Physics Ideas

Io (satellite): characteristics, composition, orbit, movement, structure

Io is part of the four Galilean satellites (Io, Europa, Ganymede, Callisto), so named because they were discovered in 1610 by Galileo Galilei with a rudimentary telescope he built himself.

It is third in size to the satellites of Galilee and the remaining 75 satellites of Jupiter. In orbital radio order, it is the fifth satellite and the first of the Galileans. Its name comes from Greek mythology, in which Io was one of the many maidens with whom the god Zeus, also called Jupiter in Roman mythology, fell in love.

Io is one-third the diameter of Earth and is similar in size to our satellite, the Moon. Compared to other satellites in the solar system, Io ranks fifth in size, preceded by the Moon.

The surface of Io has mountain ranges that stand out in the extensive plains. No impact craters are observed, indicating that they were erased due to their great geological and volcanic activity, considered the largest of all in the solar system. Its volcanoes produce clouds of sulfur compounds that rise 500 km above its surface.

Hundreds of mountains are counted on its surface, some higher than Mount Everest, which were formed due to the satellite’s intense volcanism.

The discovery of Io in 1610 and other Galilean satellites changed the perspective of our position in the universe, as we thought that at that time it was the center of everything.

Upon discovering “other worlds”, as Galileo called the satellites that revolved around Jupiter, the idea proposed by Copernicus that our planet revolved around the Sun became more viable and palpable.

Thanks to Ío, the first measurement of the speed of light was taken by the Danish astronomer Ole Christensen Rømer in 1676. He noticed that the duration of the eclipse of Io by Jupiter was 22 minutes shorter when the Earth was closer to Jupiter. than when he was at the farthest point.

It was the time it took for light to travel the Earth’s orbital diameter; therefore, Rømer estimated 225,000 km/s for the speed of light, 25% less than the currently accepted value.

General characteristics of Io

When the Voyager mission approached the Jovian system, it found eight volcanoes erupting on Io, and the Galileo mission, though unable to get very close to the satellite, brought excellent resolution images of the volcanoes. No fewer than 100 erupting volcanoes detected this probe.

Io’s main physical characteristics are:

-Its diameter is 3643.2 km.

-Mass: 8.94 x 10 22 kg.

Density -Average 3.55 g / cm 3 .

-Surface temperature: (°C): -143 to -168

-The acceleration of gravity on its surface is 1.81 m / s 2 or 0.185 g.

-Rotation period: 1d 18h ​​​​27.6m

– Translation period: 1d 18h ​​​​27.6m

-Atmosphere composed of sulfur dioxide (SO2) at 100%.

Summary of main features of Io


Io’s most striking feature is its yellow color, due to the sulfide deposited on the essentially volcanic surface. For this reason, although impacts due to meteorites that the giant Jupiter attracts are frequent, they are quickly erased. 

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It is thought that basalts abound on the satellite, as always, yellow in color with sulphur.

Molten silicates abound in the mantle (see below for details of the internal structure), while the crust is composed of sulfur and frozen sulfur dioxide.

Io is the densest satellite in the solar system (3.53 g/cc) and is comparable to rocky planets. The silicate rock in the mantle surrounds a cast iron sulfide core.

Finally, Io’s atmosphere is made up of almost 100% sulfur dioxide.


Spectral analysis reveals a weak sulfur dioxide atmosphere. Although hundreds of active volcanoes spew a ton of gases per second, the satellite cannot trap them due to low gravity, and the satellite’s escape velocity is also not very high.

In addition, the ionized atoms that leave the vicinity of Io are trapped by Jupiter’s magnetic field, forming a type of donut in its orbit. It is these sulfur ions that give a reddish color to the tiny nearby Amaltea satellite, whose orbit is below Io.

The pressure of the thin and thin atmosphere is very low and its temperature is less than -140°C.

Io’s surface is hostile to humans due to the low temperatures, toxic atmosphere and enormous radiation, as the satellite is within Jupiter’s radiation belts. 

Io’s atmosphere fades and burns

Due to Io’s orbital motion, there is a moment when the satellite stops receiving sunlight, as Jupiter eclipses it. This period lasts 2 hours and, as expected, the temperature drops.

In fact, when Io faces the Sun, its temperature is -143°C, but when it is eclipsed by the gigantic Jupiter, its temperature can drop to -168°C. 

During the eclipse, the satellite’s weak atmosphere condenses on the surface, forming ice with sulfur dioxide and disappearing completely.

Then, when the eclipse ceases and the temperature begins to rise, the condensed sulfur dioxide evaporates and Io’s weak atmosphere returns. This is the conclusion reached by a NASA team in 2016.

So, Io’s atmosphere is not formed by the gases from the volcanoes, but by the sublimation of ice on its surface.

translation movement

Io makes a complete loop around Jupiter in 1.7 Earth days, and each spin of the satellite is overshadowed by its host planet, for a period of 2 hours.

Due to the enormous force of the tides, Io’s orbit must be circular, but this is not the case due to interaction with the other moons of Galilee, with which they are in orbital resonance.

When Io turns 4, Europe turns 2 and Ganymede 1. The curious phenomenon is seen in the following animation:

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This interaction causes the satellite’s orbit to have a certain eccentricity, calculated at 0.0041.

The smallest orbital radius (periastrum or perihelion) of Io is 420,000 km, while the longest orbital radius (apoastrum or aphelion) is 423,400 km, resulting in an average orbital radius of 421,600 km.

The orbital plane is tilted from the Earth’s orbital plane by 0.040°.

Io is considered the closest satellite to Jupiter, but there are actually four more satellites below its orbit, albeit extremely small.

In fact, Io is 23 times larger than the largest of these small satellites, which are likely meteorites trapped in Jupiter’s gravity.

The names of the small moons, in order of proximity to their host planet, are: Metis, Adrastea, Amalthea and Thebe.

After Io’s orbit, the next satellite is that of Galilee: Europe.

Despite being very close to Io, Europe is completely different in composition and structure. This is believed to be the case, because this small difference in orbital radius (249,000 km) makes the tidal force in Europe considerably smaller.

Io orbit and Jupiter’s magnetosphere

Io’s volcanoes eject ionized sulfur atoms into space that are captured by Jupiter’s magnetic field, forming a ring of plasma that corresponds to the satellite’s orbit.

It is Jupiter’s own magnetic field that transports ionized material from Io’s weak atmosphere.

The phenomenon creates a current of 3 million amps that intensifies Jupiter’s already powerful magnetic field by more than twice as much as it would have been if Io did not exist.

rotary movement

The period of rotation around its own axis coincides with the orbital period of the satellite, caused by the tidal force that Jupiter exerts on Io, its value being 1 day, 18 hours and 27.6 seconds.

The tilt of the axis of rotation is negligible.

Internal structure

As its average density is 3.5 g / cm 3, it is concluded that the interior structure of the satellite is rocky. Io spectral analysis does not reveal the presence of water, making ice unlikely.

Based on calculations based on the collected data, the satellite is believed to have a small core of iron or iron mixed with sulfur.

It is followed by a deep, partially melted bedrock and a thin rock crust.

The surface has the colors of a poorly made pizza: red, pale yellow, brown and orange.

The crust was originally considered sulfur, but infrared measurements reveal that volcanoes explode lava at 1500 °C, indicating that it is not just sulfur (which boils at 550°C); there are also molten rocks.

Another evidence of the presence of rock is the existence of some mountains with heights that double Mount Everest. Sulfur alone would not have the strength to explain these formations.

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Io’s internal structure according to theoretical models is summarized in the following illustration:

Io geology

The geological activity of a planet or satellite is driven by the heat within it. And the best example is Io, the innermost of Jupiter’s largest satellites.

The massive mass of its host planet is a great attractor of meteorites, like the remembered Shoemaker-Levy 9 in 1994, however Io does not show impact craters and the reason is that intense volcanic activity erases them.

Io has more than 150 active volcanoes that spew enough ash to bury impact craters. Io’s volcanism is much more intense than Earth’s and is the largest in the entire solar system.

What drives the eruptions of the Io volcanoes is the sulfur dissolved in the magma, which when it releases its pressure drives the magma, throwing ash and gases up to 500 m high.

The ash returns to the surface of the satellite, producing layers of debris around the volcanoes.

Whitish areas are seen on Io’s surface due to frozen sulfur dioxide. In the lava crevices, molten lava flows and explodes upward.

Where does Io’s energy come from?

Being slightly larger than the Moon, which is cold and geologically dead, it’s worth wondering where this little Jovian satellite’s energy comes from.

It can’t be the remaining heat from the formation, because Io isn’t big enough to hold it. Nor is it radioactive decay in the interior, since, in fact, the energy dissipated by its volcanoes easily triples the heat from radiation emanating from such a large body.

Io’s energy source is the force of the tides , due to Jupiter’s immense gravity and due to its proximity to it.

This force is so great that the surface of the satellite rises and falls 100 m. Friction between the rocks is what produces this enormous heat, certainly much greater than that of the earth’s tidal forces, which barely move the solid surface of the continents by a few centimeters.

The enormous friction caused by the gigantic tidal force on Io causes enough heat to be generated to melt the deep layers. Sulfur dioxide vaporizes, creating enough pressure for the magma released by the volcanoes to cool and cover the surface.

The tidal effect decreases with the cube of the distance from the center of attraction, so this effect is less important on satellites farther away from Jupiter, where the geology is dominated by meteorite impacts.

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