Ganymede is the third moon of Jupiter, if viewed from Jupiter itself. Its diameter is 5270 km and its density is 1.9 g/cmXNUMX. In terms of its diameter, it is larger than Mercury and its low density, as with Callisto, can testify to its internal composition. In terms of these basic data, the two moons are essentially twin moons. Ganymede has been imaged by the Voyager, Galileo and New Horizons spacecraft.
atmosphere
In the 90s it was discovered that Ganymede has a very thin atmosphere made of oxygen, ozone and a large corona of hydrogen atoms. Observations made with the Hubble Space Telescope in 1996 revealed a subtle ultraviolet fluorescence of oxygen atoms. The moon is doubly careful at these wavelengths at the poles and red around the equator. The fluorescent warning occurs when atoms in Jupiter's magnetosphere move along magnetic field lines toward the moon's poles. In these places, they break the oxygen molecules in the lunar atmosphere and the result is the excitation of oxygen atoms that emit ultraviolet radiation at wavelengths of 1304 and 1306 angstroms. At the same time, gas "lumps" surrounding the equator give off a fluorescent glow in the red light. An observation made in August 1999 with the Keck II telescope observed the emission of red light from oxygen atoms at a wavelength of 6300 angstroms. The explanation given for the warning at the equator is as follows: from the time the oxygen atoms are excited, they need 5-6 minutes to emit a photon in the red light, a long enough time for the atoms to be diverted into space or captured by the lunar soil (1). It turned out that the closer the spacecraft got to Ganymede, the density of charged particles around the moon was 100 times greater. This meant that Ganymede was surrounded by an ionosphere (2).
dust cloud
Around the moon is a cloud of dust grains. Interplanetary meteorites hit the moon and spray dust grains from the surface into space. The impact with the ground is so fast that it explodes and vaporizes and causes balls of fragments to be thrown from the ground at high speeds that allow them to break free from Ganymede's gravity. Similar dust clouds have also been found on the other Galilean moons. This cloud was discovered when the Galileo spacecraft was less than 10,000 km from the lunar surface. Some of the dust grains sprayed into space enter a circular orbit around Jupiter and form a thin ring that surrounds it(3).
Surface
In a global view, Ganymede's surface is divided into bright and dark areas. The photographs transmitted to Israel by the Voyager spacecraft show that there are many grooves in the bright areas. The bright areas are not always in geographic sequence. Sometimes they penetrate the dark areas in the configuration of wide or narrow strips and as a result create in them configurations of polygons. In the light areas you see alternating ridges and grooves and the latter can be straight or curved. They can be single or in clusters and sometimes they are crossed by smooth spots and coughs (swaths). In the dark areas the density of craters is higher than in the light areas, which indicates according to the accepted concept that these areas are older than the bright areas. A morphological analysis of the surface raises the possibility that the places where the grooves are located grew at the expense of the crushed areas. Areas where craters are found have become areas of grooves and that vertical tectonics and shear movements of the ground are very dominant in those places where the surface has been changed. The assumption is that during a certain period the lithosphere was thin and that the upward movement of convection currents caused cracks to appear on the surface accompanied by normal faults. In places where cracking is extensive, sections of the crust have separated from each other and cut each other. The grooves seem to reflect an early stage in Ganymede's geology, a stage in which plate tectonics of ice operated which caused cracking and drifting of the ice lithosphere and, unlike terrestrial plate tectonics, caused small vertical inversions in the bright areas. In the light areas there is relatively clean water with few silicate additions, while in the dark areas there is a mixture of ice and silica (4, 5). The bright area is richer in tectonic activity. It was found that the darkness of one of the prominent areas of Ganymede Galileo Regio originates from sediments created following the evaporation (sublimation) of volatile substances from a "dirty" water lithosphere. It is also possible that the origin of Galileo Regio is an impact crater, what could support this hypothesis is its circular shape.
Unlike Callisto's monochromatic landscape, Ganymede is multi-colored. It is also crushing, but the grooves of their kind create a fascinating mosaic of landscapes that allows making stratigraphic sections and determining the relative age of geological periods. If we take for example photo 02572PIA in the center we will find the Arbela Sulcus which is oriented north south, there are thin strips and very few craters. To the right is an older area with a large number of craters. This area is called Regio Nicholson and on the left is a massively deformed area where grooves are found (7). The area visible in photo 01062PIA whose dimensions are 68 X 54 km is dark and has a dense network of cracks, some of which have light sides(8). In the photo 01618PIA whose dimensions are 664 X 518 km you can see dark areas and bright areas next to each other. The location of the photograph is 194°N°43 ( 9 ).
Makhteshim
Ganymede has a small number of large craters and a large number of small craters. One crater with a diameter of 588 km, a crater with a diameter of 301 km, 7 craters with a diameter of 200-300 km, 40 craters with a diameter of 200-100 km and 183 craters whose diameter is less than 100 km (10). For comparison, 90 craters with a diameter of less than 100 km were counted on Callisto (11). It should be taken into account that craters smaller than 10 in diameter were not counted because they are below the resolution limit of the Voyager and Galileo spacecraft. A phenomenon common to some of the craters on Ganymede and other craters in the solar system are the rays. Material thrown from the craters during their formation and their radial distribution around them, such as the Tros crater with a diameter of 94 km, the Ishkur crater with a diameter of 70 km and the Punt Facula crater with a diameter of 155 km (12).
Craters by their nature penetrate into the material of the crust at different depths and they can be used as a good tool for revealing the composition of the material below the surface. The craters that are less than 50 km in diameter and are in the dark areas do not have deposits of light material at their edges. Larger craters that have been weathered due to the flow of viscous material are brighter than the environment in which they are found. If the bright material is not the sole result of the material that was present during the formation of the craters. It is possible that these craters brought up from the depths to the surface materials that do not contain silicates. It is also possible that at depths greater than 10 km there is water ice without silicates. The albedo at the bottom of small craters is similar to that of their surroundings, which raises the possibility that they did not expose material rich in silicates. It may be that the thickness of ground made of ice in bright areas is less than 10 km. Their craters have bright rays concentrated in bright areas (5).
The circularity of craters in Marius Regio raises the possibility that very few ground-penetrating deformations occurred in this region, while impact craters of this type were formed during most of the period in which the Moon developed. Ganymede's lithosphere was rigid and was mainly affected by extensional tectonics and deformation. These forces focused on bright areas shortly after the placement (emplacement) of the bright areas (13).
To illustrate, we will present several craters. Khensu Crater is a small crater with a diameter of 13 km located at W ° 153 N °2 in the Uruk Sulcus. Its floor is very dark, with a light spot in the center and around it a thick layer of light material, which was probably thrown out with the formation of the crater. As for the dark material covering its floor, it could be leftover material from the body that hit the moon and created this crater. Another possibility is that the offending body penetrated deep into the ground and exposed what was underneath. To the right of this crater is a larger crater named L. Its diameter is 54 km and in the center is a small depression (14). The Gula crater with a diameter of 40 km and the Achelous crater with a diameter of 35 km are located between
W ° 12.5/N°65 – N °60 and both are surrounded by material thrown from them and piled up at a moderate angle. In the crater there is a bulge in the center and in the Achelous crater there is no bulge. What is visible in the center is probably the remains of a bulge. It seems that both have terraces on their inner walls (15). In the dark area Marius Regio at W °210 – N °40 on the border of the light area Byblus Sulcus there are two craters whose western and eastern sides were completely destroyed due to tectonic activity (16).
The Voyager spacecraft photographed 56 craters in the center of which is a domed configuration. A distinction was made between two types of craters. Craters in which the ratio between the diameter of the dome and the diameter of the crater is 0.25-0.56 and craters in which the ratio is 0.05-0.19. The diameter of the large domes is 10-50 km and the diameter of the small domes is 3-10 km. 75% of the craters have a domed relief and are found in dark areas, 15% in the light areas and 10% in the transition area between the dark areas and the light areas. Several explanations have been put forward for this phenomenon. According to one explanation, two large domed formations found in craters greater than 100 km in diameter were created by water volcanism and/or isostatic uplift of a thin layer of ice. According to this hypothesis, if these domes were volcanic, liquid water could have spread through a network of cracks created during the impact that created these craters, probably from a liquid water mantle tens of kilometers below the surface. According to another explanation, water ice was raised in the center of the crater upon its formation and melted due to the high impact speed of the body creating the crater and a "molten lake" was formed in the depression in the center of the impact. It expanded as it froze and formed the dome shape. It has also been hypothesized that the domes of craters in the bright areas are smaller because the stock of the material was used to create the bright ground there. On the other hand, the domes in the dark areas that were created at the same time, are larger because the stock under the dark ground is usually not emptied by surface emplacement, unless it was used to create craters of this type(17).
An unusual crater in its structure is the Neith crater with a diameter of 160 km and its location is W °9 - N °29. What sets it apart is its fragmented sides. These segments are partly pointed and it is obvious that they have undergone any kind of weathering. In the center of the crater is a domed relief with a diameter of 45 km and at its left end is a crater with a diameter of 13 km. The dome configuration is not symmetrical. Her left side is taller than her right side. It slopes towards the right and has several grooves on it (18). A palimpsest configuration found on Callisto is Buta Facula at E°24 – N ° 12 in the Marius Regio region and has a diameter of 145 km (19).
On the face of Ganymede you find 3 chains of craters. In each such chain, a row of Makhtesham interrupting each other. One such chain is called Enki Catena and is 150 km long. A similar phenomenon is also found on Callisto. It is estimated that these are secondary creators. Rocks that are thrown from an impact crater upon its formation and hit the ground one after the other and form small craters(20). This phenomenon is also found on Earth's moon.
conclusions
In preparation for the second transit flight of the Galileo spacecraft near Ganymede, a series of calculations were made to determine the configuration of its flight path. Since the spacecraft made this transit flight, it became clear that there were several fluctuations in the orbit. This phenomenon was experienced by the Lunar Orbiter spacecraft that orbited the Earth's moon and the Apollo spacecraft from the time they entered orbit around the moon. It turned out that large concentrations of mass in places where there are large basins (craters hundreds of kilometers in diameter) caused this phenomenon. These concentrations were named Mascons (Mass Concentrations) (21). The origin of mass concentrations on Ganymede is unclear. The number of giant craters on Ganymede is very small compared to those found on Earth's moon. This phenomenon is of great importance. It will be possible to prepare gravimetric maps of Ganymede, to map its interior and to plan flight paths for future spacecraft.
grooves and tectonics
The grooves in their shape resemble grooves on the earth. Their width is between 4-6 km, their depth is 700 meters. , they can extend for hundreds and thousands of kilometers and the distance between them is between 6-7 kilometers (22). They appear as straight or curved lines. They can be single or in clusters. They can be narrow and long or wide and short. Some look like fans, beams, or pegs. There are grooves bordered by deeper grooves. In the photograph taken by Voyager 2, groups of grooves with symmetrical shapes were observed with seam lines in their centers surrounded by smooth material. There are groups of grooves that cross each other by superposition or truncation. The superposition causes the groups to be denser and allows the relative age of the formation of the grooves to be determined.
The boundary line between the grooves and the crushed areas is usually sharp and indicated by one deep groove or by a narrow swath. Since in many cases the area of the grooves crosses geologically crushed areas it is younger. In several grooves areas, there are many craters including large basins. In others there is a small number of small craters on which there is also a superposition. Which brings up two possibilities. Either the flow of impacts creating the craters changed or the area of the grooves changed during the flow of these impacts.
Smooth ground is found in small areas or swaths, but elsewhere it extends over large areas similar to the Earth's Moon. Other areas where you see low relief are near the terminator. The ridges are straight or rounded to a small extent and many have sharp boundaries, although in some places they are not clear, especially in places where they are related to grooves or ridges that enter crushed areas. Many of the smooth swaths are probably young because they have a small number of superimposed craters, craters that cut grooves (4).
The vignettes show that Ganymede's surface has been partially changed from a body with a high crater density similar to that of Callisto, to one where cratered areas have been erased and smooth grooves and plains have appeared in their place. The possibility was raised that this change resulted from the expansion of Ganymede due to internal differentiation of the lunar body and melting that caused extensive cracking and breaking of the surface and their reconstruction by a massive eruption of water. The water flooded the graben similar to lava flooding low areas (rift valleys) on Earth. The problem with this hypothesis is that some of Ganymede's ground features, which appear to have been created by lateral ground movement and partial spreading of the moon. What is appropriate for these types of developments is plate tectonics. It may be that solid-state convection currents in a shell of water ice exerted pressure at the base of a relatively thin and rigid ice lithosphere. In many places, spreading forces that acted on convection currents that rose upward were not strong enough to split segments of the crust, but created wide-ranging normal grooves. The result, the crushed area is different into furrows, grooves and ridges of different areas. Along different depressions (rifts), from pockets of water in the solid shell, different materials broke out and created smooth planes.
It is possible that the lithosphere split in places where the surface was elevated and isolated segments or plates were displaced. A new membrane is formed instead along the suture lines, but there is a lack of significant evidence of reduction
(subduction) or pressures, raises the possibility that the addition of new crust was small. The many evidences of shearing forces give rise to the thought that even if a new crust was added, its disintegration and disintegration would have been strong. Small and large sections of crushed areas and clusters of grooves have been torn, displaced and rotated, giving Ganymede its complex appearance. The result, solid-state convection currents in Ganymede's ice crust formed
Plate tectonics of ice which is different from the plate tectonics of the Earth. The plate tectonics of Ganymede creates a large-scale lateral displacement, plate displacement of the lithosphere, with little subduction and vertical turnover (4) Below are some examples of grooves:
1. In the Uruck Sulcus region there are grooves crossing older grooves. Such a wide retirement of grooves indicates points of contact where tectonic zones moved away from each other, while the surface made of ice cooled and expanded. It is estimated that tectonics played a more significant role in shaping the bright surface than water volcanism(23).
2. Nippur Sulcus at °204-51 °N a bright area where clusters of ridges and grooves cross each other. You see a cluster of grooves whose direction is northwest-southeast crossing another cluster of grooves whose direction is east-west. It is possible to determine the sequence of events that led to the construction of this area(24).
3. Erech Sulcus at ° 177- S ° 16 a cluster of short grooves whose direction is south-north cuts the dark area of Marius Regio. The grooves in the Erech Sulcus were probably formed when tectonic forces pushed up Ganymede's ice sheet. Similar clusters of fragments are found in East Africa. The southern edge of the Erech Sulcus is interrupted by the bright area of the Nippur Sulcus which runs east-west. The smooth surface of the Nippur Sulcus suggests that ice volcanism covered the area in the past(25).
4. Marius Regio at ° 157- N °7- a photograph in which you can see a boundary line between a light area and a dark area in the southern part of Marius Regio. You see a narrow strip 15 km wide of bright fragments crossing a dark area. In the dark area on both sides of the strip, you can see many fracture lines in different directions. These fault lines were created by tectonic activity(26).
5. Lagash Regio at ° 156-S °17-a narrow cluster of grooves out a dark area within the Marius Regio area. In one section of the grooves, you can see an expansion of the bright area in a fan shape (27).
6. Photo from Nicholson Regio. In the photograph you see a crater that has been cut by grooves and that part of it has been displaced from its place (28). In another photograph in this area, you see large craters and areas where there are groups of fragments whose direction is parallel to the boundary line between the dark Nicholson Regio and the light Harpagia Regio. In the bright area there are parallel and refined ridge lines created by tectonic forces(29).
The configuration of the grooves reinforces the estimate that they originate from tectonic activity that came from inside the moon. They were probably formed due to tensile pressures in the crust as a response to the increase in Ganymede's volume that came after changes in the interior of the moon in the state of water accumulation that occurred during the internal differentiation of the lunar body(30). With the geographical progress of the fractures, these depressions were filled with materials relatively free of silica that penetrated into them - liquid water, semi-melted ice or ice. The change of the surface created bands of smooth ice deposits that separated older blocks in which there is more silica(31).
Detailed geological mapping of the grooves shows 3 stages in their development. In the first stage the lithosphere is cut into polygonal blocks due to initial development in main areas of weakness. Blocks of rock that are in the path of the developing grooves undergo deformation while creating networks and areas where there are hills and steeps and create a drift (degradation) in the previous topography. In the second stage, extensive flooding of the low topographic array occurs. The polygonal blocks continue to be deformed by groups of grooves acting against the development of the main grooves. All this while creating polygons with the grooves. In the third step, the grooves are removed in a repetitive process, superimposed on the polygons and concentrated along the continuous lines of weakness(22).
Volcanic activity
Examination of the Voyager and Galileo photographs shows smooth, bright areas located 100-1000 meters below a rugged environment. The topographic data combined with photographs showing sharp gulfs and ancient formations buried beneath them, indicate that the flat areas were formed following the flooding of shallow and low troughs by lava of low-viscosity water ice. The bright areas of Ganymede originate from a continuous process of flooding, flattening, cracking the ground and creating grooves. Volcanism plays an important role in these processes and is consistent with the assessment that inside Ganymede there was a partial melting of material(32). In several photographs, you can see formations on the ground reminiscent of calderas. In one of the photographs you can see such a configuration that is 5-20 km wide. These formations probably served as a source of volcanic and bright flow of liquid water and semi-melted ice. Since there are no rims raised above the surface around this formation, it is difficult to consider it as having been created by a meteorite impact. The resulting impression is that a lava rich in water flowed out of it towards the east (33). The smooth surface in the Sippar Sulcus can indicate that this area also had ice volcanism in the past(25).
Poles
Voyager images revealed a thin layer of ice at Ganymede's poles. The thickness of the layer at each pole is a few millimeters (34). According to one explanation, the origin of this ice is the thermal migration of water vapor to high latitudes, and according to another explanation, a plasma bomb brightened the poles. From the moment the Galileo spacecraft began photographing and measuring Ganymede, it became clear that the magnetic field of this moon blocks the impact of the plasma at the equator and channels it to the polar regions. The analysis of the soil color on a global scale of the high resolution photographs taken by Galileo showed a high correlation between the boundaries of the polar ice caps and the open and closed boundary lines of the magnetic field, it became clear that there is a connection between the plasma bombardment and the brightening of the poles. High-resolution images showed that the bright areas at the poles are actually divided into bright spots and dark spots, raising the possibility of continuous reflection and distribution of cold water molecules. Small differences between the location of the boundary of the open and closed magnetic field lines and the boundary lines of the polar caps, probably originate from the interaction between Ganymede and Jupiter's magnetosphere. It is estimated that the difference in brightness between the side of the moon facing Jupiter, and the side hidden from it at low latitudes is due to the intensification of the magnetic flux towards the side facing Jupiter and is higher than the darkening on the other side of the moon(35).
On Earth, particles that reach the poles come into contact with the atmosphere and create the aurora borealis. This phenomenon was also observed in Ganymede. Jupiter's plasma particles are fragments ejected from the volcanoes of the moon Io that are accelerated by Jupiter's powerful magnetic field to speeds of 650 km/h. When they hit the ice layer of the poles, they create a spray of water molecules that fall back to the ground as thin and delicate snow that shimmers more brightly than the ice at the equator(000).
ice
It is widely believed among researchers that water-ice frosts are the cause of the brightening observed around circular formations at higher latitudes from N°57 onwards. In photo 00946PIA taken by Galileo, an area first imaged by one of the Voyager spacecraft in 1979, the sun is due south. The slopes of ridges and sides of craters facing north are brighter than those facing the sun. It is estimated that this brightness originates from the frost of the water ice that covers these surfaces(37). In photo 01058PIA the slopes of the Arctic ridges, facing east are covered with light material and the flanks facing west are dark. The reason for the brightness is probably differences in the size of the grains and differences in the composition between the original surface and the material beneath them that originates from frost deposition, or due to the effects of light(38).
ocean
Following the many findings that were broadcast to Israel, the researchers began to believe that beneath the surface of the earth there is a huge ocean made of salt water. An ocean can be a better electrical conductor than water ice. This ocean is supposed to be 192 km deep below the crust and several kilometers deep. The radioactivity of the internal rocks provides heat that supports a stable layer of liquid water between two ice layers below the surface, one at a depth of 144 km and the other at a depth of 192 km. Support for this hypothesis is found in high-resolution photographs from Galileo, which suggest that muddy water or ice reached the ground through cracks in the crust and flattened the surface between separated areas of the crust (39).
Internal structure
Following the discovery of Ganymede's magnetic field, it became clear that the interior of the moon is divided into 3: core, mantle and ice shell. Estimates regarding the size of the nucleus range from 400-1280 km and it may be pure iron or an alloy of iron and iron sulphide. It is still not known which estimate is correct, so the estimates regarding the mass of the nucleus range between 1.4% of Ganymede's mass or 2/3 of Ganymede's mass. The nucleus is covered by a silicate mantle and above it is a layer of ice 800 km thick (40).
magnetic field
The strength of Ganymede's magnetic field is 1/40 that of the Earth(41) and the obvious question is what is its origin? It is not enough that the nucleus has iron. For there to be a magnetic field, the nucleus must be molten and in motion. Since its formation billions of years ago Ganymede was supposed to cool and apparently it didn't. According to one estimate the nucleus was continuously regenerated by a pulse of heat. If, for example, Ganymede during its movement around Jupiter is in resonance with the moon Europa, tidal forces can melt its interior. With the re-solidification of the nucleus, slow internal agitation releases heat towards the surface and at the same time a dynamo is formed (42). According to another estimate in the distant past, Ganymede was closer to Jupiter than it is today and over time wandered outward until it settled into its current orbit. In this orbit, strong tidal forces are applied to it that distort its shape and the core remains molten(41). The magnetic field creates a bubble around the moon that is twice its diameter. On the surface this magnetic field is larger than its counterparts on Mercury, Venus and Mars and it behaves like a magnetic rod inclined at an angle of 10° from the spin axis (42). The magnetic field is strong enough to hold Jupiter's magnetosphere above the equator, where the magnetic field lines form closed loops. Above the poles, the field lines spread out and merge with Jupiter's magnetic field lines, creating an open passage that allows high-energy electrons and ions to reach the ground of the lunar poles(43). The two magnetospheres of Jupiter and Ganymede work inside each other while interacting with each other.
hydrogen escape
During a flyby by Galileo near Ganymede in 1996, the ultraviolet spectrometer detected a large emission of hydrogen from the Moon into space. If the source of the hydrogen is in water molecules, then the oxygen atoms are trapped within Ganymede's ice crust or floating above it. The hydrogen escape corresponds to the traces of oxygen observed at the beginning of 1996 by the Hubble telescope(42).
geological mapping
A geological map for Ganymede was drawn using the set of photographs of the Vigers and Galileo. A project that took 7 years. This map allows a better understanding of the geological processes that shaped the lunar surface and the factors that led to the formation of these processes (44).
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One response
Very interesting!
If Ganymede does have an underground ocean of salty water, there is also a good chance of life...