Currently, a significant part of the research on the planetology of the solar system is devoted to satellites of giant planets. Interest in them increased at the turn of the seventies and eighties, after the very first pictures from Voyager spacecraft opened up for scientists the amazing diversity and complexity of these distant worlds. One of the promising objects of study is the largest satellite of Jupiter - Ganymede.
Briefly about the Jupiter system
Speaking of satellites, as a rule, they do not take into account the difference in the number of small objects that make up the ring system - huge in Saturn and much more modest in Jupiter. With this in mind, the largest planet in the solar system has the most numerous, according to modern data, retinue.
The number of known satellites is constantly increasing. So, by 2017, it was known that Jupiter has 67 satellites, the largest of which are comparable to planets, and the smallest ones are about a kilometer in size. At the beginning of 2019, the number of open satellites reached 79.
Galilean satellites
The four largest bodies, in addition to the planet itself, in the Jupiter system were discovered in 1610 by Galileo Galilei. In honor of him, they got their collective name. The largest moons of Jupiter bear the names of the beloved supreme deities of the Greco-Roman pantheon: Io, Europe, Ganymede and Callisto. They are not difficult to see using a small telescope or binoculars. Each of these satellites is of great interest to planetologists.
Io - the closest to the planet - is remarkable for being the most active object in the solar system. Due to the tidal effects of Jupiter, as well as Europe and Ganymede, more than four hundred volcanoes act on Io. The entire surface of the satellite, slightly larger in diameter than the moon, is covered by emissions of sulfur and its compounds.
Europe is the second largest satellite, somewhat inferior to the moon in size. It is covered with ice crust, intersected by faults and cracks. There are signs of liquid water beneath this crust of the ocean. Europe is one of the main candidates for the discovery of extraterrestrial life.
The third most distant large satellite is Ganymede. Details of its features will be discussed below.
Callisto is the farthest satellite from Galilee from Jupiter. In diameter, it is very close to the planet Mercury. The surface of Callisto is extremely ancient, characterized by a huge number of impact craters, which indicates the absence of geological activity. Some structural models allow the existence of a liquid ocean below the surface of Callisto.
In the photo below - the largest moons of Jupiter in order of distance from it and in comparison with the size of the Earth and the Moon.
Ganymede: dimensions and orbit
The diameter of Ganymede is 5268 km, which is almost 400 km more than that of Mercury. It is not only the largest satellite of Jupiter, but also the largest and most massive satellite in the solar system. Ganymede is one and a half times larger and twice as massive as the moon.
The satellite is a little more than a million kilometers from Jupiter, moving in an almost circular orbit, making a complete revolution in 7.15 Earth days. Ganymede's own rotation occurs synchronously with the revolution around the planet, so that it is always turned to Jupiter in the same hemisphere - the same as the Moon to the Earth.
Composition and structure of the satellite
In addition to rock and iron, Ganymede contains a large amount of water (mainly in the form of ice) with an admixture of volatile substances, such as ammonia. The data of spectral analysis also indicate the presence on its surface of carbon dioxide, sulfur compounds and, probably, organic substances in the form of a mixture (the so-called tolins).
The structure model of Ganymede is based on the results of studying the features of its rotation and magnetic field. It is assumed that the satellite consists of the following distinct layers:
- iron-enriched core;
- silicate inner mantle;
- the outer predominantly ice mantle;
- subsurface salty ocean, interbedded by ice;
- bark of complex composition and structure.
Surface features
The images of the largest satellite of the planet Jupiter, taken during the Voyager missions and especially Galileo, demonstrate the diversity and complex structure of the surface. About a third of Ganymede Square is occupied by dark, apparently ancient sites with a large number of craters. The lighter areas are slightly younger, since there are significantly fewer shock formations. They have a furrowed nature, covered with many cracks and ridges.
It is believed that these light wrinkled areas arose as a result of tectonic activity that has occurred in the past. Probably, these processes were caused by a number of factors. First, during the gravitational differentiation of the satellite interior and the formation of its core and other layers, heat was generated and the surface was deformed. In addition, the action of tidal forces should be taken into account during instability of the orbits in the early system of Jupiter.
The largest satellite of the giant planet has weak polar caps formed by particles of water frost.
The subtle atmosphere of Ganymede
With the Hubble Space Telescope, Ganymede discovered an extremely rarefied gas shell of molecular oxygen. Its presence is most likely associated with the dissociation of water molecules in surface ice under the influence of cosmic radiation. In addition, hydrogen in atomic form was detected in the atmosphere of Ganymede.
The concentration of particles in this weak atmosphere is of the order of hundreds of millions of molecules per cubic centimeter. This means that the pressure at the surface of Ganymede can be tenths of a micropascal, which is a trillion times less than on Earth.
Magnetic field and magnetosphere
As a result of measurements made by the Galileo station, it turned out that the largest satellite of Jupiter has its own rather strong magnetic field. The magnitude of its induction ranges from 720 to 1440 nT (for comparison, it is 25â65 ΞT at the Earth, that is, an average of 40 times more). The presence of a magnetic field served as a serious argument in favor of the model according to which the iron core of Ganymede, like that of our planet, is differentiated into a solid central part and a molten shell.
Ganymede's magnetic field forms the magnetosphere - the region within which the movement of charged particles obeys this field. This area extends over a distance of 2 to 2.5 diameters of Ganymede. It interacts in a complex way with the magnetosphere of Jupiter and with its extremely long ionosphere. At the poles of Ganymede, auroras are sometimes observed.
About further research
After the Galileo spacecraft, the moons of Jupiter were studied mainly through telescopes. A certain number of images were also taken during the flights of the Cassini and New Horizons stations. At the beginning of the XXI century it was supposed to carry out several special space projects to study these celestial bodies, but for several reasons they were closed.
Missions such as the EJSM (Europa Jupiter System Mission), which includes the launch of several devices for the study of Io, Europe and Ganymede, Europa Clipper, as well as JUICE (Jupiter Icy Moons Explorer), are now planned. In the latter's program, special attention is paid to the largest satellite of Jupiter.
Which of these projects will be realized, time will tell. If the declared missions take place, we will learn a lot of new and exciting interesting things about distant worlds in the Jupiter system.