Ganymede (Ganymede, Ga

ymede, Γανυμήδης) is a moon orbiting Jupiter with an orbital period of about 7 days. Ranked by distance from Jupiter from near to far, it ranks seventh among all Jupiter's moons and third among Galilean moons. It maintains a 1:2:4 orbital resonance relationship with Europa and Europa.

Ganymede is the largest moon in the solar system. Larger in diameter than Mercury and about half the mass of Mercury, Ganymede is composed mainly of silicate rocks and ice, with a distinctly layered star and an iron-rich, fluid core. It is larger than Mercury and is the only known moon in the solar system that has a magnetosphere.

Ganymede was not first discovered by Galileo. Between 400 and 360 B.C. (most likely in the summer of 364 B.C.), Gander was quoted from Jupiter in the Tang Kaiyuan Zhanjing: "If there is a small red star attached to its side", the famous astronomer Mr. Xi Zezong pointed out that Gander observed Europa, the last moon of Jupiter, in the middle of the 4th century BC.

As for the discovery of Jupiter's moons, the modern era was discovered by the Italian scientist Galilei in 1610 after the invention of the telescope in the early 17th century. It is a miracle that Gander, almost two thousand years earlier, Galileo and without a telescope, discovered the moons of Jupiter with the naked eye alone. Later, the astronomer Simon Marius named it after Ganymedes, Zeus's lover in Greek mythology. The Voyager spacecraft accurately measured the size of the satellite, and the Galileo probe discovered its subsurface ocean and magnetic field.

On March 12, 2015, NASA announced that under the ice cap of Europa, the largest moon of the solar system, there is an ocean of saltwater with more liquid water than Earth.

Between 400 BC and 360 BC (most likely in the summer of 364 BC), Gander had already discovered Gander during the Warring States period in China, more than 2,000 years before Galileo.

On January 11, 1610, Galileo Galilei observed three stars close to Jupiter, and the next night he noticed a shift in the three stars, and then he discovered a fourth star, later Ganymede. By the evening of January 15, 1610, Galileo determined that the stars were orbiting Jupiter. He claimed the right to name the satellites and had considered the "Co**ia" satellite

Sta

s), but eventually named it "Medicea".

Sta

s)，

The French astronomer Nicolas Claudie Fabry de Pereste suggested naming the moons of the Medici family separately, but his suggestion was not accepted.

Simon Marius, who originally claimed to have discovered Galileo's moons, tried to name them "Satu of Jupiter."

of Jupite

), Jupiter of Jupiter

of Jupite

, referring to Ganymede), "Jupiter's Venus" (Ve

us of Jupite

and "Mercury of Jupiter" (Me

cu

y of Jupite

But it was never adopted, and it was later suggested that it be named after Ganimedes, the god of Greek mythology and the lover of Zeus.

This nomenclature was not universally accepted for a long time, and it was not until the mid-20th century that it became widely used, and in early astronomical literature, the moon was referred to by Roman numerals (a system proposed by Galileo) and was called Jupite

III) or "Jupiter's third moon" (thi

d satellite of Jupite

）。

Later, with the discovery of Saturn's constellation of moons, a naming system based on the suggestions of Kepler and Marius began to be used to refer to Jupiter's moons. Europa is the only one of Galileo's moons to be named after a male figure.

Ganymede is the only known moon in the solar system to have a magnetosphere, which may have been created by the convective motion of an iron-rich flowing core. A small number of these magnetospheres overlap with Jupiter's larger magnetic field, creating field lines that spread outward.

Europa has a thin oxygenated atmosphere that contains atomic oxygen, oxygen and ozone, and atomic hydrogen is also a component of the atmosphere, and it has not yet been determined whether Ganymede has an ionosphere. Ganymede is composed mainly of silicate rocks and ice, with distinct stratification and an iron-rich, fluid core. Scientists speculate that 200 kilometers below the surface of Europa there is a saltwater ocean sandwiched between two layers of ice.

There are two main types of topography on the surface of Europa: the darker areas, which occupy about one-third of the total area of the stars, are densely covered with impact craters and are estimated to be 4 billion years old, and the rest are brighter, crisscrossed by a large number of trenches and ridges, and are slightly younger in geological age. The cause of the fractured geological formations in the bright area remains a mystery to this day, and it is possible that it was caused by tectonic activity caused by tidal heat.

Europa is the only known moon in the solar system that has a magnetosphere. It possesses a thin oxygenated atmosphere that contains atomic oxygen, oxygen, and ozone. Whether or not Ganymede has an ionosphere has not yet been determined.

The average density of Europa's internal structure is 1.936 g/cm³, indicating that it is composed of nearly equal amounts of rock and water, the latter mainly in the form of ice bodies, which account for 46-50% of the total mass of the satellites.

Slightly lower than Europa. In addition, there may be certain unstable ice bodies, such as ammonia ice bodies. The exact composition of the Ganymede rocks is unknown, but it is likely to be close to the L-type or LL-type common chondrites, which contain less all-iron and metallic iron than H-chondrites, and more iron oxides on Ganymede, with an abundance ratio of 1.05-1.27 for iron and silicon by mass, and 1.8 in the Sun.

The albedo on the surface of Ganymede is about 0.43. Ice water is widely present on its surface, with a specific gravity of 50-90%, which is much higher than the overall proportion. Using near-infrared spectroscopy, scientists have found intense adsorption bands of ice water in the wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 microns.

The trench structure in the bright zone may contain more ice and therefore appear brighter. In addition to water, analysis of high-resolution near-infrared and ultraviolet spectroscopy results taken by Galileo and ground-based observatories has revealed the presence of other substances, including carbon dioxide, sulfur dioxide, and possibly cyanide, bisulfate, and several organic compounds. In addition, Galileo also found magnesium sulfate, sodium sulfate and other materials on the surface of Ganymede, which may have come from the ocean beneath the surface

The surface of Europa is asymmetrical: the side in the same orbital direction is brighter than the side in the opposite orbital direction. This situation is similar to Europa, but the opposite is true of Ganymede. In addition, Europa appears to be rich in sulfur dioxide on the same orbital side. Carbon dioxide is relatively evenly distributed in both hemispheres, although its presence is not observed in the polar regions. The impact crater on Callista (except for one) is not rich in carbon dioxide, unlike Callisto. Europa's carbon dioxide may have been depleted in the past.

The stratigraphic structure of Ganymede is sufficiently differentiated, containing a core of ferrous sulfide and iron, an inner culvert made of silicates, and an outer culvert composed of ice. This structure is supported by the fact that Europa itself has a low dimensionless moment of inertia of 0.3105 ± 0.0028, as measured by Galileo in several flybys, and is in fact the smallest solid object of moment inertia in the solar system. The magnetic field inherent in Europa detected by Galileo is related to its iron-rich, fluid core. Convection of liquid iron, which has high conductivity, is the most reasonable mode to generate a magnetic field.

The thickness of the different layers within Ganymede depends on the composition of the silicate (some of which are olivine and pyroxene) and the amount of sulfur in the core. The most likely scenario is that the inner core has a radius of 700-900 km, the outer ice culvert is 800-1000 km thick, and the rest is a silicate culvert.

The density of the core is 5.5–6 g/cm³ and that of the silicate culvert is 3.4–3.6 g/cm³. Similar to the structure of the Earth's inner core, some models that generate magnetic fields require the presence of a solid core composed of pure iron within the iron-ferrous sulfide liquid core. In the case of this type of core, the radius may be up to 500 km. The temperature of the Ganymede core can be as high as 1500-1700 K and the pressure can be as high as 100 kilobar (10 billion Pa).

Europa has been found to contain the largest amount of liquid water in the solar system. By analyzing Ganymede's aurora spectrum, the Hubble telescope estimated that its ocean was as deep as 400 kilometers. There are also scientists who suspect that this may be only a small part of the ocean of Ganymede, which may have three oceans, each layered on top of each other, separated by a layer of high-pressure ice, and the bottom layer of the ocean may be in direct contact with the rocky core of Ganymede. So Europa's ocean depth may be more than 1,000 kilometers, containing a huge body of water of more than 15 billion cubic kilometers, which contains more than 30 times the amount of water on Earth.

There are two main types of terrain on the surface of Europa: dark areas that are very old and densely packed with impact craters, and bright areas that are slightly younger than their predecessors (but still very old in geological age) and full of trenches and ridges. The dark zone covers about one-third of the sphere's total area and contains clay and organic matter, possibly caused by the meteorite that hit Ganymede.

The heating mechanism that generates the groove topography is still a major problem in planetary science. The current view is that trench topography is essentially formed by tectonic activity, and that if ice volcanoes play a role in it, it is only a minor role.

In order to cause this tectonic activity, Callisto's lithosphere must be pressured sufficiently strongly, and the forces that cause this pressure may be related to tidal heat that has occurred in the past—this action may occur when Ganymede is in an unstable orbital resonance The gravitational deflection of the ice body heats the interior of the star, stresses the lithosphere, and further leads to the formation of fissures, ramparts, and grabens that replace the ancient dark areas that make up 70% of Ganymede's surface area.

The formation of the trench topography may also be related to the formation of the early inner core and the subsequent tidal thermal processes in the interior of the star, which may cause the phase transformation and thermal expansion and contraction of the ice body caused by them, which may lead to a slight expansion of Calmede by 1-6%. As the star develops further, hot water jets are squeezed from the inner core to the surface of the star, causing tectonic deformation of the lithosphere.

The heat energy generated by the radioactive decay inside the star is the most likely source of heat, and it may depend on the formation of Ganymede's subterranean ocean. Models have been shown to show that if Ganymede's orbital eccentricity had been much higher in the past than it is today, and indeed it may have been, then tidal heat could have been replaced by radioactive decay heat as the main heat source for Ganymede.