In astronomy, ice moons are natural satellites whose surface consists primarily of ice (mostly, but not exclusively, water ice ). Such a celestial body has a cryosphere that takes up the entire surface of the body and can sometimes be very voluminous.
All known ice moons are located in the outer solar system at a distance from the central star beyond the so-called ice line . When the planets and moons emerge from the protoplanetary disk, the water ice resublimates from the gas in the disk beyond this line.
Many of the moons in this outer region of the gas planets have a large proportion of water, which is in the form of ice because of the low temperatures on the surface of the moons. In their interior, however, water can exist in liquid form due to the high pressure and heat sources such as tidal forces or radioactive nuclides . These liquid bodies of water, extraterrestrial oceans , could provide the conditions for extraterrestrial life .
Ice moons can have structures similar to rock moons on their surface, such as craters, ditches and furrows. They can also have a type of "cold" volcanism known as cryovolcanism (cold or ice volcanism), as well as geysers .
In their interior, ice moons can have a differentiated structure and thereby have a subglacial ocean under the ice cover and a rock core made of silicate or metallic material. More complex shell structures are also conceivable. They can experience internal warming through tidal forces ( tidal friction ).
Some ice moons are assumed to have layers of ice modifications under their surface that only occur at high pressure ( high pressure ice ).
The space probe JIMO ( Jupiter Icy Moons Orbiter ) of NASA was planned from 2003 for the exploration of the ice moons Ganymede, Callisto and Europe. However, the mission was stopped again in 2005 for budget reasons.
ESA has been developing the JUICE ( Jupiter Icy Moon Explorer ; German Jupiter Ice Moon Explorer ) space probe for exploring all four Galilean moons since 2012 . It is scheduled to start in June 2022.
Ice moons of Jupiter
Most of the known 67 (as of September 28, 2014) moons of Jupiter consist mainly of silicate rock. However, three of the four Galilean moons are ice moons , as well as probably at least one of the smaller satellites .
Ganymede is the largest of Jupiter's moons. The analysis of the data from the Galileo space probe indicates that it is a differentiated body, the shell structure of which consists of four layers: A relatively small core made of iron or iron sulfide is surrounded by a coat of silicate rock . Above it is an approximately 800 km thick layer of soft water ice and an outer hard ice crust.
According to a new model of the crust, it would also be possible that there is a slightly salty ocean under the ice surface, in the lower area of which crystals of a denser type of ice form due to high pressure. The contained salts are released and sink to the bottom, where they form a lower, more salty part of the ocean with water. This floats on another layer made of an even denser type of ice, which in turn floats on an even more salty and therefore denser ocean that rests on Ganymede's rock mantle.
Callisto is the second largest of Jupiter's moons and has the highest density of impact craters in the entire solar system . Concentric, ring-shaped elevations created by the impacts characterize the surface; larger mountain ranges are not available. This suggests that the surface of Callistus is predominantly composed of water ice. The ice crust has given way over geological time, leveling older craters and mountain ranges.
The visible surface lies on a layer of ice that is estimated to be 200 km thick. Underneath there is probably a 10 km deep ocean of liquid salt water, as indicated by magnetic measurements from the Galileo space probe. Another indication of liquid water is the fact that on the opposite side of the Valhalla crater there are no fractures and faults visible, as can be seen on massive bodies such as the Earth's moon or the planet Mercury . A layer of liquid water apparently dampened the seismic shock waves before moving through the interior of the moon.
Europe is the fourth largest of Jupiter's moons and, with an albedo of 0.64, has one of the brightest surfaces of all known moons in the solar system. Europe's surface is made of ice. Their most noticeable feature is a network of reddish criss-cross trenches and furrows ( lineae ) that cover the entire surface. The lineae have a strong resemblance to cracks and faults on earthly ice fields. The larger ones are about 20 kilometers wide and have indistinct outer edges and an inner area made of light-colored material.
The lineae could have been created by cryovolcanism or the eruption of geysers from warm water, which pushed the ice crust apart.
These lineae are also for the most part in different places than you would expect. This can possibly be explained by the fact that there is water between the ice crust and the rock surface. This could have arisen because due to the eccentric orbit of the moon around Jupiter, its gravitational effect on Europe is constantly changing, so that it is constantly being deformed. As a result, Europe is warming up and some of the ice is melting.
Europe's surface temperature reaches a maximum of −150 ° C. The smooth surface and the structures are very reminiscent of ice fields in polar regions on earth . It is believed that under Europe's ice crust there is liquid water that is heated by tidal forces. At the very low surface temperatures, water ice is as hard as rock. The largest visible craters have apparently been filled in with fresh ice and leveled. This mechanism, as well as calculations of the warming caused by the tidal forces, suggest that Europe's crust of water ice is about 10 to 15 kilometers thick. The water layer below could be up to 100 kilometers thick. The amount of liquid water would be more than twice that of the terrestrial oceans. However, from about 3 kilometers below the surface, there could be water bubbles trapped in the ice.
Detailed recordings show that parts of the ice crust have shifted against each other and broken, creating a pattern of ice fields. The movement of the crust is caused by tidal forces that raise and lower the surface by 30 m. The ice fields should have a certain, predictable pattern due to the bound rotation. Instead, further recordings show that only the geologically most recent areas show such a pattern. Other areas deviate from this pattern with age. This can be explained by the fact that Europe's surface is moving slightly faster than its inner mantle and core. The ice crust is mechanically decoupled from the interior of the moon by the water lying in between and is influenced by Jupiter's gravitational forces . Comparisons of images from the space probes Galileo and Voyager 2 show that Europe's ice crust would have to move completely around the moon in around 10,000 years.
Amalthea is a very irregularly shaped body with a low density of only 0.86 g / cm³. This speaks for a porous structure made of water ice. It is possible that it is a "run-up" object that either comes from the outer Jupiter system or originated at a great distance from the Sun and was captured by Jupiter's attraction.
Ice moons of Saturn
All 62 (as of September 28, 2014) known moons of Saturn (whose density is known) presumably consist of a predominant proportion of water ice and silicate rock. Their density varies from about 0.5 g / cm³ (water ice with a low proportion of silicate rock) to about 2.3 g / cm³ (water ice with a high proportion of silicate rock). Since little is known of many of Saturn's 62 moons, it cannot be ruled out that there are moons among them that are not largely made of ice.
Titan is the largest of Saturn's moons. Half of its solid body consists of a mantle of water ice and the other half of a core of silicate rock. Radar measurements from the Cassini probe indicate that an ocean of liquid water exists beneath the ice crust. The thickness of the ice crust is estimated to be around 80 kilometers.
According to a model that was transferred from Jupiter's moon Europa to Titan, the heat generated by tidal friction may also have led to the formation of this molten layer under its ice crust. It would have to be detectable with the Cassini probe by means of gravitational field measurements .
Ammonia , which is about 10% in the water, would act as an anti-freeze agent (see lowering of freezing point ), so that despite the expected temperature of −20 ° C at this depth, a liquid ocean could have formed - especially in connection with the high pressure there.
From a geological point of view, the existence of an ocean in the depths means that the crust above can be much more mobile than on celestial bodies that are continuously solid, such as the Earth's moon. The crustal mobility leads to the observed large tectonic structures and also to cryovolcanism, whereby it can be assumed that water from the subterranean ocean is also directly involved in ice volcanism, as is the case with the earth with magma from the mantle. As has already been demonstrated on Enceladus , the crustal movements alone can generate so much heat locally that significant amounts of ice in the movement zones are liquefied and create cryovolcanism.
Cassini discovered that the attraction over titanium mountains is weaker than over flat areas. The researchers therefore suspect that the ice under mountains extends deeper into the ocean than under plains. The evaluation of gravity field measurements by Cassini showed that the presumed ocean must be very salty. He is about to freeze, which is why the outermost layer of ice above him should be very rigid.
Rhea , the second largest of Saturn's moons, is composed of around two thirds of water ice and a core of silicate rock. Its thin ice crust has given way over geological time. On the following hemisphere of Rhea, light streaks are visible on a dark surface as well as some impact craters. The stripes were formed by cryovolcanism at an early stage of development, when the interior of the moon was still liquid.
Iapetus is the third largest of Saturn's moons and is made up almost entirely of water ice with a small amount of silicate rock. Iapetus' surface can be divided into two distinctly different regions based on its discoloration. The leading hemisphere (called Cassini Regio ) is very dark and reddish in color with an albedo of 0.03 to 0.05. The following hemisphere (called Roncevaux Terra ) is as bright as Jupiter's moon Europa with an albedo of 0.5. The difference in brightness is so striking that Cassini reported that he could only observe the moon with his telescope on one side of Saturn. If the moon turned the dark region towards the earth, it remained invisible. Iapetus has the greatest brightness contrast of all bodies in the solar system . The bright side is icy and heavily cratered. According to the latest research, Iapetus is said to have been rotating rapidly in its youth and not yet frozen because it was heated by radioactive substances ( 26 aluminum and 60 iron ) with a relatively short half-life. The rapid rotation gave it a bulging shape. The activity of the isotopes decreased and Iapetus froze before the rotation duration increased to its present value.
Dione is the fourth largest of Saturn's moons and is mainly composed of water ice. Inside, there must be larger proportions of denser material, such as silicate rock. Research by the Cassini spacecraft suggests that Dione, similar to Enceladus, may have a layer of liquid material beneath the surface. Bends in the crust beneath the Janiculum Dorsa mountain range are evidence of internal warming in what was, for astronomical standards, more recent.
Tethys is the fifth largest of Saturn's moons and an icy celestial body, similar to the large Saturn moons Dione and Rhea. Its low density indicates that it is largely composed of water ice. The mass of rock cannot exceed 6% of the total mass of the moon. One of the noticeable features on Tethys is a vast valley, Ithaca Chasma , which is approximately 100 km wide and three to five kilometers deep. With a length of 2,000 km, it runs around three-quarters around the moon. According to one theory, it could have been formed when liquid water inside the moon froze and tore the surface as a result of expansion.
Enceladus is the sixth largest of Saturn's moons and is presumably composed mainly of water ice, which reflects 99% of the incident sunlight, making it appear extraordinarily bright. Part of its surface appears to be relatively young, with an estimated age of 100 million years. This suggests that Enceladus is geologically active. The cause is obviously cryovolcanism, in which water leaks from the interior of the moon and spreads over the surface.
Gravimetric measurements suggest that there is an ocean of water beneath the ice of the South Pole region. Cassini's flyby was used for this: The mass distribution inside the moon influences the trajectory of the probe, which can be measured using the Doppler shift of the radio signals. A region of higher density was discovered, which is interpreted as a water ocean with a depth of 10 km under 30 to 40 km of ice.
Mimas is the seventh largest of Saturn's moons and is probably composed mainly of water ice with a small amount of silicate rock. Since the density of Mimas is slightly higher than the density of water - which was the only component that could be proven spectroscopically with certainty - it is possible that Mimas is a differentiated body that has a small rock core with a thick coat of water ice. Mimas has a very high albedo of around 0.962.
In 2010, NASA published a temperature map of Mimas, according to which, contrary to expectations, the temperatures are unevenly distributed, as the warmest regions of the moon are near the morning terminator (the area where the sun was just rising) and in the two polar regions is located.
Studies of Cassini data in October 2014 suggest that the core of the moon is either misshapen and shaped like a football , or Mimas has a liquid subglacial ocean.
For its size, Hyperion is one of the most irregularly shaped bodies in the solar system. It is mainly made up of porous water ice with a small amount of silicate rock. In contrast to most of Saturn's moons, Hyperion has a dark surface with an albedo of 0.25. According to investigations by the Cassini space probe in 2005, the dark deposits are hydrocarbon compounds that contaminate the grains of water ice.
Until 2000 Phoebe was considered the outermost of Saturn's moons. With 1.63 g / cm 3 it has the second highest density of the large Saturn moons after Titan. Apart from ice, their interior must have a larger proportion of dense material, such as silicate rock. Most of Saturn's large moons have a very light surface, while Phoebe's, with a geometric albedo of 0.081, is extremely dark. Presumably (similar to Hyperion) the ice on the surface is contaminated.
Ice moons of Uranus
As with the moons of Saturn, probably all of the 27 (as of September 28, 2014) known Uranus moons consist largely of water ice. Their density varies relatively slightly from about 1.3 g / cm³ to a maximum of 1.5 g / cm³. Only three of the four largest moons, Titania , Oberon and Ariel, have a slightly higher density.
Titania is the largest moon of Uranus and has an average density of 1.71 g / cm³. The low density and the high albedo suggest that Titania is composed of around 50% water ice, 30% silicate rock and 20% carbon compounds and other heavy organic compounds. The presence of water ice is supported by infrared spectroscopic studies from 2001 to 2005 that revealed crystalline water ice on Titania's surface. This seems to be more prominent on Titania's leading hemisphere. The reason for this is unknown, but it appears to have come from the bombardment of charged particles from Uranus' magnetosphere , which is more represented in the following hemisphere due to the co-rotation of the plasma.
The water-ice-rock mixture and the possible presence of salt or ammonia - which lower the freezing point of water - indicate that an underground ocean like the one on Jupiter's moon Europa could possibly exist between the core and ice mantle of Titania. The depth of this ocean would be around 50 km in this case, the temperature would be around −83 ° C (190 K). The current internal structure depends heavily on Titania's thermal history, which is not well known.
Oberon is the second largest moon in Uranus and has an average density of 1.63 g / cm³. The low density and the albedo suggest that Oberon is composed of around 50% water ice, 30% silicate rock and 20% carbon compounds and nitrogen compounds as well as other heavy organic compounds. The ratio of water ice to rock is in line with the other larger moons of Uranus. The presence of water ice is supported by infrared spectroscopic studies that revealed crystalline water ice on Oberon's surface. This seems to be more prominent on Oberon's subsequent hemisphere; this is in great contrast to the other major moons of Uranus, which have the greater proportion on the leading hemisphere.
The current status of the ice mantle is still unclear. If the ice contains enough substances that lower the freezing point of water, such as salt or ammonia, a subterranean ocean like on Jupiter's moon Europa could possibly exist between the core and the ice mantle of Oberon. The depth of this ocean would be around 40 km in this case, the temperature would be around −93 ° C (180 K). The current internal structure depends heavily on Oberon's thermal history, which is not well known.
Umbriel is the third largest moon of Uranus and has an average density of 1.39 g / cm³. Based on the low density, it is assumed that Umbriel is composed of around 60% water ice. It also has shares of silicate rock and carbon compounds such as methane and the organic heavy tholine . The presence of water ice is supported by infrared spectroscopic studies that revealed crystalline water ice on Umbriel's surface. This seems to be stronger on Umbriel's leading hemisphere. The reason for this is unknown, but it appears to have come from the bombardment of charged particles from Uranus' magnetosphere, which is more represented in the following hemisphere due to the co-rotation of the plasma.
According to previous studies, it is unlikely that an underground ocean like that on Jupiter's moon Europa could exist in the ice mantle of Umbriel.
Ariel is the brightest and fourth largest moon of Uranus and has an average density of 1.66 g / cm³. Based on the high albedo of 0.39 and the low density, it is assumed that Ariel is composed of around 50% water ice, 30% silicate rock and 20% carbon compounds such as methane and the organic heavy tholine. The presence of water ice is supported by infrared spectroscopic studies that revealed crystalline water ice on Ariel's surface. This seems to be more prominent on Ariel's leading hemisphere. The reason for this is unknown, but it appears to have come from the bombardment of charged particles from Uranus' magnetosphere, which is more represented in the following hemisphere due to the co-rotation of the plasma.
The size, the water-ice-rock mixture, and the possible presence of salt or ammonia - which lower the freezing point of water - suggest that Ariel is a differentiated body, with a rock core and a coat of water ice. According to previous studies, it is unlikely that an underground ocean like that on Jupiter's moon Europa could exist in the ice mantle of Ariel.
Miranda is the fifth largest moon of Uranus and has an average density of 1.21 g / cm³. It is predominantly (around 80%) composed of water ice, with parts of silicate rock, and carbon compounds such as methane. Miranda has some noticeable features that are probably due to cryovolcanism or upwelling .
Ice moons of Neptune
Presumably, all of the 14 (as of September 28, 2014) known Neptune moons consist largely of water ice. Their density varies relatively slightly from about 1.2 g / cm³ to a maximum of 1.5 g / cm³. Only Triton has a slightly higher density, while Galatea has a very low density of only 0.75 g / cm³ and thus probably belongs to the Rubble Piles .
Triton presumably consists of a differentiated structure, a core of silicate rock and a crust of water ice. Research from 2012 also suggests the possibility that a thin, ammonia-rich ocean may exist beneath the surface. The energy to keep the ocean below the surface liquid at −90 ° C is said to come from the decay of radioactive substances in Triton's interior and the tidal friction that arises during the orbit around Neptune.
Nereid is the third largest moon in Neptune. Nereid appears gray in the spectrum. Spectrally, Nereid moves between the Uranus moons Titania and Umbriel, which indicates a surface composition of water ice - which was identified in 1998 by Michael E. Brown's group - and spectrally neutral material. In terms of spectrum, Nereid is more similar to Proteus than Triton and is noticeably different from the asteroids of the outer solar system, suggesting a formation in the Neptune system rather than a captured asteroid.
Trans-Neptunian ice moons
Charon ( Pluto )
Charon's mean density was determined to be 1.71 g / cm³. It should therefore consist of around 55–60% rock and 40–45% water ice; an obvious difference to Pluto, whose rock content is around 70%. There are two theories about the internal structure of Charon: either Charon is a differentiated body with a rock core and ice cover, or it consists of a uniform mixture of ice and rock. With the discovery of evidence of cryovolcanism, the first theory is favored. Charon's surface appears to be made of water ice.
Hiʻiaka and Namaka ( Haumea )
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