Jupiter's Moon Europa: Summary of Findings

The second moon among the Galilean moons, if viewed from Jupiter and beyond, is Europa. Its diameter is 3138 km and its size is slightly smaller than the Earth's moon. Its distance from Jupiter is 670,900 km, its density is 3.01 g/cm3.5 and it orbits Jupiter once every XNUMX days. The first photographs from the Voyager spacecraft confirmed telescopic observations that large parts of its surface are covered in water ice, raising many thoughts about the discovery of life on the moon.

atmosphere

Europe has a very thin atmosphere. The atmospheric pressure is 10-12 bar and it contains oxygen(1). Despite its thinness, it also has an ionosphere. It originates from the sun's ultraviolet radiation or from energetic particles from Jupiter's magnetosphere. It is most likely that charged particles from Jupiter's magnetosphere hit the lunar soil with great force and break up water molecules. The maximum density of the ionosphere is 10,000 electrons per cubic meter compared to 20,000-250,000 per cubic meter in Jupiter's ionosphere(2).

Surface

Europa has the smoothest surface in the solar system and is among the strongest reflectors of light. His relief is very low. No part of its terrain route exceeds the height of 1 km. The most noticeable thing are dark lines crossing it in every direction. In this sense, it reminds of the cracking of the ice in the Earth's arctic ocean and as we will see later its ice surface is indeed cracked ice(3). The surface is divided into two. One part is ice plains with a high albedo and another part with many steeps but a low albedo(4). In the Northern Hemisphere, you see reddish spots and pits, each 10 km long, which received the name lanticulae (a Latin word meaning freckles) (5). They originate from reddish ice rising to the surface. The identical size of these formations and the intervals between them, which are also of equal length, raise the possibility that the ice shell of Europe is shrinking at the same time as the water ice, which is under the ice shell, rises to the surface, while colder ice sinks upwards(5). The colors in the different areas are a combination of ice coming under the semi-melted ice rising up, filling cracks and penetrating the ground covering the surface. Another source is ions of sulfur, a product of the volcanic activity of Jupiter, transported by Jupiter's magnetosphere(3).

Makhteshim

One of the most striking things about Europa compared to other planets and moons is the paucity of craters. A total of 28 craters were counted on its surface, the largest of which is Pwyll, which is 24 km in diameter. The height of its sides is 500 meters and in the center there is a bulge with a diameter of 5 km. Since bumps in the center of craters are material that is brought up from the depths, it is likely that the meteorite that created this crater could not penetrate through the ice shell to the water layer below. Based on a built model, it is estimated that the depth of the crust at this location is at least 3-4 km(6). The small number of craters, according to the accepted approach, indicates that Europe is still geologically active and that the land surface is constantly changing. The soil as it appears in the photographs revealed that it is a small number of millions of years old (7). Beside these, there are places where craters with a diameter of hundreds of meters or less have been found (8). There are also formations that look like remnants of craters such as the Tire Macula formation which is 40 km in diameter and has at least 5-7 concentric rings around it(9). There is also evidence of a crater with a diameter of 80 km (10).

cracks (or channels)

A striking phenomenon on Europa and which distinguishes it from the rest of Galilee is the extensive system of cracks that covers large parts of its surface (Saturn's moon Enceladus resembles these features, although it is a very small body, 500 km in diameter). These are cracks in the lunar crust and their width can reach up to 30 km. The largest ones are thousands of kilometers long. They can be straight and curved and in many places cross each other. In total they cover 5-15% of the surface of the moon. Most of them have a simple strip structure and their depth is no greater than 100 meters. There is no significant return from them and therefore they are not clean ice. It is likely that what is found in them is semi-melted ice that was brought up from below to the surface(3).

The crack strips can be divided into several types:

Dark wedge-shaped bands - they are rough and dark. are found in the equatorial region and their direction is southeast. They are among the widest of all the lunar fissure bands. Their width does not decrease gradually as they move along them, but are cut off sharply. Their edges are sharp and the material filling them is relatively dark, so their peg shape is distinct. This configuration and the displacement of some of the cracks adjacent to them raises the possibility that the expansion and rotation of lumps of the crust that existed before are related to their formation. It is also possible that primary plate tectonics and strike-slip faults deformed the lunar crust and that a slight rotation of the crustal plates opened spaces into which a semi-melted crust mixed with rocky material entered from below, thus creating small strips of new crustal material.

Triple bands - cracks whose edges are dark and light and narrow bands in the center. The central strip is actually a ridge line. They usually merge with a brown streak line. These cracks are global in scope. Since in many cases they cross each other, it is possible to learn about their relative age. Another obvious conclusion is that the stress fields had different directions of action.

Usually the triangular bands disappear when they are swallowed up in brown soil. This means that they are older than the brown soil. This soil upon its formation mimics the section of triangular strips that was previously in this place, or the physical properties of a soil are different to the point of preventing the formation of cracks in that place.

Gray bands - found in the southern hemisphere of the moon and are among its widest bands. They are crossed by other formations and are considered ancient. Since they are limited to a certain area. The impression is that they are a product of local tectonics(7).

Flexi straps - a special strap configuration. These are chains of arcuate strips, arcs 100 km long each, one following the other. The name given to them is flexi. The Galileo cameras, which have a higher resolution than those of the Voyager spacecraft, allow different types of bands to be distinguished. The simplest are those that look like cracks or dents. In the high resolutions they look like double ridges. According to most of the models developed to explain the cracks in Europe as a whole, the arcuate double ridges are actually a configuration of cracks at the beginning of their formation. Other arcuate formations are extensions of the lithosphere. Examples of this can be seen in Tynia Linea. It is possible that the Astypella Linea copy fragment also began to develop as an arcuate fissure.

According to many studies these bands or cracks were formed due to tensile cracks and are closely related to daily changes (Europa day) of tides, the influence of the gravity of Jupiter and the nearby moons Io and Ganymede. The amplitude and direction of the peak tides change during the 85 hours that the Moon orbits Jupiter. The height of the ice shell during this day changes by 30 meters with the amplitude of the tides during half a day - 42.5 hours. The annular cracks develop in the following way: when the tensile strength of the ice reaches its peak, a crack forms perpendicular to the local direction of the tensile strength and it begins to spread. Since the daily tidal pressure varies, the cracks propagate along changes in the pressure field in both amplitude and direction. The expansion can follow the curving path until a tensile pressure is created in it that is not strong enough to continue the expansion and the latter stops. This process can last several hours until the moment when the pressure at the end of the crack, where the tensile stress is strengthened again. The expansion is strengthened in the direction perpendicular to the new direction of the pressure field. The shape of the crack therefore takes the shape of a sickle's point (11). Compared to known geological processes in which the various landscape formations develop over hundreds of years, these tensile cracks develop quickly. The duration of the development of an arcuate ligament is 3.5 days - the duration of Europa's orbit around Jupiter (12).

Ridges

There are many ridges on Europe and some of them cross each other. They can be double - triple or more and there are some that are parallel to each other. Between them there are dark strips and at their ends they are feather-like. The longest ones are half a moon long (13). Their formation is probably related to the strong tides that operate regularly on the moon. Tidal forces appear to cause fresh ice to rise below the surface. New, frozen ocean water is probably pushed up and above the surface, where it is slowly enriched with oxygen. Since ridges rise on top of previous ridges, older material is buried and pushes this oxygen-rich material downwards. (14) Some of the ridges were created by stretching the ice sheet. When two plates are slightly separated from each other, hot material rises from below, freezes and forms a ridge. Other ridges were probably created as a result of the compression of two plates that are pushed together with each other and the place where they meet creases and forms a ridge(15). Examples of such ridges can be found in photograph 01126PIA (16), in photograph 01180PIA one of the most impressive due to the high resolution in which it was taken, 6 meters per pixel. The feeling is that you see the surface from a bird's eye view (17), in photo 01179PIA which was made with a resolution of 26 meters per pixel and in photo 01664PIA (19).

Ice domes

There are large ice caps on the surface of Europe and it is not clear how they were formed. It is estimated that they are the product of the combination of two factors - rising to the surface of warm ice from the ice shell and small amounts of sodium chloride (NaCl) and sulfuric acid (H2SO4) (20).

Examples from the surface of Europe

1. Photo 00592PIA (21). Photographed at a resolution of 26 meters per pixel. Dimensions 16 X 11 km. You see a smooth area in the left section of the photo. Formed as a result of flooding by liquid material under the ground and covered clusters of ridges and grooves. In the photo you see several small craters and several grooves crossing each other.

2. Photo 00877PIA (22). Dimensions 675 X 675 km. Location W ° 210 S ° 40. In the upper part of the photo you see an area of ​​wedges. A place that has undergone extensive earth breaking to the south of this area. Is part of the Agenor Linea strip (which is 1000 km long) similar in length and width to hot strips in the Northern Hemisphere, except that it is brighter.

3. Photo 00294PIA (23). Dimensions 860X 700 km. In the center you can see a crater with a diameter of 30 km. The location of the crater W ° 239 N ° 2. The impact creates a crater that excavates white material. You can also see the dark strip Belus Linea that runs east-west. This form falls into the category of triple bands. The outer fringes of this and other bands raise the possibility that the dark material is the product of geysers. The X configuration on the left side of the photograph represents a crack in the ice crust and is accompanied by semi-melted ice frozen in place.

4. Photograph 01640PIA(24). Dimensions 180X 175 km. Location W ° 80 N ° 20 Resolution 235 meters per pixel. A configuration reminiscent of a boxing glove with a texture similar to chaotic terrain is seen in many places on the moon. The material inside the formation looks like frozen semi-melted ice. The formation protrudes above the surface and it interrupts bands, which means that it was created in a later period than them. There are small craters in it.

5. Photo 01182PIA (25). Dimensions 4 X 1.7 km. Location W ° 274 N ° 9. Resolution 9 meters per pixel. The place Canamara Chass sees ice plates that have broken off and shifted aside. At the bottom of the photo you can see a crack running along it.

6. Photo 01401PIA (26). Dimensions 215 X 240 km. Location W ° 179 S ° 23. Resolution 460 meters per pixel. On the side that we do not turn to justice is a very complex area. In terms of the prominent formations in it, two are dark. Each one is 20 km wide and 50 km long. They appear to have formed when the lunar crust cracked and were filled with semi-melted ice that rose to the top. At the bottom of the photo you see a crater with a diameter of 15 km. Next to it you see an area of
chaotic terrain.

surface temperature

In the 60s, spectroscopic measurements were used to determine the surface temperatures of Europa. The temperature measured near the equator was -20°C and near the poles -113°C. The conclusion they reached was that the moon has a layer of ice with the hardness of a rock (223). The measurements made by the Voyagers were not accurate because the resolutions were low, a spatial resolution of 27 kilometers per pixel. The measurements made by Galileo were more effective thanks to a higher resolution of 900-80 km per pixel. It turned out that during the day in low latitudes the temperature range is -200 - 141 °C. The temperatures in the bright areas around Pwyll Crater and at the equator after sunset do not match the surface albedo. It is possible that the origin of this anomaly is due to local temperature changes from an unknown source, probably internal(187).

Volcanic activity

No volcanic activity has been observed on Europe. However, the possibility that such activity occurred in the past cannot be ruled out. Phenomena observed in different photographs indicate different currents. These can give evidence that such activity did exist in the past. Ice flow and dark scratches in some cracks look like remnants of geysers or volcanoes(15). There can also be the activity of underwater volcanoes that send heat waves that melt the ice crust of the moon and are manifested in the form of brown spots and ice blocks surrounded by smooth planes(29). It is possible that the dark spots scattered along the length of Rhadamanthys Linea originate from ice volcanism(30). Bellus linea margins are diffuse. They are probably the product of underground activity that released gas and rock fragments from inside the moon(23).

tectonics

A panoramic view of Europe's ice sheet shows that in many places it is cracked and many processes typical of plate tectonics have acted on it(15). According to one hypothesis, the one who contributed significantly to this is Tzedek himself. Europa's orbit around Jupiter is not completely circular, it is eccentric. In parts of its movement around Jupiter it approaches it and in other parts moves away from it. The result is strong tidal forces that crack the surface and create an array of tectonic plates. In addition, the water rising from below through the cracks freezes near the surface in a short time. The cracks close and sprinkle a large amount of ice on the surface(13).

The Astypa Linea fault line is at least 800 km long. The two sides of the fault moved away from each other more than 48 km in a manner reminiscent of strike-slip faults. In some of the photographs you see a turning point in the fault line similar to the San Andreas fault in the USA (31). Two other examples that illustrate the tectonic activity are one Agenor Linea, a broad and bright strip in relation to its surroundings and within it several narrow strips. The band cuts and is cut by narrow fissures(32). The second example is in photo 00592PIA. You see groups of cracks crossing each other (strike-slip faults). In many photographs you see huge blocks of the crust that have been detached from their surroundings, turned and are sliding on thin ice, or they float like icebergs in a more liquid material(33).

Polar migration

The Voyager spacecraft spotted two strange arcuate depressions, each hundreds of kilometers long and 40 kilometers wide. The mapping of the moon was done using the set of photographs of the Voyager, Galileo and New Horizons spacecraft. These formations form two almost identical giant circles thousands of kilometers long. They are each exactly on the opposite side of the moon, offset from the equator and the Europa-Zeptius axis. The unsynchronized rotation of the ice sheet and the high daily tidal wave (30 meters amplitude) are probably the ones that give the explanation for many of Europe's fractures and ridges, but it does nothing to solve this mystery. The hypothesis that was put forward back in 1989 has something to explain these formations and the reference is to polar migration. According to this explanation, Europa's outer shell can slowly change its position, while the rocky core of the moon rotates normally. Low polar temperatures can thicken the ice crust and lead to the movement of the poles. When a correlation was found between the pole migration pressure and the concentric circles, the researchers were very surprised. It turned out that the other linear formations of the moon also correspond to the pressure of the migration of the poles(34).

Internal structure

According to Europa's density and gravity, it is estimated that the moon has a metallic core rich in iron, a silicate mantle, above it a layer of water between 80-170 km thick and above it a layer of ice. Heat generated by the tides is the source of energy that probably allows the ocean to sustain water (27,35). Another possible source of heat is the decay of radioactive elements and the release of energy stored inside, heat created in an earlier period, of greater heat(36).

magnetic field

During Galileo's passage near Europa in December 1996, the magnetometer found a significant magnetic signature and it was also found that the moon's north magnetic pole was pointing in a strange direction. It could be that the moon has its own magnetic field and its strength is ¼ of the strength of Ganymede's magnetic field(37). The direction of Jupiter's magnetic field in Europe changes every 5.5. hours, this change can affect an electric current within a conductor such as an ocean. These currents create a magnetic field similar to Earth's magnetic field, but with its north magnetic pole near the equator and is constantly in motion (38).

ocean

As mentioned earlier, the data that Galileo transmitted indicates that Europe has its own ocean of enormous dimensions. On the surface you see many cracks that break the crust into plates with an average length of 30 km each. Some of the plates were detached from their surroundings and rotated in different directions. It seems that the movement of the plates was lubricated by soft ice or liquid water from below(39). The ice shelves are similar to the ice floes on Earth in the arctic oceans when they melt in the spring. The ice sheets on Europa seem to float and resemble glaciers(40). These floating lumps were named Chaos Land(41). Further strengthening the belief that Europa has an ocean beneath the ice blocks is the fact that electric currents have been observed. Ice is not a good electrical conductor. The most likely option is salt water(38).

Sailors

Another evidence that confirms the existence of an ocean below the surface is the discovery of various minerals. In the observations of the Vigers and Galileo. For example, salt deposits were found near the cracks on the surface. Some minerals have a crystalline structure containing water molecules(42). Another salt found in various spots on the surface is magnesium sulfate (MgSO4). On Earth it is found in the layers of lakes that have dried up(43). Minerals rich in sulfur were also found. It seems that their origin is from the moon Yu as a product of the volcanic activity that occurs on its surface and that they reached Europe through the magnetosphere of Jupiter. It is possible that there is also an internal source that is transported in some form to the surface(35). On the hidden side of Jupiter, sulfur dioxide (44) and hydrogen peroxide were found. Hydrogen oxide is formed when high-energy particles from Jupiter hit the lunar surface and break down molecules present there. But from the moment they are created, they begin to break down by ultraviolet radiation or due to contact with other chemicals. Their lifespan ranges from several weeks to several months. The hydrogen oxide turns into a reactive chemical called hydroxyl that can form hydrogen and oxygen(45).

Life

All the findings indicate that Europa has a very large ocean of water below the surface. The mind gives that there is a probability of finding life on the moon. Clues to this are actually found on Earth. In recent years, on the world we live in, new life forms have been found in the depths of the oceans that can give an indication of what may be found on Europe. A place where conditions similar to those prevailing in Europe's ocean probably prevail is Lake Vostok located 3.6 km below the Antarctic ice sheet. The little sunlight that reaches Europe can reach the upper layers of water through the cracks between its ice blocks, which can enable photosynthesis, although not to the extent found on Earth and, in addition, the moon contains various chemicals that are essential for the existence of life(46).

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