Ganymede is also called Callistoəˈlɪstoʊ/、Greek : ώ is a satellite orbiting JupiterGalileo Galilei It was first discovered in 1610.Ganymede is the third largest satellite in the solar systemJupiterThe second largest satellite, second only toGanymede。
Ganymede is also called Callistoəˈlɪstoʊ/、Greek : ώ is a satellite orbiting JupiterGalileo Galilei It was first discovered in 1610.Ganymede is the third largest satellite in the solar systemJupiterThe second largest satellite, second only toGanymede。Ganymede has a diameter ofMercuryThe diameter is 99%, but the mass is only one third of it.The orbit of the satellite is fourGalileo satellitemiddle distanceJupiterThe farthest is about 1.88 million kilometers.Ganymede is not like the three stars in the inner layerGalileo satellite（Io、EuropaandGanymede）So inTrack resonanceState, so there is no obviousTidal heateffect.Ganymede belongs toSynchronous rotation Satellites always face Jupiter in the same direction.Ganymede's surface is affected by Jupiter due to its far orbitmagnetic fieldThe influence of is less than that of inner satellites.
Ganymede is composed of nearly the same amount of rock and water, with an average density of about 1.83g/cm.Astronomers know that Ganymede's surface materials includeice、carbon dioxide、silicateAnd various organics.GalileoThe detection results of Ganymede indicate that there may be a smallsilicateAt the same time, there may be an underground ocean composed of liquid water 100 kilometers below its surface[1]。
Ganymede
Because Ganymede may haveoceanSo there may be living things on the satellite, but the probability is less than that of another nearby satelliteEuropa。MultipleSpace probeThe satellite has been explored, includingPioneer 10 、Pioneer 11 、Galileo Jupiter probeandCassini。For a long time, people have believed that Ganymede is the setting of further explorationJupiterThe best location for the system base.[2]
Ganymede's surface was once hit violently, and its geological age is very old.Since there is no evidence of any plate movement, earthquake or volcanic eruption and other geological activities on Ganymede, astronomers believe that its geological characteristics are mainly caused by meteorite impact.The main geological features of Ganymede include multi ring structure, various forms of impact craters, impact crater chainsSteep cliff, ridges and sedimentary topography.After careful investigation by astronomers, it was found that the surface topography of the satellite was changeable, including the small and bright ice deposits on the top of the uplifted terrain, and the surrounding areas with gentle edges (composed of darker materials).Astronomers believe that this terrain is caused by the sublimation of small geological structures. Small impact craters have generally disappeared. Many lumpy terrain is left behind traces. The exact age of this terrain has not yet been determined.
Ganymede has a very thin atmosphere, mainly composed ofcarbon dioxideThe composition may also include oxygen. In addition, Ganymede has an activethe ionosphere。Scientists believe that Ganymede is caused by the disk of gas and dust around Jupiteraccretion Formed slowly by action.Due to the slow formation process of Ganymede and the lack of tidal heat effect, its internal structure did not undergo rapid differentiation.The thermal convection in Ganymede began shortly after its formation, which led to the partial differentiation of the internal structure. The underground ocean and small rocky cores located 100 to 150 kilometers deep on the surface may be formed as a result.
Discovery and naming
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Italian astronomerGalileoIn January 1610, Ganymede and three other large moons of Jupiter were discovered（Io、EuropaandGanymede）。Ganymede's name comes from Callisto, one of Zeus's lovers in Greek mythologyArtemis, the goddess of the moonClosely relatedNymph (Sometimes also considered aslycaon Daughter).Simon Marius The name was put forward shortly after the star was discovered, and Marius thought it wasJohannes Kepler Recommendations.However, astronomers did not welcome this name for a long time, and it was not widely adopted until the middle of the 20th century.In many early astronomical literaturesRoman numeralsTo call this satellite (the system isGalileoThe proposed) is called Jupiter IV or the fourth satellite of Jupiter.[3]
physical characteristics
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Constituents
GanymedeImpact craterPlainNear infrared lightSpectra.The average density of Ganymede is 1.83g/cm ³, which indicates that Ganymede is composed of nearly the same amount of rock and ice water. In addition, there may be some unstable ice bodies, such as ammonia ice bodies.The specific gravity of ice body is between 49-55%.The exact composition of Ganymede's rocks is unknown, but it is likely to be close to the L-type or LL typeOrdinary chondrite, these two types of meteoritesH Chondrite, contains less total iron and metallic iron, andIron oxideMore.On Ganymede, the abundance ratio of iron to silicon in terms of mass is 1.05-1.27, while in the sun it is 1.8.
The surface of Ganymede is asymmetric: its side in the same orbital direction is darker than its side in the opposite orbital direction, which is just the opposite of that of other Galileo satellites.In addition, it seems to be rich incarbon dioxide, and its side in the same track direction contains moresulfur dioxide。Many of Ganymede's younger impact craters are rich in carbon dioxide.In general, the composition of the material on the surface of Ganymede, especially in the dark area, is very close to that of the D-type asteroid, whose surface is made of carbon based materials.
The albedo of Ganymede's surface is 0.2.It is speculated that the material composition of its surface is roughly the same as that of its whole.Utilize nearinfrared spectrumScientists have found strong absorption of ice water in the wavelength range of 1.04, 1.25, 1.5, 2.0 and 3.0 μ m.Ice bodies are common on the surface of Ganymede, with a proportion of 25-50%.yesGalileoAnd ground-based observation stationsnear infrared spectroscopyAfter analyzing with ultraviolet spectrum photos, scientists found a variety of water-insoluble substances, such as magnesium and iron containing hydrated silicate, carbon dioxide, sulfur dioxide, and possibly ammonia and a variety of organic compounds.[Spectral analysis data show that even in a very small area, the material composition on the surface of the star is extremely mixed. Small, bright patches made of ice are mixed with patches made of rock and ice mixture, while the vast dark area is made of non ice material.
internal structure
The internal structure of Ganymede.
The internal structure of Ganymede.Below the surface of Ganymede, which has been heavily bombarded, is a layer of cold, hard ice lithosphere with a thickness of 80-150 kilometers.A study of the magnetic field surrounding Jupiter and its moons shows that there is a salt water ocean 50-200 kilometers below the crust of Ganymede: scientists found thatmagnetic fieldGanymede in is like an ideal conductive sphere, that is, the magnetic field cannot penetrate the core of the satellite, which means that there is a layer of high conductivity liquid at least 10km thick in the star.The sea may also contain a small amount of ammonia or other antifreeze substances, the proportion of which reaches 5%, thus preventing the freezing of the sea.In this case, the thickness of the ocean will reach 250-300km.If there is no ocean, its icy lithosphere will be thicker, perhaps up to 300 kilometers thick.
The interior of the stars under the lithosphere and the hypothetical ocean may not be a homogeneous whole or a completely differentiated type.GalileoDetection data (especially measured in close flyoverDimensionless moment of inertia——Its value is 0.3549 ± 0.0042), which indicates that its interior is composed of compressed rock and ice body, and due to the partial deposition of composition, the proportion of rock gradually increases with the increase of depth.That is to say, the internal structure of Ganymede is only partially layered.At this density and moment of inertia, there may be a small silicate core in the center of the star.The radius of such cores cannot exceed 600km, and their density may be between 3.1-3.g/cm ³.
Impact crater plain
The geological age of Ganymede's surface is very old, and it is also one of the most heavily bombarded objects in the solar system. [31] The density of its impact crater is close to saturation: any new impact crater may cover the old one.The large geological structure on Ganymede is relatively simple: there are no large mountains, volcanoes or other endogenous structural features.Impact craters and multi ring structures, as well as fractures, cliffs and sedimentary topography are among the few large geological structures found on the surface of the star.
The surface of Ganymede can be divided into several different geological units: impact crater plain, brighter plain, bright and gentle plain, and various terrain units composed of multi ring institutions and impact pits.The impact crater plain covers most of the surface of Ganymede, which is a typical representative of the ancient lithosphere. Its composition is a mixture of ice and rock.In the brighter plain, there are bright impact craters, remnants of ancient impact craters known as metamorphic structures, and the central part of the polycyclic structure. Scientists suspect that this terrain is formed by the deposition of ice impact craters.The bright and gentle plain covers a small area, often appearing inWallHalaIn the ridge and trench zones of the impact crater and the Asgard impact crater, the isolated spots in the impact crater plain also belong to this terrain.The formation of this terrain was initially considered to be related to endogenous geological activities, butGalileoThe returned high-resolution photos show that this bright and gentle plain terrain is related to the fracture and tumor like terrain, and there is no sign that the surface of stars has been covered for many times.Galileo's photos also show that Ganymede's small dark flat area covers less than 10000 square kilometers and is surrounded by the surrounding terrain.This terrain may beIce volcanoSedimentary topography.The geological age of these relatively bright or gentle plains is slightly younger than that of the impact crater plain.
Hal impact crater and its central arch structure
The diameter of the impact crater on Ganymede ranges from 100 meters - the maximum resolution of the probe image - to more than 100 kilometers, while the multi ring structure is not included.Smaller impact craters with a diameter of less than 5 km have a simple bowl or flat bottom structure.The impact crater with a diameter of 5-40 km has a central peak.Many impact craters with a diameter of 25-100 kilometers, such as TingDelTindr crater, in which the central mountain peak is replaced by the collapsed terrain.The arch structure may exist in the center of the large impact crater with a diameter of more than 60 km, which may be caused by the tectonic uplift after the impact event.A few bright impact craters with a diameter of more than 100 km have distinctive arched structures.These impact craters are shallower than similar structures on the moon, which may be transitional terrain to multi ring mechanism.[4]
Polycyclic basin
The impact crater plain on Ganymede.
The largest impact terrain on Ganymede isPolycyclic basin。Two of them are huge, and the Volhalla impact crater is the largest of them. Its bright central zone has a diameter of 600 kilometers, while the annular structure continues to extend outward for 1800 kilometers.The second largest multi ring structure is the Asgard impact crater, with a diameter of about 1600 km.The multi ring structure may be caused by concentric ring fractures caused by soft or flowing materials such as the lithosphere above the ocean after the impact event.The crater chain is a long chain of craters distributed in a straight line on the surface of the star. It may be that Ganymede was too close to Jupiter and was disintegrated by gravitational tideCelestial impactIt may be formed after small angle impact.The former case is obtainedComet Shoemaker Levy 9Confirmation of the impact event.
As mentioned above, there is also a patch terrain composed of pure ice with albedo up to 0.8 on Ganymede, surrounded by darker materials.GalileoThe high-resolution photos of, show that these brighter patches are mainly located on the uplifted terrain: such as impact crater rim, cliff, ridge and nodular terrain.This patch may be a thin layer of frost deposit.Dark materials are usually located in the surrounding low and flat terrain, such as the low-lying area between the bottom of the impact crater and the impact crater. They cover the original frost deposits, so the area appears dark, forming dark spots with a diameter of more than 5 kilometers.
Degenerate feature
At the level of several kilometers, compared with the surface of other Galileo satellites, Ganymede's surface topography shows more degradation characteristics.For example, compared with the dark area of other satellites, such as Ganymede, the surface of Ganymede lacks impact craters with a diameter of less than 1 km, and is replaced by ubiquitous small tumor like terrain and craters.Nodular terrain is considered to be the remnants of crater margin formed by the impact crater through a hitherto unknown degradation process. This degradation may be caused by the slow sublimation of ice body - when Ganymede moves toSundown pointWhen the temperature on the sunny side reaches above 165K, the ice body will sublimate: the bedrock will cause the decomposition of the dirty ice on it, which will sublimate the ice water and other volatile substances.The non ice residues in the debris collapsed and fell from the slope at the edge of the impact crater.This kind of collapse often occurs near and inside the impact crater, and is called "debris aprons".
Hal impact crater and its central arch structure.
In addition, the edges of some impact craters are cut by some winding canyon like cuts (they are called gullies), which look a bit like the canyons on the surface of Mars.In the ice body sublimation hypothesis, dark matter in low-lying areas is interpreted as an overburden mainly composed of non ice matter from the edge of the degenerated impact crater, which covers most of the ice body bedrock on the surface of Callisto.
Through the density of impact craters covered by each geological unit, people can infer their relative ages of collapsed terrain and nodular terrain: the greater the distribution density of impact craters, the greater the relative age of this geological unit.However, their absolute ages are still uncertain. However, according to theoretical prediction, the geological age of the impact crater plain is believed to be 4.5 billion years long, almost dating back to the formation of the solar system.The geological age of the multi ring structure and the impact crater depends on the density of the impact crater in the area where they are located, and the estimated ages from this range from 1 billion to 4 billion years.
Atmosphere and ionosphere
Inductive magnetic field around Ganymede
The induced magnetic field around Ganymede has a very thin atmosphere, mainly composed ofcarbon dioxideComposition.GalileoThe Near Infrared Mapping Spectrometer (NIMS) at 4.2 μ m measured the absorption characteristics of the atmosphere, thus confirming its existence.It is estimated that the surface pressure is 7.5 × 10 ⁻⁻ ² bar, and the particle density is 4 × 10 ⁸ cm ⁻ ³.This layer of atmosphere is so thin that it only takes four days for its constituent substances to escape, so the atmosphere must be continuously supplemented. The source of the supplement may be dry ice sublimated from the icy crust of the star, which is also consistent with the ice sublimation formation hypothesis of the nodular terrain on the bright surface of the star[5]。
Ganymede's ionosphere isGalileoIt was found for the first time in several flyovers, and its high electron density is 7-17 × 10 ≮ cm ⁻ ³, which is similar to that in the atmospherecarbon dioxideThe effect of photoionization is inconsistent.So someone predicted that Ganymede's atmosphere should be composed of oxygen (10 to 100 times the content of carbon dioxide), but oxygen has not been detected in the atmosphere.
Collapsed terrain and nodular terrain
track
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Ganymede (lower left corner)JupiterandEuropa(Located at the lower left of Jupiter's Great Red Spot).Ganymede is the farthest from JupiterGalileo satelliteIts orbit is about 1.88 million kilometers away from Jupiter (26.3 times the radius of Jupiter - 71398 kilometers), which is much farther than Ganymede's orbit radius - 1.07 million kilometers - which is the second closest to Jupiter.Because the track radius is large, it is not in the track resonance state, and may never be in this state.
Ganymede does not participate in orbital resonance, which means that it will never produce an obvious tidal heat effect, which is an important driving force for the differentiation and development of the internal structure of stars.Because it is far away from Jupiter, its surface comes from the magnetic field of JupiterCharged particle flowWeak - 300 times weaker than the charged particle flow on Europa's surface.Therefore, compared with other Galilean satellites, the photoosmosis effect of charged particles on the surface of Ganymede is weaker.
Like most satellites, Ganymede is a synchronous rotation satellite, that is, Ganymede's rotation period is equal to itsRevolution period, about 16.7 Earth days.hisEccentricity of trackIt is very small, and the orbital inclination is also very small, close to the equator of Jupiter. At the same time, the eccentricity and inclination of the orbit will be affected by the gravitational perturbation of the sun and Jupiter in the form of periodic functions during the period of hundreds of years.The variation range is 0.0072-0.0076 and 0.20-0.60 ° respectively.The change of this kind of track makes the inclination of its rotating shaft vary from 0.4 to 1.6 °.
Origin and evolution
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Relevant interpretation
Partial layering of Ganymede's internal structure (the conclusion is made byDimensionless moment of inertiaIt indicates that the star was never sufficiently heated to partially melt its ice.Therefore, the most likely formation model is the slow accretion process in the low-density Jupiter sub nebula.This long lasting accretion process makes the star finally cool, but cannot maintain the heat accumulated in the accretion process, radioactive element decay process and star contraction process, thus blocking the process of ice melting and rapid differentiation.Its formation stage takes about 100000 to 10 million years.
The further evolution of Ganymede depends on the competition between the heat generation mechanism of radioactive decay and the cooling mechanism of heat conduction near the surface of the star, and whether the interior of the star is in a solid or sub solid convection state.The specific motion of sub solid convection of ice body is the biggest uncertainty factor in all ice satellite models.Based on the influence of temperature on ice viscosity, when the temperature is close toYu BingThe sub solid convection will occur at the melting point.In sub solid convectionMovement speedVery slow, about 1 cm/year, but in the long run, the sub solid convection is actually a very effective cooling mechanism.In the cold and hard surface layer of Ganymede, known as the "seal lid", heat conduction does not take the form of convection;In the ice body below this layer, heat is conducted in the form of sub solid convection.For Ganymede, the outer conductive layer is the cold and hard lithosphere with a thickness of about 100 kilometers.Its existence explains why there is no evidence of endogenous tectonic activity on the surface of Ganymede.In the interior of Ganymede, thermal convection may be hierarchical, because under high pressure, ice water will have a variety of crystal phases, from the first state of ice on the surface of the star to the seventh state of ice in the center of the star.In the early days, the operation of the sub solid convection mechanism inside Ganymede prevented the large-scale melting of ice body, which would lead to the differentiation of the interior of the star, thus forming a large rock core and ice mantle.Also due toConvectionThe partial differentiation of ice and rock has lasted for billions of years and is still in progress slowly.
About Current Interpretation
The current point of view for explaining the formation of Ganymede considers that there may be an underground ocean under its surface, and its formation is related to the abnormal melting point of the first crystal phase of the ice body - its melting point decreases with the increase of pressure. When the pressure reaches 2070 bar, the melting point can be as low as 251K.In all the realistic models of Ganymede, the temperature of the stratum at the depth of 100-200 km is very close, or even slightly exceeds the melting point of this anomaly.The presence of a small amount of ammonia, with a specific gravity of about 1-2%, can increase the possibility of the existence of liquid at this depth, because ammonia can further reduce the melting point of ice body.
Can be used as a basic reference object
Although Ganymede andGanymedeVery similar, but the geological history of the former is relatively simple.The surface of the star was basically shaped before the impact event and other external forces.Adjacent satellite with trench structureGanymedeIn contrast, there are few signs of tectonic activity found on Callisto.This relatively simple geological historyplanetScientists are of great significance. They can use this star as a good basic reference object to compare other more complex stars.
probe
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The artistic imagination of the future human base on Ganymede [40] in the 1970s,Pioneer 10 andPioneer 11 Approaching Jupiter successively, I obtained a small amount of new information about Ganymede.The real breakthrough came from 1979-1980Voyager 1andVoyager 2Inspection.They photographed more than half the surface of Ganymede, with an image resolution of 1-2 kilometers. At the same time, they accurately measured the surface temperature, quality and size of Ganymede.The second survey was carried out from 1994 to 2003, whenGalileoEight close range flybys of Ganymede, the last one in 2001, when Galileo was in orbit C30, only 138 kilometers from the surface of Ganymede.Galileo completed the global mapping of Ganymede's surface and sent back a large number of photos of specific areas with a resolution of 15 meters.In 2000,CassiniGo toSaturnOn the way, four Galileo satellites, including Ganymede, were detected with high-precision infrared spectra.From February to March 2007,New Horizon detectorGo toPlutoOn the way, we passed Ganymede and photographed and analyzed its spectrum.
NASAandEuropean Space AgencyOne of the cooperation aims to detectJupiter satelliteThe "Europa Jupiter Plan", a plan of.In February 2009, NASA and the European Space Agency confirmed that the plan will take precedence over“titan-Project Saturn was implemented.However, the funding for the ESA program still faces competition from other ESA programs.The "Europa Jupiter Program" includes the "Jupiter Europa Orbiter" hosted by NASA and the "Jupiter Ganymede Orbiter" hosted by the European Space Agency, and may also includeJapan Aerospace Research and Development AgencyHosted by the "Jupiter Magnetic Field Probe".
Possibility of life
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Just like Ganymede and Ganymede, some people think that alien life may exist in the salt water ocean below Ganymede's surface.However, compared with Europa and Ganymede, the environment on Ganymede is relatively harsh, mainly because of the lack of accessible rock materials and low heat flux from the core of stars.Scientist Terrence Johansen discussed the possibility of existence of life on Ganymede and other Galileo satellites in this way: "The basic materials that constitute life - what we call" pre life materials "-celestial bodies , such ascomet, asteroids and ice satellites are very rich.Biologists believe that liquid water and heat are essential to support life, so it is very exciting to find liquid water on another celestial body.However, for Ganymede, at least at this stage, heat is a big problem. The ocean above it is mainly heated by the decay of radioactive elements, while Europa, which is relatively close to Jupiter, can generate more heat by gravitational tides. "
Spanish scientists provided a new explanation for the oceans inside Ganymede in the British journal Nature published on the 26th.Its calculation shows that Ganymede's temperature and pressure conditions make the ice layer on its surface less heat-transfer, prevent the internal heat loss, and the heat preservation effect is better than previously thought.
According to calculations, Ganymede may have a huge ocean 20 kilometers deep 150 kilometers below its surface.Jupiter's other two big moons Europa and Ganymede may also have underground oceans for similar reasons.
Ganymede is the second largest moon of Jupiter, with a diameter of 4800 kilometers.In 1998,Galileo detectorThe data sent back shows that Ganymede's magnetic field fluctuates with Jupiter's rotation.The most likely explanation for this is that Ganymede has a large saline ocean under its surface.Salt containing water can conduct electricity, causing the interaction between Ganymede's magnetic field and Jupiter's magnetic field to produce such waves.The new results of Spanish scientists provide further support for the above conjecture.
People usually think that where there is water, there may be life.But scientists say the deep ocean inside Ganymede is full of dense ice and rocks.These ice and rocks prevent the flow of heat energy, making the possibility of life on Ganymede very slim
Based on the above considerations and other scientific observations, Europa is considered to be the object most likely to have life in Galileo's satellite.
Colonial plan
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Artistic Imagination of the Future Human Base on Ganymede
In 2003, NASA carried out a conceptual research called Human Outer Planets Exploration (HOPE) for the future exploration of the outer solar system.In detailed consideration, the target will be Ganymede.Scientists believe that it is possible to establish a base on the surface of Ganymede, so as to provide fuel support for the exploration of deeper space in the solar system.
The advantage of building a base on Ganymede lies in its low radiation (Ganymede is far from Jupiter) and geological stability.At the same time, it can also provide convenient support for further exploration of Europa, which is also set to go farther in the Jupiter systemOuter solar systemAn ideal location for a space shuttle repair station - after leaving Ganymede, the spacecraft can get gravity boost by flying close to Jupiter.
