IO (SATELLITE): CHARACTERISTICS, COMPOSITION, ORBIT, MOVEMENT, STRUCTURE - ARTE - 2025

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 that he himself built.

It is the third largest of the Galilean satellites and of the remaining 75 Jupiter satellites. In order of orbital radius, 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.

Figure 1. Io is part of the four satellites discovered by Galileo Galilei in 1610 and of the four it is the closest to the planet. (wikimedia commons).

Io is one third the diameter of the Earth and about the size of our satellite the Moon. Compared to the 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 from the vast plains. No impact craters are observed, which indicates that they have been erased by 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.

There are hundreds of mountains on its surface, some higher than Mount Everest, which have been formed due to the intense volcanism of the satellite.

The discovery of Io in 1610 and the other Galilean satellites changed the perspective of our position in the universe, since at that time we were thought to be the center of everything.

By 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 feasible and palpable.

Thanks to Io the first measurement of the speed of light was made by Danish astronomer Ole Christensen Rømer in 1676. He realized that the duration of the eclipse of Io by Jupiter was 22 minutes shorter when the Earth was closer to Jupiter than when it was at its furthest point.

That was the time it took for light to travel the Earth's orbital diameter, from there Rømer estimated 225,000 km / s for the speed of light, 25% less than the currently accepted value.

General characteristics of Io

By the time the Voyager mission approached the Jovian system it found eight erupting volcanoes on Io, and the Galileo mission, while unable to get too close to the satellite, did bring up excellent resolution images of the volcanoes. No less than 100 erupting volcanoes detected this probe.

Figure 2. Surface of Io showing the extensive plains and abundant volcanoes, in true colors photographed by the Galileo probe. Source: NASA.

The main physical characteristics of Io are:

-Its diameter is 3,643.2 km.

-Mass: 8.94 x 10 ²² kg.

-Average density 3.55 g / cm ³.

-Surface temperature: (ºC): -143 to -168

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

-Period of rotation: 1d 18h ​​27.6m

-Translation period: 1d 18h ​​27.6m

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

Summary of the main characteristics of Io

Composition

The most outstanding characteristic of Io is its yellow color, which is due to the sulfur deposited on the essentially volcanic surface. For this reason, although the impacts due to meteorites that the giant Jupiter attracts are frequent, they are quickly erased.

Basalts are thought to be abundant in the satellite, as always, colored yellow by sulfur.

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

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

Finally, the atmosphere of Io is composed almost 100% of sulfur dioxide.

Atmosphere

Spectral analyzes reveal a thin atmosphere of sulfur dioxide. Even though hundreds of active volcanoes spew a ton of gases per second, the satellite cannot retain them due to the low gravity and the escape velocity of the satellite is not very high either.

Additionally, ionized atoms that leave Io's vicinity are trapped by Jupiter's magnetic field, forming a kind of donut on its orbit. It is these sulfur ions that give the tiny and nearby satellite Amalthea, whose orbit is below that of Io, the reddish color.

The pressure of the thin and thin atmosphere is very low and its temperature is below -140ºC.

The surface of Io is hostile to humans, due to its low temperatures, its toxic atmosphere and the enormous radiation, since the satellite is within the radiation belts of Jupiter.

Io's atmosphere fades and ignites

Due to the orbital movement of Io there is a time in which the satellite stops receiving the light of the Sun, since Jupiter eclipses it. This period lasts 2 hours and as expected, the temperature drops.

Indeed, when Io faces the Sun its temperature is -143 ºC, but when it is eclipsed by the gigantic Jupiter its temperature can drop to -168 ºC.

During the eclipse, the thin atmosphere of the satellite condenses on the surface, forming sulfur dioxide ice and disappears completely.

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

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

Translation movement

Io makes one complete revolution around Jupiter in 1.7 Earth days, and each turn of the satellite is eclipsed by its host planet, for a period of 2 hours.

Due to the enormous tidal force the orbit of Io should be circular, however this is not the case due to the interaction with the other Galilean moons, with which they are in orbital resonance.

When Io turns 4, Europa turns 2 and Ganymede 1. The curious phenomenon can be seen in the following animation:

Figure 3. Orbital resonance of Io and its sister satellites: Ganymede and Europa. Source: Wikimedia Commons.

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

The smallest orbital radius (periaster or perihelion) of Io is 420,000 km, while the largest orbital radius (apoaster or aphelion) is 423,400 km, giving a mean orbital radius of 421,600 km.

The orbital plane is inclined relative to the Earth's orbital plane by 0.040 °.

Io is considered the closest satellite to Jupiter, but in reality there are 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 probably meteorites trapped in Jupiter's gravity.

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

After Io's orbit, the next satellite is a Galilean one: Europa.

Despite being very close to Io, Europe is completely different in composition and structure. This is believed to be so because that small difference in the orbital radius (249 thousand km) makes the tidal force on Europa considerably less.

Orbit of Io and magnetosphere of Jupiter

Volcanoes on Io blow ionized sulfur atoms into space that are trapped by Jupiter's magnetic field, forming a plasma conductor donut that matches the satellite's orbit.

It is Jupiter's own magnetic field that carries the ionized material from Io's thin atmosphere.

The phenomenon creates a current of 3 million amperes that intensifies Jupiter's already powerful magnetic field to more than double the value it would have if there were no Io.

Rotatory motion

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

The inclination of the axis of rotation is negligible.

Internal structure

Since its average density is 3.5 g / cm ^{3, it} is concluded that the interior structure of the satellite is rocky. Spectral analysis of Io does not reveal the presence of water, so the existence of ice is unlikely.

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

It is followed by a deep and partially molten rocky mantle, and a thin, rocky crust.

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

The crust was originally thought to be sulfur, but infrared measurements reveal that volcanoes erupt lava at 1500ºC, indicating that it is not only composed of sulfur (which boils at 550ºC), there is also molten rock.

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

The internal structure of Io according to the theoretical models is summarized in the following illustration:

Figure 4. Structure of Io. Source: Wikimedia Commons.

Geology of Io

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

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

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

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

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

Whitish areas are observed on the surface of Io due to frozen sulfur dioxide. In the crevices of the faults the molten lava flows and explodes upwards.

Figure 5. Sequence taken by the New Horizons probe, showing a volcano erupting on the surface of Io. Source: NASA.

Where does Io's energy come from?

With Io being a little larger than the Moon, which is cold and geologically dead, one wonders where the energy of this small Jovian satellite comes from.

It cannot be the remaining heat of formation, because Io is not large enough to retain it. Nor is it the radioactive decay of its interior, since in fact the energy dissipated by its volcanoes easily triples the radiation heat that a body of such size emanates.

Io's source of energy is the tidal force, 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. The friction between the rocks is what produces this enormous heat, certainly much greater than that of the terrestrial 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. The sulfur dioxide vaporizes, generating enough pressure for the magma spewed by volcanoes to cool and cover the surface.

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

References

1. 20 minutes. (2016) The observation of an eclipse in Io reveals its secrets. Recovered from: 20minutos.es
2. Kutner, M. (2010) Astronomy: A physical perspective. Cambridge University Press.
3. Seeds and Backman. (2011). The solar system. Cengage Learning.
4. Wikipedia. Io (satellite). Recovered from: es. wikipedia.com
5. Wikipedia. Jupiter satellites. Recovered from: es. wikipedia.com
6. Wikipedia. Galilean satellite. Recovered from: wikipedia.com