, the radiation of Jupiter removed any surface water.

Important Events:

1610: Galileo Galilei discovers Io as well as three additional moons that he found were orbiting Jupiter.

1973-1974: The Pioneer 10 became the first manmade spacecraft to have an up close exploration of Jupiter. This was followed by the Pioneer 11 that made the first measurements of Io close up.

1979: Voyager 1 does a flyby of Jupiter and catches images of one of Io’s volcanic eruptions.

1979: Voyager 2 collects more scientific data during a distant flyby of Io and takes two additional images.

1992: The Ulysses spacecraft makes use of Jupiter to slingshot itself into orbit so that it can study the poles of the sun. Scientists are surprised when Ulysses passes through a particle stream from Io that turns out to be a particle stream traveling faster than most that our known in the solar system so far.

1995-2002: The Galileo spacecraft accomplishes multiple Io flybys collecting the most detailed observations and records to date.

2000: On its way to Saturn, the Cassini spacecraft passes both Jupiter and Io and studies Io on its continued journey.

2007: The New Horizons spacecraft collects more information on Io’s constantly changing surface.

Using the observations by the Voyager 1 and 2 spacecraft and the Galileo, a published work by the U.S. Geological Survey in 2012 included a full map of the entire surface of Io.

Facts about Io Moon for Kids:

  • Io is the most volcanically active moon in the solar system and has over 400 moons.
  • Io’s volcanoes are in a constant state of eruption.
  • Jupiter’s intensely strong gravity stretches Io one way, and the gravitational pull of some of Jupiter’s other moons are pulling Io the other way.
  • The plumes from Io’s volcanoes rain down and cover the landscape with sulfur, sulfur dioxide, and rocky as.
  • The “Io plasma torus” is a ring of charged particles around Jupiter that originate from Io.
  • The extreme conditions on Io make it a moon that people won’t be visiting, but future robotic missions could learn more about Io.

Pop Culture:

Io may be most well-known in pop culture as the reference from 2010: The Year We Make Contact, which was the sequel to 2001: A Space Odyssey. In the movie, astronauts accomplish a very dangerous spacewalk over the volcanoes of Io to board the previously abandoned spacecraft.