Space
Ten strange moons of the solar system
Ten strange moons of the solar system
Except for the two planets Mercury and Venus, the rest of the planets of the solar system have natural moons. Earth’s moon is a beautiful and dead world full of ancient volcanoes and countless impact craters. Today, we are more familiar with the Earth’s moon; But there are more interesting surprises in the distance. Each of the giant planets in the outer part of the solar system benefits from a huge collection of moons, many of which formed from icy material at the same time as the planet itself. Although these moons are far away from the Sun, they have various characteristics that distinguish them from each other.
Next, we take a look at some of the strangest and most exciting moons in the solar system. Some of these moons, such as Callisto from Jupiter or Mimas from Saturn, are frozen and solid and were formed billions of years ago, and strange scratches were created on them due to space bombardments. Other moons such as the shepherd moons, Pan, Atlas, and the Nereid moon of Neptune have been influenced by their neighbors throughout history. Some of these strange worlds were heated by their planet’s powerful tidal forces, causing intense phases of activity that formed moons like Miranda. These forces, which created other fascinating moons like Jupiter’s Ivy and Saturn’s icy moon Enceladus, are still active today. Enceladus’ calm outer shell hides a great secret: extraterrestrial life.
Enceladus
NASA’s Cassini probe reached Saturn in 2004. Since then, Enceladus, the small inner moon of this planet, has been in the spotlight and has become one of the most controversial worlds in the solar system. This moon owes its fame to the discovery of glaciers that are released into space from its southern hemisphere.
These glaciers are one of the evidences of the existence of liquid water under the thin icy shell of Enceladus. Of course, the strange activity of Enceladus was revealed even before Cassini thanks to the early images. These images showed a strange and bright surface and impact craters that resembled a snow cover; However, the discovery of glaciers proved that Enceladus is an active world.
Enceladus, with a diameter of 504 km, is a mixture of rock and ice, and like many of its neighbors in the Saturn system, it has remained frozen for billions of years. But the tidal forces created by the gravitational pull between Saturn and its largest moon, Dione, kept the interior of Enceladus warm and active. Therefore, this moon is one of the important targets for the search for life in the solar system.
Although most of the ice on Enceladus returns to its surface, a small portion of the ice also escapes from the moon’s weak gravity and enters Saturn’s orbit. These ices are usually distributed in Saturn’s donut-shaped ring called the E ring, which is the outermost and most diffuse ring of Saturn.
Glaciers are spreading out from fissures in Enceladus’ south pole
Callisto
Callisto is the outermost moon of the Galilean group of Jupiter’s moons and the third largest moon in the solar system, only slightly smaller than Mercury. Callisto has the largest number of impact craters; Therefore, it has gained a lot of fame. The reason for the many impact craters is Callisto’s position in the Jupiter system. Jupiter’s massive gravity deflects comets. One of the famous collisions of comets with Jupiter was the massive collision of comet Shoemaker Levi 9 in 1994.
Jupiter’s largest moons are directly in the line of fire; But Callisto’s inner neighbors were affected by geological processes, which caused craters to disappear from their surface. However, Callisto’s surface has remained unchanged since 4.5 billion years ago.
An image of Callisto captured by NASA’s Galileo spacecraft
Dactyl
243 Ida is an asteroid of the Kronis asteroid family located in the asteroid belt of the solar system. This asteroid has a small moon called Dactyl, the width of its longest axis reaches 1.6 km. Due to the weak gravity of Ida 243, it is unlikely that Dactyl’s moon was trapped by it; Rather, it can be said that Aida and Dactyl were formed together.
Ida is a member of the Cronis family of over three hundred asteroids that all share the same orbit. This group was formed one or two billion years ago during an asteroid impact. Dactyl is also a small part of the collision that entered the orbit around Ida; But here’s a problem: According to computer models, Dactyl must have been nearly destroyed by another asteroid impact; So, how could it be more than a billion years old?
According to one theory, the Kronis family is younger than it appears, and the Ida impact craters were formed by a storm of collisions during the early breakup process. According to another theory, the Dactyl has already experienced a destructive encounter; But it has returned to its orbit. This explanation can be the reason for the spherical and strange shape of this moon.
Dactyl photographed by NASA’s Galileo Orbiter
Iaptus
According to two claims of Iapetus, Saturn’s moon is included in the list of strange moons of the solar system. This moon was discovered in 1671 and one side is dimmer than the other side. The part of the hemisphere facing Saturn’s orbit is dark brown; while the other hemisphere is light gray. According to a theory explaining the color difference of this moon, the side facing Saturn is covered with dust that was spread by small meteorite impacts on other small outer moons of Saturn.
Meanwhile, the Cassini images tell a more complicated story. Most of the dark material on the surface of Iapetus originates from inside this moon and leaves behind dark streaks by the sublimation of dusty ice from the moon’s surface (solid to vapor). This process probably begins with the accumulation of dust from foreign moons.
Also, Iapetus has a mountain range 1 km high and 20 km wide at the equator, which gives it a distinctive walnut-like appearance. The origin of this mountain range is confusing. According to some theories, this mountain is a fossil from the time of the faster rotation of Iaptus, which arose in the equatorial part; While others believe that this mountain is the result of pebbles from the ancient ring system around this moon that collapsed and landed on the surface.
The image on the left shows the hemisphere facing Saturn and the image on the right shows the hemisphere behind it.
Ten strange moons of the solar system
Nereid
Nereid was the second moon discovered in the orbit of Neptune. The fame of this moon is due to its strange orbit. The distance between Nereid and Neptune varies between 1.4 million and 9.7 million kilometers. This orbit is usually common for trapped moons (asteroids and comets that are trapped by the planets’ gravity); But the unusual and large size of Nereid tells a strange story.
According to Voyager 2 findings in 1989, Triton’s moon entered Neptune’s orbit from the Kuiper belt. Triton disturbed the orbit of Neptune’s main moons and caused many of them to leave, But many astronomers also believe that Nereid could be a remnant that remained in the gravitational boundary of Neptune.
In 1949, Gerald Kuiper discovered the Nereid
Ayo
Io is the innermost moon of the four large Galilean moons, orbiting Jupiter, the largest planet in the solar system. While the three outer moons are calm, frozen worlds with a mixture of rock and ice, Io’s landscape is a harsh mix of yellows, oranges, and browns, filled with strange and variable mineral forms that are a mixture of sulfur and scattered in various forms across Io’s surface. have been Io is the most volcanic moon in the entire solar system. The strange surface of this moon was first observed during the flight of the Pioneer spacecraft in the early 1970s, But its volcanic nature was predicted just weeks before Voyager 1 reached it in 1979.
Io is caught in a gravitational tug of war between its outer neighbors and the planet Jupiter, preventing it from entering a full orbit. Small changes in Io’s distance from Jupiter (less than 0.5% in orbit) lead to enormous eddy currents that pound the moon’s interior in all directions. The rocks are heated due to friction and keep the core of this moon molten; As a result, a source of underground magma is created.
Although most of Io’s rocks are similar to Earth’s silicate rocks, they have very high melting points; As a result, they often form as hot magma oceans that flow tens of kilometers below Io’s surface. A large part of Io’s surface activity depends on sulfur rocks, which remain molten even at lower temperatures. Volcanic activity on Io has removed any icy material on Io, creating a dry, ice-free world. Meanwhile, the average temperature of Io’s surface can reach minus 160 degrees Celsius.
Image of Io from NASA’s Galileo spacecraft
Hyperion
Hyperion has a strange appearance: its surface is like a sponge or coral with dark pits and sharp grooves formed by ice and lighter rocks. However, this is not the only strange feature of Hyperion: Hyperion was the first discovered non-spherical moon and has an eccentric orbit.
Hyperion’s rotation does not coincide with its orbital period and orbits Saturn in an irregular pattern; so that its rotation axis fluctuates unpredictably. Like other moons of the solar system, Hyperion is made of water ice; But its surface is strangely dark. According to Cassini spacecraft estimates, the density of Hyperion is 55% of that of water; As a result, a large part of its interior is empty.
According to a popular theory, one of the reasons for Hyperion’s strange properties is that it was a remnant of a larger moon that was probably located between Titan and Iaptus and was destroyed by a collision with a large comet. The remaining material condensed again and formed Hyperion.
A false-color image of Hyperion captured during Cassini’s flyby of the planet on September 26, 2005
Titan
Titan, Saturn’s largest moon in the solar system, has some unique features. Titan is the only moon with a thick atmosphere. The Cassini orbiter penetrated the opaque atmosphere of this moon with radar and infrared instruments and revealed views of rivers and lakes of this moon that are unique in the entire solar system. Titan is located at a distance of 1.4 billion kilometers from the sun, and the average temperature of its surface reaches minus 179 degrees Celsius.
Most of Titan’s atmosphere consists of inert nitrogen gas. This gas is one of the main components of the earth’s air; But the foggy atmosphere and distinctive color of its clouds are formed due to the small percentage of methane. Titan’s conditions are perfect for the methane cycle (transformation of methane from gas to liquid to solid). This cycle is similar to the water cycle in the Earth’s climate.
At low temperatures, Titan’s surface freezes, and at moderate temperatures, liquid droplets fall on the surface in the form of rain, accumulate in lakes, evaporate in warmer regions, and return to the atmosphere. Additionally, Titan has variable seasons and is similar to Earth in this respect; Of course, the year of Titan is equal to 29.5 Earth years. In each titanic season, the lakes are moved from one pole to another. Due to these characteristics, Titan is one of the targets of the search for extraterrestrial life; Although most biologists find it unlikely to find life and organisms in such a harsh atmosphere and chemical conditions, they often believe that Enceladus, Titan’s blue neighbor, offers a more promising prospect.
A color view of Titan shows sunlight reflecting off the moon’s arctic seas
Ten strange moons of the solar system
Miranda
Miranda, the moon of Uranus, is one of the strangest moons in the entire solar system. Voyager images show strange patches and streaks on the surface of this moon. In some parts of Miranda, impact craters can be seen, and in some other parts, no impact craters can be seen, which shows that this part is younger and less exposed to meteorite bombardments. One of the outstanding features of Miranda is the presence of a pattern of concentric ovals that resemble a racing track; While in other parts, parallel V-shaped patterns have formed scratches.
According to one theory, Miranda is Frankenstein’s world; That is, the old lunar fragments condensed in the orbit of Uranus and formed Miranda. Astronomers believe that the previous moon was probably disintegrated due to an interplanetary collision and this event may have occurred due to a severe deviation in the axis of Uranus.
However, further research showed that such a theory is unable to describe Miranda’s diverse surface characteristics. Instead, it can be attributed to Jezromadi’s powers as the reason for Miranda’s strange appearance.
Today, Miranda is in an almost circular orbit; But in the past, its orbit was in resonance with Umberil, the larger moon of Uranus; As a result, the two moons were regularly aligned with each other, and this caused Miranda’s orbit to enter a longer ellipse and experience strong tidal forces. Tension thrust and internal heat are some of the causes of Miranda’s surface cracks and streaks.
A view of Miranda captured by NASA’s Voyager 2 probe
Mimas
The Voyager probes shocked scientists by capturing detailed images of Saturn’s moon Mimas in the 1980s. This moon is very similar to the dead star in the sci-fi movie Star Wars. A large impact crater covers one of the hemispheres of this moon, and it is exactly the same size and similar to the destructive laser plate that George Lucas mentioned; But Mimas is more than just an element of popular culture.
Mimas is Saturn’s innermost moon, orbiting closer to Saturn than Enceladus and farther than Pan and Atlas. The diameter of this moon reaches 396 km; For this reason, the smallest object in the solar system is spherical in shape.
Herschel impact crater on the surface of Mimas. The name of this impact crater is derived from the name of William Herschel, who discovered this moon in 1789.
Pan and Atlas
Saturn’s moons Pan and Atlas are the smallest moons in the solar system. Despite their small size, these moons can influence Saturn’s ring system. These small worlds are perhaps the best-known examples of shepherd moons. Shepherd moons are small moons that are located in the ring systems of giant planets. As their name suggests, these moons help the particles in the Saturn system stick together while cleaning out other particles.
Pan and Atlas with UFO shapes
Pan is the cause of the Encke Gap; A prominent resolution seen in the bright ring A; While Atlas is located outside the ring A. The most important feature of both moons is their smooth surface, which looks like a flying saucer or a walnut. Bonnie Borrati of NASA’s Jet Propulsion Experiment believes these moons are covered in tiny particles that clear the space between the rings.
Why is Jupiter not a star due to its large size?
The smallest known main-sequence star in the Milky Way is a red dwarf called EBLM J0555-57Ab, located 600 light-years from Earth. With an average radius of nearly 59,000 kilometers, this star is only slightly larger than the planet Saturn. Therefore, this red dwarf is the smallest known star that has hydrogen fusion in its core; The process that provides the star’s energy to burn until the end of its life.
In the solar system, there are two objects bigger than the mentioned star. One of them is the sun; But the other is the planet Jupiter , whose radius reaches 69,911 kilometers; But why is Jupiter a planet and not a star according to these dimensions?
The answer to the above question is simple: Jupiter does not have enough mass to support the hydrogen-to-helium fusion process. The star EBLM J0555-57Ab is nearly 85 times more massive than Jupiter. If this star was a little lighter, it would not be able to perform the hydrogen fusion process; But if the solar system had a different structure, would it be possible for the planet Jupiter to shine as a star?
Jupiter and the Sun are more similar than you might think
Jupiter may not be a star, but it has a huge influence on the solar system. The mass of this gas giant is 2.5 times the total mass of other planets in the solar system. On the other hand, Jupiter has a low density of 1.33 grams per cubic centimeter. While the density of the Earth is close to 5.51 grams per cubic centimeter, which is four times more than the density of Jupiter.
But it is interesting to point out the similarities between Jupiter and the Sun. The density of the sun is 1.41 grams per cubic centimeter. These two crimes are also very similar in composition. In terms of mass, nearly 71% of the sun is made up of hydrogen and 21% of it is made up of helium, and traces of other elements can be seen in it. On the other hand, 73% of Jupiter is made of hydrogen and 24% of it is made of helium.
Illustration of the planet Jupiter and its moon Io
For the above reasons, Jupiter is sometimes called a failed star; But again, Jupiter is unlikely to even come close to being a star. Stars and planets form in two completely different mechanisms. Stars form when a dense knot of matter in an interstellar molecular cloud collapses under its own gravity. This material begins to rotate in a process called cloud collapse. As rotation continues, more material from the surrounding cloud enters the stellar accretion disk.
With the increase in mass and as a result of gravity, the core of the baby star becomes more and more compact, which causes the temperature to increase and make it hotter. Finally, this mass becomes so compressed and hot that its core ignites and the process of thermonuclear fusion begins.
Based on our understanding of the star formation process, when a star runs out of accretion material, a full portion of its accretion disk remains. Planets form from this residue. According to astronomers, for gas giants like Jupiter, this process, called accretion, begins with small clumps of icy rocks and dust in the disk. With the rotation of these materials around the baby star, their density starts gradually and they stick to each other with the force of static electricity. Finally, these growing masses reach the size of nearly 10 times the mass of the earth; So that they can gravitationally absorb more gases from the surrounding disk.
From this stage, the gradual growth of the customer and its current mass began. The current mass of Jupiter is 318 times the mass of the Earth and 0.001 times the mass of the Sun. When a gas giant absorbs all its available matter, its growth stops. As a result, Jupiter has never even approached the mass of a star. The reason why Jupiter’s composition is similar to the Sun is not that it is a failed star; Rather, the reason for being born in the molecular gas cloud is the same as the sun.
Real failed stars
There are different groups of objects that can be classified as failed stars. These objects are called brown dwarfs and can fill the gap between gas giants and stars. The mass of brown dwarfs starts at 13 times the mass of Jupiter. These objects are heavy enough to support nuclear fusion, but this fusion is not of ordinary hydrogen but of deuterium or heavy hydrogen. Deuterium is an isotope of hydrogen that has one proton and one neutron in its nucleus instead of just one proton. The temperature and pressure of deuterium fusion is lower than the temperature and pressure of hydrogen fusion.
Since deuterium fusion occurs at lower mass, temperature and pressure, it is one of the steps to reach hydrogen fusion for stars whose accretion process continues and absorb the surrounding mass; But some objects never reach the required mass for hydrogen fusion.
Shortly after the discovery of brown dwarfs in 1995, these objects were called failed stars or ambitious planets, but numerous studies show that the formation of these objects like stars was from cloud collapse, not core accumulation; Some brown dwarfs do not even have enough mass to fuse deuterium, making them difficult to distinguish from planets.
Jupiter has exactly the lower mass limit for cloud collapse; The minimum mass required for cloud collapse is approximately equal to the mass of the planet Jupiter. As a result, if the planet Jupiter was formed from the collapse of a cloud, we could place it in the group of failed stars; But data from NASA’s Juno probe suggests that Jupiter at least once had a solid core, which is more consistent with the theory of core formation.
Modeling shows that the upper limit of planetary mass and formation by core accretion method is less than 10 times the mass of Jupiter. As a result, the planet Jupiter is not included in the group of failed stars; But by thinking about the cause of this issue, we can get a better understanding of how the universe works. In addition, the planet Jupiter has a stormy, striped and twisted appearance, and the existence of humans would probably not be possible without this gas giant.
Why do none of the moons of the solar system have rings?
We have many strange moons in our solar system. hot and cold moons; Moons with liquids and dusty moons. One lunar planet is walnut-shaped and another is potato-shaped; But among almost 300 moons that have been discovered so far, not even one of them has rings. This is really strange.
Of the eight planets in the solar system, half have rings of dust and ice that orbit their equator. It is thought that Mars once had a ring, and according to new research, even our blue planet probably had a ring similar to Saturn’s ring about 500 million years ago, which lasted for tens of millions of years.
In addition, some dwarf planets also have rings; Although astronomers have not yet been able to understand how these rings are formed. Even some asteroids have their own rings.
While investigating the concept of ringed moons outside our solar system, Mario Socercchia, an astrophysicist at the Universidad Adolfo Ibánez in Chile, and his colleagues became involved in the question of why moons in our own cosmic neighborhood lack rings. In an interview with Science Alert, he explains:
If the giant planets of the solar system have rings, and if the asteroids beyond the orbit of Jupiter and the non-Neptunian bodies also have rings, why don’t the moons of the solar system have rings? This absence is illogical considering the presence of rings in other places. As a result, it is better to investigate whether there are underlying dynamical reasons that prevent the formation of rings or their long-term stability around moons.
James Webb Space Telescope image of the rings of the planet Uranus.
We have yet to definitively discover an extrasolar moon, but in 2021 Soserkia and his colleagues hypothesized that if a moon had a large ring system, it could engineer its existence by blocking enough starlight. But the group later realized that we have yet to see any ringed moons, so the likelihood of their existence is very low.
When you’re an astronomer with a question in mind and a simulation tool at hand, there’s only one thing you can do: build models of cosmic systems and see what happens when you set them in motion.
There are many raw materials from which rings can form around the moons of the solar system. Some of these materials are dust resulting from the formation of impact craters. Some other moons emit steam or gas of their own, so there seems to be no problem with ring formation.
Considering the gravitational influence of the moon, host planet and other moons, researchers designed and tested physical N simulations and realized that due to these variables, ring formation around moons is difficult.
For example, Saturn’s moon Enceladus releases water vapor, ice particles, and gases from its glaciers in the Antarctic region with its remarkable surface activity. However, instead of forming a ring around this moon, these materials are transported into Saturn’s orbit by strong interactions with neighboring moons, feeding Saturn’s E ring.
In other words, even though the moons produce part of the raw materials necessary for the ring, their surrounding environment makes a large part of these materials available to the host planet and prevents the formation of the ring around the moons themselves.
So far, NASA has discovered 293 moons in the orbit of the planets of the solar system, most of which belong to the planets Jupiter and Saturn. Also, moons around dwarf planets and even asteroids have been discovered.
Soserkia and his team simulated the moons of a variety of solar system objects, from the Earth’s moon to the larger moons of Jupiter and Saturn, over millions of years of evolution. They sought to investigate the stability of these objects, their gravitational environment, possible ring systems, and their changes over time. The results of the investigation were contrary to the expectations of the researchers. Susarkia explains about this:
At first I expected rings to be completely unstable, which directly answered our question. However, contrary to expectation, we found that these structures have maintained their stability in many conditions. Indeed, in a previous paper we showed that isolated moons can have stable rings, but we did not predict that moons would remain stable in harsh gravitational environments despite the large number of other moons and planets that have distributed their rings. Another surprise came when we realized that these harsh environments, instead of destroying the rings, beautified them by creating structures like cracks and waves, which were just like what we see in Saturn’s rings.
Saturn’s moon Iapetus with its prominent equatorial ridge.
Some features of the moons of the solar system are signs of the past of the rings. The simulations suggest that the pebbles found orbiting Saturn’s moon Rhea could be the last remnants of a complete ring system. Also, Saturn’s moon Iaptus has a equatorial groove, which could be the remnant of a ring that fell on this moon, and in this sense, it is just like Saturn’s rings that slowly fall on this gas giant.
The findings show that the reason we do not see rings in the solar system today is that we are not in the right time and place. Solar radiation pressure, magnetic fields, internal heating, and magnetospheric plasma all contribute to the loss of once-existing lunar rings. According to Susarkia:
I believe we are unlucky to some extent; Because we started observing the universe during a period when these structures no longer exist. After doing this research, I was convinced that these rings probably existed in the past.
On the other hand, the only reason we see Saturn’s rings is because we are in the right place and time. For this reason, we see solar and lunar eclipses; Because the moon is gradually moving away from the earth and at some point it will be so far that it can no longer completely cover the sun.
The glory of Saturn’s rings.
The researchers believe that further simulations that take into account more parameters, such as beam pressure and magnetic fields, can help us understand the absence of lunar rings in more detail. We should also look more closely at the moons and look for evidence of the past, such as the craters on Iaptus.
At the same time, Suserkia and his colleagues are looking to expand their search and look for moons of rings around alien extrasolar worlds. He explains:
I wonder what mythical and epic stories we will hear from the inhabitants of other worlds about ringed moons. I mean, how will their stories and culture about the moons of the rings be different from our stories? There are infinite possibilities.
The scientists’ research has been accepted for publication in the Journal of Astronomy and Astrophysics and is available in the archive database.
Why is Jupiter not a star due to its large size?
Why doesn’t Jupiter have big and bright rings like Saturn?
Why do none of the moons of the solar system have rings?
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