Currently, a significant part of the research on the planetology of the solar system is devoted to the satellites of the giant planets. Interest in them increased at the turn of the seventies and eighties, after the very first images from the Voyager spacecraft revealed to scientists the amazing diversity and complexity of these remote worlds. One of the promising objects of study is the largest satellite of Jupiter - Ganymede.
Jupiter system in brief
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 systems - huge on Saturn and much more modest on Jupiter. Given this consideration, the largest planet in the solar system also 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, andsmall ones are about a kilometer in size. At the beginning of 2019, the number of open satellites has already reached 79.
The four largest, in addition to the planet itself, bodies in the Jupiter system were discovered in 1610 by Galileo Galilei. In honor of him, they received their collective name. The largest satellites of Jupiter bear the names of the lovers of the supreme deity of the Greco-Roman pantheon: Io, Europa, Ganymede and Callisto. They are easy to see with a small telescope or binoculars. Each of these satellites is of great interest to planetary scientists.
Io - the closest to the planet - is remarkable in that it is the most active object in the solar system. Due to the tidal influence of Jupiter, as well as Europa and Ganymede, more than four hundred volcanoes act on Io. The entire surface of the satellite, slightly larger than the Moon in diameter, is covered with emissions of sulfur and its compounds.
Europa is the second largest satellite, somewhat smaller than the Moon. It is covered with an icy crust crossed by faults and cracks. There are signs of an ocean of liquid water under this crust. Europa is one of the top candidates for finding extraterrestrial life.
The third largest moon is Ganymede. Its features will be discussed in more detail below.
Callisto is the Galilean satellite farthest 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 indicatesabout the absence of geological activity. Some models of the structure allow the existence of a liquid ocean under the surface of Callisto.
The photo below shows the largest moons of Jupiter in order of distance from it and in comparison with the size of the Earth and the Moon.
Ganymede: size 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 moon of Jupiter, but in general 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 slightly more than a million kilometers away from Jupiter, moves in an almost circular orbit, making a complete revolution in 7.15 Earth days. Ganymede's own rotation occurs in synchronism with the revolution around the planet, so that he always turns to Jupiter with the same hemisphere - just like the Moon to the Earth.
Composition and structure of the satellite
In addition to rocks and iron, Ganymede contains a large amount of water (mainly in the form of ice) with an admixture of volatile substances, such as ammonia. Spectral analysis data also indicate the presence of carbon dioxide, sulfur compounds and, probably, organic substances in the form of a mixture (so-called tholins) on its surface.
The model of Ganymede's structure 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 pronounced layers:
- iron-enriched core;
- silicate inner mantle;
- outer predominantly icy mantle;
- subsurface s alty ocean interbedded with ice;
- bark of complex composition and structure.
Images of the largest satellite of the planet Jupiter, obtained during the Voyager and especially Galileo missions, demonstrate the diversity and complex structure of the surface. Approximately a third of the area of Ganymede is occupied by dark, apparently ancient areas with a large number of craters. Lighter areas are somewhat younger, since there are significantly fewer impact formations there. They have a furrowed character, covered with many cracks and ridges.
These light wrinkled areas are believed to be the result of past tectonic activity. Probably, these processes were caused by a number of factors. First, during the gravitational differentiation of the satellite's interior and the formation of its core and other layers, heat was released and the surface was deformed. In addition, one should take into account the effect of tidal forces during the instability of orbits in the early system of Jupiter.
The largest moon of the giant planet has faint polar caps, believed to be formed by particles of water frost.
The thin atmosphere of Ganymede
With the help of the Hubble Space Telescope, an extremely rarefied gaseous envelope of molecular oxygen was discovered near Ganymede. Its presence is most likely associated with dissociationwater molecules in surface ice under the influence of cosmic radiation. In addition, atomic hydrogen has been detected in the atmosphere of Ganymede.
The concentration of particles in this faint atmosphere is on 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 carried out by the Galileo station, it turned out that the largest satellite of Jupiter has its own rather strong magnetic field. The value of its induction ranges from 720 to 1440 nT (for comparison, for the Earth it is 25–65 µT, that is, on average, 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.
The magnetic field of Ganymede forms the magnetosphere - the region within which the movement of charged particles obeys this field. This region extends for a distance of 2 to 2.5 Ganymede diameters. It interacts in a complex way with Jupiter's magnetosphere and with its extremely extended ionosphere. The poles of Ganymede occasionally show auroras.
On further research
After the Galileo apparatus, Jupiter's satellites were studied mainly through telescopes. Some amountThe images were also obtained during the flybys of the Cassini and New Horizons stations. At the beginning of the 21st century, several special space projects were supposed to be carried out to study these celestial bodies, but for a number of reasons they were closed.
Now planned missions such as EJSM (Europa Jupiter System Mission), involving the launch of several vehicles to explore Io, Europa and Ganymede, Europa Clipper, and JUICE (Jupiter Icy Moons Explorer). In the program of the latter, especially great attention is paid to the largest satellite of Jupiter.
Which of these projects will come true, time will tell. If the announced missions take place, we will learn a lot of new and exciting things about the distant worlds in the Jupiter system.