In the first part of the material on satellites there were only three of them — Earth’s Moon and two satellites of Mars. In the second part we will discover Jupiter, which has currently as many as 67 registered satellites!



This giant has a special place in the Solar System, since it is almost twice as large as all of the planets combined. Jupiter is so massive that their common center of mass with the Sun lies above the Sun’s surface.


The common center of mass of Jupiter with the Sun is shown with a dot

Jupiter has a very powerful radiation emission, second after the Sun in the Solar System. A huge number of satellites rotate around Jupiter in comparison to other planets.

Thanks to ground-based observations of the Jupiter system, by the end of the 70s thirteen moons were known. In 1979, while making a flyby of Jupiter, the Voyager 1 spacecraft has found three more satellites. Later, with the help of a new generation of ground-based telescopes, additional 51 moons of Jupiter were discovered.

The vast majority of the satellites have a diameter of 2–4 kilometers. Scientists believe that Jupiter has not less than a hundred satellites, but, as already mentioned, to date 67 moons are recorded and 63 are well studied.

Jupiter’s satellites are divided in three groups: Galilean, inner and irregular. Let’s start with the Galilean.

Galilean Moons

The four biggest moons — Io, Europa, Ganymede and Callisto were discovered by Galileo Galilei in 1610, and so now there are called “Galilean”. These satellites formed from gas and dust that surrounded Jupiter after its formation.


Galilean moons of Jupiter, from left to right in order of distance from Jupiter: Io, Europa, Ganymede, Callisto


Size comparison; in the top row, from left to right in order of distance from Jupiter: Io, Europa, Ganymede, Callisto. The Earth and the Moon are at the bottom


Io is the fifth moon of Jupiter and it is the most volcanically active body in the Solar System. It is four and a half billion years old, about the same age of Jupiter. The moon is always turned to its planet with one side. The distance from the surface of Jupiter to Io is 350 thousand kilometers. Its diameter is equal to 3642 kilometers — a little more than that of the Moon (3473 km). It is the fourth largest satellite in the Solar System.



The volcanic activity on satellites is an extremely rare phenomenon in the Solar System and Io is undoubtedly the leader in our System on this activity. Io is one of the four currently known celestial bodies in the Solar System which have processes of the volcanic activity. Apart from Io: Earth, Triton (satellite of Neptune) and Enceladus (moon of Saturn). Venus is also “suspected” in volcanism (the Beta Regio), however, at this moment no active volcanoes have yet been found there.

Eruptions on Io are large and they are clearly visible from the space. Volcanoes erupt sulfur to a height of three hundred kilometers. Lava flows and more than a hundred calderas are clearly seen on the moon’s surface; the entire surface is covered with sulfur in various colored forms. Io’s atmosphere contains mainly sulfur dioxide, this is due to high volcanic activity.


An animation of the Tvashtar Paterae eruption composed of five images taken from the New Horizons spacecraft in 2007

Because of its proximity to Jupiter enormous gravitational forces of the planet are acting on the satellite, causing tidal forces that create great friction inside the satellite as a result of which heating occurs of both the subsoil and surface of Io. The gravitational forces of the planet constantly stretch and deform the satellite. Some parts of the moon are heated to three hundred degrees Celsius; also twelve volcanoes spewing magma to a height of three hundred kilometers are found on Io.


Eruption of the Pele volcano on Io, captured on the Voyager 2 spacecraft

Besides Jupiter other forces of attraction of both Europa and Ganymede satellites have an influence on Io. Europa has the main influence by providing to Io additional heating. Unlike volcanoes on the Earth that have long “sleeping” periods and a relatively short period of eruption, volcanoes of an incandescent satellite are always active. Constantly flowing molten magma forms rivers and lakes. The largest molten lake has a diameter of twenty kilometers and it has an island of frozen sulfur.

Io’s movement through Jupiter’s magnetosphere produces powerful electricity, causing a severe thunderstorm in the upper part of the atmosphere of Jupiter. But their interaction is bad not only for Jupiter — the planet’s powerful magnetic belts take every second 1000 kilograms of substances from Io. This further strengthens Jupiter’s magnetosphere by increasing it twice as much.


Europa is the sixth satellite in terms of the distance from Jupiter. Its surface is covered with a layer of ice; scientists believe that there is a liquid ocean underneath. The moon is about four and a half billion years old — roughly the same age as that of Jupiter.

Because the moon’s surface is young (about a hundred million years), it has almost no meteorite craters which did arise in large numbers 4.5 billion years ago. Scientists had found a total of five craters on Europa’s surface with the diameters between 10–30 kilometers.



The orbital distance of Europa to Jupiter is equal to 670,900 kilometers. All of the time the satellite is turned to the planet with one side, its diameter is 3100 kilometers, therefore, Europa is smaller than the Moon, but larger than Pluto. The temperature of Europa’s surface never rises above minus 160 degrees Celsius at the equator and above minus 220 degrees Celsius at the poles.


Two models of the structure of Europa

Scientists suggest that an ocean exists deeply under the satellite surface and that life forms can be detected there. They may exist due to thermal sources near underground volcanoes same as on the Earth. The amount of water on Europa is twice as much as on our planet.

Fluctuations in the form of Europa associated with the tides, forcing it to stretch out and then become round again

The moon’s surface is covered with cracks. Many believe that it is caused by the tidal forces on the ocean beneath the surface. It is possible that the water below the ice rises higher than usually when the satellite comes closer to Jupiter. And if so, then the constant raising and lowering of the water level caused many cracks observed on the surface. Many scientists believe that the ocean under the surface sometimes erupts through the cracks (like lava from the volcano) and then freezes. Icebergs observed on the surface of the satellite can be a proof of this theory.

Europa is one of the smoothest bodies in the Solar System — it has no hills higher than a hundred meters. The atmosphere on the satellite is rarefied and consists essentially of molecular oxygen. This is probably the result of the expansion of ice into hydrogen and oxygen under the influence of solar radiation and other hard radiation. Molecular hydrogen quickly disappears from the surface of the satellite as it is relatively light and the gravity force on Europa is weak.


Ganymede is the largest satellite in the Solar System. Its diameter is equal to 5268 kilometers — more than 2 % than that of the Titan (the second largest satellite in the Solar System) and is 8 % larger than that of Mercury. If it orbited the Sun and not Jupiter it would be classified as a planet. The distance from Ganymede to Jupiter’s surface is approximately 1.07 billion kilometers. It is the only satellite in the Solar System that has its own magnetosphere.

The surface of Ganymede is divided into two groups. The first — strange stripes of ice generated by active geological processes three and a half billion years ago, that take 60 % of the surface. The second group (the remaining 40 % of the surface, respectively) — the ancient thick ice crust covered with numerous craters.




Possible internal structure of Ganymede

The heat that comes from the core and the silicate mantle allows the existence of the underground ocean. It is believed that it is located at a depth of two hundred kilometers beneath the surface, unlike the Europa satellite, which has a large ocean closer to the surface.

The atmosphere of the moon is thin and consists of oxygen, it is similar to that found on Europa. The craters on Ganymede have almost no rise and are very flat, compared with craters on other satellites. They have no central cavity that is distinctive to the craters on the Moon. This is probably due to the slow and gradual movement of the soft ice surface.


Callisto is the third largest satellite in the Solar System. Its diameter is equal to 4820 km, which is about 99 % of Mercury’s diameter, and weight — only a third of the mass of the planet. Callisto is about 4.5 billion years old, about the same age as Ganymede, Europa, Io and Jupiter itself. The satellite is removed from the planet at a distance of nearly 1.9 million kilometers (1,882,700 km). Due to the large distance from the planet it is out of the hard radiation field of the gas giant.



Callisto has one of the oldest surfaces in the Solar System — its age is about four billion years. It is covered with craters and each new impact with a meteorite did certainly fall into the already formed crater. The ancient surface has reached our days thanks to the absence of violent tectonic activity and warming of the surface of the moon since its formation.

Many scientists believe that Callisto is covered with a huge layer of ice, under which an ocean is located and the center of Callisto contains rocks and iron. The atmosphere of the moon is ratified and consists of carbon dioxide.

One of the most remarkable places on Callisto is the Valhalla crater. It consists of a bright central region of 360 kilometers in diameter around which striking concentric rings of a radius up to 1900 kilometers are located: they diverge from it like the rings that emerge from a stone thrown into water. In general, the diameter of Valhalla is about 3800 kilometers. This is the largest area formed around an impact crater in the entire Solar System. The crater’s size itself is only on the thirteenth place in the Solar System. Such a structure has arisen because of the satellite collision with a relatively large asteroid the size of 10–20 kilometers.


Valhalla — an impact basin on the Callisto satellite

Since Callisto is out of the hard radiation field of Jupiter, it is regarded as a priority object (after the Moon and Mars) for the construction of a space base. The water can be extracted from the satellites’ ice and the crew will be able to carry out a study of another Jupiter’s satellite — Europa. The flight to Callisto may take two to five years. It is assumed that the first manned mission to the satellite will not launch before 2040 and possibly even later.


The model of the internal structure of Callisto; shown: icy crust, possible water ocean and a core of rocks and ice

Inner Satellites of Jupiter

Why are they inner? The fact is that the orbits of these satellites are located very close to Jupiter and all of them are inside the orbit of Io — the closest Galilean satellite to the planet. There are only for inner satellites: Metis, Amalthea, Adrastea and Thebe.


The leading side of Amalthea (right — Jupiter, up — north). The Pan crater is visible on the upper right edge, Gaea (with bright slopes) — on the bottom. Color photo made by Voyager 1 (1979)


Amalthea, 3D model

These satellites as well as a number of yet unseen internal small moons supplement and support the weak system of Jupiter’s rings. Metis and Adrastea help maintain the base ring of Jupiter, while Amalthea and Thebe maintain their own weak outer rings.

Amalthea produces the greatest interest in this group as the surface of this satellite has a dark red color which has no analogues in the Solar System. Scientists suggest that it consists mainly of ice with inclusions of minerals and sulfur components, but this hypothesis does not explain the color of the satellite. Most likely Jupiter captured the satellite from space as the planet regularly does with comets.

Irregular Satellites of Jupiter

This group consists of small satellites whose diameter is between one and 170 kilometers. They move along elongated and strongly inclined to the equator of Jupiter orbits. Currently, there are 59 irregular satellites. Those satellites that are close to the planet move in their orbits in the direction of Jupiter’s rotation while most remote satellite move in the opposite direction.


Jupiter’s satellites orbits

Some small satellites move in almost identical orbits, it is believed that they are the remnants of the larger satellites that were destroyed by the gravitational force of Jupiter. All external satellites which have been observed by spacecraft resemble shapeless lumps. Most likely, some of them flew freely in space until they were captured by the gravitational field of Jupiter.

Rings of Jupiter

In additional to the satellites Jupiter has a system of rings. Yes, Jupiter also has rings. Moreover, all four gas giants of our Solar System have them. But unlike Saturn, which has shiny ice rings, Jupiter’s rings have an insignificant dusty structure. That is why Saturn’s rings were discovered back in 1610 by Galileo, and the minor rings of Jupiter only in 70s, when the spacecraft first visited the Jupiter system.


Image of the main ring obtained by Galileo in forward-scattered light

The ring system of Jupiter consists of four main components: halo — a thick torus of particles, a relatively bright and a very thin main ring as well as two broad and faint outer rings known as gossamer rings.

The main ring and halo consist mainly of dust from the Metis, Adrastea and perhaps a few more satellites. Halo has the shape of a donut, it width is about twenty to forty thousand kilometers, although most of its material is in the range of several hundred kilometers away from the plane of the ring. Its shape is believed to be linked to the electromagnetic forces in the Jupiter’s magnetosphere, acting on the dust particles of the ring.

The gossamer rings are thin and transparent rings as a spider web. They are called after the satellites’ material that forms them: Amalthea and Thebe. The outer edges of the main ring are outlined by Adrastea and Metis satellites.


Jupiter rings and inner moons

We say goodbye to Jupiter and its satellites and continue our journey further. In the following article we will discover the moons and rings of Saturn.