American astronomer Asaph Hall was born on October 15, 1829. In 1877, he made his most important discovery: during the closest approach of Earth and Mars, he discovered two satellites of the latter - Deimos and Phobos.
In honor of the scientist, we found several interesting facts about the objects he discovered.
1. Deimos and Phobos always face the same side towards Mars. This is due to the fact that they are so-called synchronous satellites: the period of revolution of each of them coincides with the corresponding period of revolution around Mars. In this regard, Deimos and Phobos are similar to the Moon, the far side of which is also never visible from the surface of the Earth.
2. One day Deimos will fall to Mars, scientists believe. This is due to the fact that the movement of this satellite slows down due to the tidal influence of the planet. It has been proven that every hundred years Phobos becomes 9 cm closer to Mars, and in about 11 million years it will collapse on its surface. However, thanks to the same processes, Phobos may simply collapse in 7.6 million years.
3. Unlike the Moon and other satellites of the Solar System, Deimos and Phobos have a distinctly irregular shape, and in appearance they look more like a pair of cobblestones. More strictly speaking, their shape is close to a triaxial ellipsoid.
4. The sizes of Deimos and Phobos are very small. For comparison, the radius of the Moon is 158 times the radius of Phobos and approximately 290 times the radius of Deimos. The latter, until the 21st century, was considered the tiniest satellite of the solar system. The same applies to the distance from the “host”: the Moon is located at a distance of 384 thousand km from the Earth, Deimos and Phobos are 23 and 9 thousand km from Mars, respectively.
5. The names of the satellites were not chosen by chance: in ancient Greek mythology, Phobos (“fear”) and Deimos (“horror”) were gods who accompanied the god of war Ares in battles. In Roman mythology, Mars took the place of Ares. Thus, Phobos and Deimos were also satellites of Mars in ancient beliefs.
6. There is practically no attraction on Phobos, or rather, there is practically no gravity on the “Martian” side. This is caused primarily by the proximity of the satellite to the surface of Mars and the strong gravity from the planet. In other parts of the satellite, the gravitational force varies.
7. The issue of the appearance of the satellites of Mars remains the subject of heated debate to this day. The unusual shape of Deimos and Phobos and some other features make popular the version of Mars capturing two asteroids and turning them into satellites. However, the difference in their structure from the objects of the group of asteroids of which they could be part speaks against this version. According to one hypothesis, Deimos and Phobos may be parts of a once split apart single satellite.
8. Some similarity of Deimos and Phobos with asteroids, as well as their close location to the surface of Mars, will help conquerors of interplanetary space in the future colonization of space. It is on Martian satellites that they will probably test means of asteroid colonization after Mars itself has been relatively developed.
9. Even before the official discovery in 1877, there were speculations about two satellites of Mars. An interesting theory was put forward by Johannes Kepler in 1610: looking at the Moon and Jupiter, of which four satellites were known at that time, Kepler suggested that the number of satellites of planets increases exponentially with distance from the Sun. Thus, Mars must have had two. Writers Voltaire and Jonathan Swift also spoke about two companions. By the way, the only two objects on Deimos (the Swift and Voltaire craters) that have their own names are named after them.
“...In addition, they discovered two small stars, or two satellites, orbiting around Mars, of which the inner one is 3 times its diameter from the center of the planet, and the outer one is 5; the first rotates in space in 10 hours, and the second in 21.5 in such a way that the ratio of the squares of these periods is very close to the ratio of the cubes of their distances from the center of Mars; this was convincing evidence for them of the manifestation of the same law of gravity that governs movement around other massive bodies.”
There are many mysteries associated with Mars, and one of them is contained in this phrase from Jonathan Swift's novel about the adventures of Gulliver. One hundred and fifty years before the discovery of the satellites of Mars, an English writer managed to predict their existence!
The most surprising detail of this prediction is the short 10-hour orbital period of the inner satellite. It is significantly shorter than the 42-hour period for Io, the fastest of the 10 moons known at Swift's time, and at the same time roughly corresponds to Phobos's true 8-hour orbital period. Swift was not as clairvoyant as it seems at first glance. The choice of distance values of three and five planetary diameters coincides very closely with the distance from Jupiter to its moons Io and Europa. However, it is more difficult to explain why Swift predicted a 10-hour period for the first satellite. Even if we take the system of Jupiter’s satellites as a model for the spatial arrangement of the satellites of Mars, the periods cannot be derived by simple analogy. If Mars had the same density as the Earth, then the first satellite at a distance of three planetary diameters should orbit in about a day; if the density were the same as that of the planets of the Jupiter group, then the orbital period should be close to two days. A passage from Newton's Principia states that "the smaller planets, other things being equal, have a much greater density." The diameter of Jupiter is approximately 22 times larger than the diameter of Mars. If we accept Mars' density to be 22 times that of Jupiter (which now seems an absurdly high value), then the inner satellite should have a 10-hour period. Swift correctly applied Kepler's third law, but it seems that he had help from a professional.
By the way, Swift was not the only great writer of the 18th century who
"discovered" the satellites of Mars. Francois Marie Voltaire - master of thoughts of a brilliant century
Enlightenment, writing in 1752. I also mentioned the fantastic story “Micromegas”
"two moons of Mars." But in passing, without the details that Swift listed,
the only “proof” is this consideration: one moon would be
not enough to illuminate a planet so far from the Sun at night! (He says: “... travelers would have seen this planet Mars have two of its moons, which were not discovered by our astronomers. I am sure that Father Castel will argue against the existence of these two moons, and even quite wittily, but I agree with those who argue by analogy. The best philosophers know how difficult it would be for Mars to have less than two moons, since he is next from the Sun").
Even earlier, Fontenelle, in his Discourses on the Plurality of Worlds, mentioned that Mars may have satellites. There, the student in a dispute gives the following arguments: “Nature has given so many moons to Saturn and Jupiter - this is a kind of proof that Mars cannot lack moons.”
The intuition that Mars has two moons can be found in the writings of Johannes Kepler, who repeatedly argued on principles based on harmony or analogy. In a letter to Galileo, Kepler wrote: “I am so far from doubting the discovery of the four planets surrounding Jupiter that I passionately desire to have a telescope so that I can, if possible, precede you in the discovery of two orbiting Mars (the number meets the requirements of proportionality), six or eight around Saturn and probably one nearby
Mercury and Venus." However, before the genuine, and not “sci-fi” discovery of the satellites of Mars, humanity had to wait until 1877, which became truly “Martian”. Giovanni Schiaparelli at this time literally brought the entire astronomical world to its feet, reporting the existence of “channels” and “seas” on the Red Planet. This “Martian fever” also had an objective basis: 1877 was the year of the great confrontation, in which Mars and Earth came very close to each other. An experienced astronomer could not neglect such favorable conditions. Asaph Hall (1829-1907), who had already earned considerable prestige as one of the best observers and calculators at the Harvard Observatory and professor of mathematics at the Naval Observatory (Washington). August 12, 1877 In the evening, Hall looked through the 26-inch telescope of the M. Observatory and saw an object that he called the “Martian Star”. A week later, Hall was able to verify that this “star” was in fact a satellite of Mars, and, in addition, he discovered a second Martian satellite (August 17). From Earth, Phobos and Deimos are visible only through a large telescope as very faint points of light near the bright Martian disk. (It is possible to photograph them using a ground-based telescope only by covering the image of bright Mars with a special mask.)
Having learned about the discovery from the newspapers, one English schoolgirl suggested Hall names for new celestial bodies: the god of war in ancient myths is always accompanied by his offspring - Fear and Horror, so let the innermost of the satellites be called Phobos, and external Deimos, for this is how these words sound in ancient Greek. The names turned out to be successful and stuck forever.
Phobos orbits Mars at a distance of 9,400 km from the center of the planet, and its speed of revolution is so high that it completes one revolution in a third of a Martian day, overtaking the daily rotation of the planet. Because of this, Phobos rises in the west and sinks below the horizon in the east. Deimos behaves more familiar to us. Its distance from the center of the planet is more than 23 thousand kilometers, and it takes almost a day more to complete one revolution than Phobos.
The latest determinations of the orbits of Phobos and Deimos were published in the works of Sinclair (1972), Shore (1975) and Born and Duxbury (1975). The first two works are based on ground-based observations, the third on photo-television filming from Mariner 9. All three definitions have comparable accuracy, and ephemeris based on them make it possible to predict satellite positions with an error of 50 to 100 km.
Until more accurate data on the satellites of Mars were obtained, scientists tried to determine the mass of Phobos, erroneously assuming that the reason for the slowdown was its braking in the Martian atmosphere. However, the first results discouraged astronomers: it turned out that, despite its large size, the satellite was very light. The famous astrophysicist Joseph Samuilovich Shklovsky (1962) noted that atmospheric braking would be sufficient at a very low density of Phobos, and in connection with this he put forward a bold and unexpected hypothesis, according to which the satellites of Mars ... are empty inside and, therefore, are of artificial origin. Confirmation by Shklovsky was not confirmed, but it stimulated research into other possible reasons for the secular acceleration of Phobos. One of them may be tides caused in the Martian crust by the gravity of the satellite. Solar radiation pressure can also cause a noticeable effect (Vinogradova and Radzievsky, 1965).
This point of view had to be abandoned after space probes transmitted images of the Martian moons to earth. In 1969, the same year when people landed on the Moon, the American automatic interplanetary station Mariner 7 transmitted to Earth a photograph in which Phobos accidentally appeared, and it was clearly visible against the background of the disk of Mars. Moreover, there was a noticeable shadow in the photo
Phobos on the surface of Mars, and this shadow was not round, but elongated!
More than two years later, Phobos and Deimos were specially photographed by the Mariner 9 station. Not only were television films with good resolution obtained, but also the first results of observations using an infrared radiometer and an ultraviolet spectrometer. Mariner 9 approached the satellites at a distance of 5,000 km, so the images showed objects with a diameter of several hundred meters.
Indeed, it turned out that the shape of Phobos and Deimos is extremely far from the correct sphere. Both satellites look like oblong potatoes. Phobos has dimensions of 28*20*18 km. Deimos is smaller, its dimensions are 16*12*10 km. Telemetric space technology has made it possible to clarify the dimensions of these celestial bodies, which will no longer undergo significant changes. According to the latest data, the semi-major axis of Phobos is 13.5 km, and that of Deimos is 7.5 km, while the minor axis is 9.4 and 5.5 km, respectively. They consist of the same dark rock, similar to the substance of some meteorites and asteroids. The surface of the satellites of Mars turned out to be extremely rugged: they are almost all dotted with ridges and meteorite craters, obviously of impact origin. Probably, the fall of meteorites onto a surface unprotected by the atmosphere, which lasted for an extremely long time, could lead to such furrowing.
Nomenclature for the names of craters on Phobos and Deimos
The largest crater on Phobos is called Stickney(in honor of astronomer Hall's wife Angelina Stickney-Hall). Its dimensions are comparable to the size of the satellite itself. The impact that led to the appearance of such a crater must have literally shaken Phobos. The same event likely caused the formation of a system of mysterious parallel grooves near the Stickney crater. They can be traced over distances of up to 30 km in length and have a width of 100-200 m with a depth of 10-20 m.
Another feature of Phobos's topography is of interest. We are talking about some mysterious furrows, as if made by a plowman, unknown, but very careful. Moreover, although they cover more than half of the satellite’s surface, all such “ridges” are concentrated only in one area of Phobos in its northern part.
The furrows stretch for tens of kilometers, their width in different areas varies from 100 to 200 m, and their depth also varies in different places. How were these grooves formed? Some scientists blamed the gravity of Mars, which could distort the face of the satellite with such wrinkles. But it is known that in the initial era of its existence, Phobos was further from its central body than it is now. Only about one billion years ago, gradually approaching Mars, did it begin to really feel its tidal force. Therefore, the grooves could not have appeared earlier, and this contradicts data according to which the age of the grooves is much older, perhaps 3 billion years. In addition, the gravitational influence of Mars on Phobos continues today, which means that very fresh grooves should exist on its surface, but they are not there.
Other scientists believed that the grooves were made on the surface of the satellite by rock fragments ejected from some as yet unknown large crater. But not all scientists agreed with this. Some experts consider another hypothesis more plausible, according to which at first there was a single large proto-moon of Mars.
Then this “parent” of both “brothers” - Phobos and Deimos - split into two current satellites, and the furrows are traces of such a cataclysm.
An analysis of photographs sent to Earth by the Viking 2 orbital compartment, in which the surfaces of the Martian satellites are painted in dark colors, showed that such coloring is most often characteristic of rocks containing a lot of carbonaceous substances. But in those relatively close regions of the solar system where the orbit of Mars lies from its
satellites, carbonaceous substances are not formed in abundant quantities. This means Phobos and Deimos are most likely “aliens” and not “natives”. If they really formed somewhere in a relatively distant corner of the solar system, then by the time they were captured by the gravitational field of the Red Planet, they, apparently, were a single body, which then split into several fragments. Some of these fragments fell on the surface of Mars, some went into space, and two fragments became satellites of the planet.
However, we should also listen to opponents who reject the emergence of the satellites of Mars by capturing a previously independent body and breaking it apart.
The leading cosmogonist, Academician O.Yu. Schmidt, at one time developed a hypothesis for the formation of the Solar System, according to which the planets arose through the accretion (sticking together) of solid and gaseous particles that originally made up a protoplanetary cloud. Soviet followers of O.Yu. Schmidt believe that the satellites of the planets were formed in a similar way. A significant confirmation of their correctness is a detailed mathematical model showing exactly how such processes can occur. These researchers consider the capture of especially large celestial bodies by planets to be a very unlikely event.
The craters on Phobos and Deimos are almost equal in size to the satellites themselves. This means that the collisions were catastrophic events for them. The shape of the satellites is very irregular: it cannot be called anything other than clastic. Therefore, Phobos and Deimos, in principle, can be fragments of a once existing larger body. It was even possible to estimate the approximate dimensions of this body. If its radius reached approximately 400 km, then the “bombardment” of meteorites would not lead to its destruction, and bodies around Mars today would be not ten to fifteen in size, but hundreds of kilometers in size.
There is another hypothesis related to the asteroid belt. It is possible that in ancient times some asteroid flew into the atmosphere of Mars, was slowed down by it and turned into its satellite. However, the Martian atmosphere would have to be very dense for this to happen.
Proponents of contradictory hypotheses for the origin of Mars' satellites have weighty arguments, and it is only a matter of time to decide which of them is right.
One of the most important discoveries of the space age is the confirmation of the existence of the solar wind. These are powerful streams of charged particles emitted by the Sun. They rush through outer space at supersonic speed, falling on everything that comes their way. And only those celestial bodies that, like our Earth, have a sufficiently strong magnetic field, which serves as a strong shield from such a magnetic flux, are not fully exposed to the influence of the solar wind.
Soviet interplanetary stations “Mars-2” and “Mars-3” launched in 1971-1972. conducted observations of how the solar wind interacts with the Red Planet. The stations sent information to Earth, according to which the solar wind does not reach the surface of Mars, but encounters an obstacle and begins to flow around the planet from all sides. This flow began either closer to Mars or further from it (depending on the strength of the “attacking” particles and the resistance of the planet’s “defending” magnetic field), but on average the distance from the center of the planet was about
4800 km. Further studies showed that in a certain region of near-Martian space the accumulation of ions is more than ten times less than in others. And the energy spectrum of these charged particles is completely different.
The strange area did not stay in one place. When her movements were examined, it turned out that she was moving along with Deimos, all the time hiding behind his back at a distance of about 20,000 km. Soviet astrophysicist A.V. Bogdanov suggested that, obviously, there is a strong release of gases from the surface of Mars that interact with the space surrounding it. When Deimos passes directly between Mars and the Sun, the area where the solar wind collides with the Martian magnetosphere moves away from the planet, as if the "defending" side, having received reinforcements, can drive off the "attackers", and the size of the Martian magnetosphere becomes significantly larger. But until now it was believed that small bodies of our solar system, such as, for example, asteroids or small satellites of planets like Deimos, were powerless to influence the powerful flow of solar wind.
Another oddity that researchers of Mars’ satellites noticed: large craters, the diameter of which exceeds 500 m, are found on Deimos about as often as on Phobos. But there are very few small craters with which Phobos is simply strewn, there are very few on Deimos. The fact is that the surface of Deimos is strewn with finely crushed stones and dust, and small craters are filled to the edges, so the surface of Deimos looks smoother. The question arises: why doesn’t anyone, figuratively speaking, fill up the pits on Phobos? There is a hypothesis that Phobos and Deimos are subject to powerful meteorite bombardment - after all, they do not have an atmosphere that would serve as a reliable shield. When a meteorite body hits the surface of Phobos, the resulting dust and small stones mostly fly away from its surface: the strong gravity of the relatively nearby Mars “takes” them away from the satellite.
But Deimos is located much further from the planet, so meteorite stones and dust thrown out when falling on its surface largely hang in Deimos’ orbit. Returning to its previous point in the orbit, the “Horror” gradually again collects fragments and dust, they settle on its surface and bury many fresh craters above them, and primarily those that are smaller.
The upper, loose layer of the Moon, Mars, and its satellites, that part of their surface that corresponds to soil on Earth, is called regolith. It can now be considered established that the regolith of the Martian moons is similar to what is observed on our “earthly” Moon. In fact, the presence of regolith on Phobos and Deimos surprised scientists at first. After all, the second cosmic speed, upon reaching which any object goes into interplanetary space, on such small celestial bodies is only about 10 m/s. Therefore, when a meteorite hits, any cobblestone here becomes a “space projectile.”
Detailed photographs of Deimos have revealed an as yet unexplained fact: it turns out that some of the crater shafts and approximately ten-meter stone blocks scattered across the surface of Deimos are decorated with a long train. These plumes look like a rather long strip, formed as if by fine-grained material thrown out from the depths. There is something similar on Mars, but it seems that these stripes look a little different there. In any case, specialists again have something to puzzle over...
In 1945, astronomer B.P. Sharpless became convinced that Phobos had a secular acceleration in its motion around Mars. And this meant that the satellite was moving faster and faster in a very, very gentle spiral, gradually slowing down and getting closer
approaching the surface of the planet. Sharpless's calculations showed that if nothing changes, then in just 15 million years Phobos will fall to Mars and die.
But then the space age came, and the problems of astronomy became closer to humanity. The general public learned about the processes of braking of artificial satellites in the Earth's atmosphere. Well, since Mars also has an atmosphere, albeit a very rarefied one, could it not, through its friction, cause a secular acceleration of Phobos? In 1959, I.S. Shklovsky
performed the appropriate calculations and came to a conclusion that caused ferment both in the minds of scientists and in the minds of the general public. The secular acceleration that we observe in the rarefied upper atmosphere of Mars can only be explained if we assume that Phobos has a very low density, so low that it would not allow the satellite to fall into pieces if it were hollow. As befits a scientist, I.S. Shklovsky did not make any categorical statements; he himself considered the question he posed to be a “very radical and not entirely ordinary” assumption.
In 1973, Leningrad scientist V.A. Shor and his colleagues at the Institute of Theoretical Astronomy of the USSR Academy of Sciences completed the processing of over five thousand comprehensive data collected over almost a century since the discovery of Phobos and Deimos. It turned out that Phobos is still accelerating. True, much weaker than Sharpless thought.
And since there is acceleration, we can predict the fate of Phobos: in no more than 100 million years it will get so close to Mars, cross the disastrous Roche limit and be torn apart by tidal forces. Some of the debris from the satellite will fall on Mars, and some will probably appear to our descendants in the form of a beautiful ring, similar to the one for which Saturn is now famous.
As for Deimos, no one has any doubts: it does not possess secular acceleration.
Both satellites experience strong tidal influence from Mars, so they always face the same side towards it. Phobos and Deimos move in almost circular orbits lying in the plane of the planet's equator.
Does Mars have any other satellites that are hitherto unknown? This question was posed by J.P. Kuiper, director of the Lunar and Planetary Observatory at Arizona State University. In order to answer this question, he developed a special photographic technique that allows him to capture even very faintly luminous objects. All his research did not lead to the discovery of a new satellite of Mars.
Then the search for the unknown satellite of Mars was carried out by J.B. Polak, an employee of the NASA Ames Research Center in California. His research was also unsuccessful. So we can still assume that only Fear and Horror accompany the heavenly incarnation of the god of war.
Some researchers believe that the satellites of Mars came to him “not of their own free will,” but were captured from the asteroid belt. As you can see, the god of war is not dangerous for the Earth, but is harsh with his entourage.
The Phobos-Grunt space mission is being prepared in Russia - a repeat attempt after Phobos-2 to land a spacecraft on the surface of a Martian satellite, study its rocks and - something that was previously unthinkable - deliver the extracted samples to Earth. The estimated time for the entire operation is 2005-2008.
Material used:
1. Encyclopedia “Astronomy. Comprehension of the Universe, mysteries of stars and galaxies,
space and life."
2. Physics of the planet Mars. V.I.Moroz.
3. Satellites of Mars. Edited by P. Seidelman (Director Nautical Naval Office. U.S.
Theories of the origin and nature of the satellites of Mars, the history of their research were accompanied by hoaxes, rumors and legends. Phobos and Deimos were also called alien ships. And scientists have suggested that the number of satellites of the Earth’s twin could have been different.
Martian moons Phobos and DeimosPrediction of Mars satellites
The idea that Mars has two accompanying bodies was first proposed by the German astrophysicist Johannes Kepler in 1611. He assumed this based on references to observations of a triple planet in the writings of Galileo Galilei. This entry was later interpreted as a description of Saturn with its famous rings.
Despite the incorrect premises, Kepler's assumption turned out to be true. To perpetuate the memory of the astronomer, craters on Mars and the Moon are named after him.
Calculations of the physical parameters of the satellites of Mars, even before their discovery, were given by Johnath Swift in 1726 in the book Gulliver's Travels, based only on his imagination. 8 craters on the surface of Phobos are named after the characters in this work.
Asaph Hall discovered the moons of Mars
In the 16th century, all developed observatories in Europe were puzzled by the search for the Moons of a neighboring planet. But Deimos and Phobos were officially discovered on August 11 and 17, 1877, respectively, in the United States by Asaph Hall. We managed to do this during the period of maximum rapprochement between our planet and its neighbor, which occurs every 15 years.
How did the moons of Mars get their name?
Phobos and Deimos received their official names in February 1878 after a long search for successful names. He corresponded with many representatives of the scientific world, and among the many proposed names, he chose the option of Henry George Madan. He, in turn, took the name from Homer's Iliad. According to another version, these names were suggested by an unknown schoolgirl from Great Britain.
Among the planets of the solar system, asteroids, stars and other cosmic bodies, many names are taken from ancient mythology. The red planet, due to its aggressive surface color, was symbolically named after the ancient Roman god of bloody war. Mars, or, as the Greeks called him, Ares. He was considered one of the most revered gods, despite the fact that he symbolized cruelty, strength, and the desire for war for the sake of pleasure. It is logical that the satellites of the planet Mars were named after the children of the god of war of Greek mythology.
According to the beliefs of the Greeks, Ares's children accompanied him and Aphrodite in all battles. Therefore, it is obvious why the satellites today are called Phobos and Deimos, that is, fear and horror.
How many moons does Mars have?
How many moons Mars had remained unknown until 1877. Despite the mathematical calculations of astrologers, suggesting the presence of bodies in the orbit of its neighbor, it was believed that the planet had no natural satellites at all, since they could not be detected. The discovery of these objects became a sensation in the scientific world.
It has now been proven that the planet Mars has two natural satellites called Phobos and Deimos. The first is located in the inner orbit of the planet at an average distance of 9380 km, the second is two and a half times further.
Distance from Mars to its moonsTo the question “ How many natural satellites does Mars have?“We can say with confidence that Mars has 2 satellites, which were discovered by the American astronomer Asaph Hall. This happened on August 12, 1877 during the great confrontation, although their existence was suggested by Kepler back in the 17th century.
According to Greek mythology, the god of war Mars had two sons: Phobos (“Fear”) and Deimos (“Horror”), who always accompanied him. This is exactly what the natural satellites of the red planet were named. They are irregularly shaped and very small in size, both having circular orbits. Scientists believe that Phobos and Deimos are asteroids that were pulled by the gravity of Mars hundreds of millions of years ago. Let's get to know these “terrible and terrible” planets better.
Phobos.
The dimensions of the Phobos satellite are only 9.6 x 10.7 x 13.5 km; it was first photographed in 1969 against the backdrop of Mars. The period of rotation around Mars is 7 hours 39 minutes at a distance of 6 thousand km. During a Martian day, the rising and setting of Phobos can be observed 3 times, and it rises in the west, not in the east.
Interestingly, the acceleration of gravity on this satellite is 5 mm/s 2 , while the average density is less than two g/cm 3. As a result, if we were on Phobos, we would weigh less than 100 g. and, having jumped, could easily fly into space.
Images of this satellite showed that its surface is very uneven, there are many craters on it, the largest of which is as much as 8 km in diameter, and this is despite the small size of Phobos. The last major crater on Phobos is believed to have formed more than a million years ago. A mystery for astronomers are the furrows, several tens of kilometers long, 100-200 m wide and 20-90 m deep. Their origin is still unknown.
Deimos.
The second satellite of Mars, Deimos, is almost 2 times smaller than Phobos, its dimensions are 5.5x6.0x6.5 km. It flies over Mars at a distance of 20,000 km, and its rotation period is 30 hours and almost 18 minutes. Deimos always faces one side towards the main planet. In general, “Terror” is the smallest of all the natural satellites of the solar system. Deimos and Phobos have a major axis directed toward the center of Mars.
Deimos has an irregular shape due to its very low gravity. It consists mainly of rocks, which are rich in carbon, and ice. The surface of Deimos, like Phobos, is covered with darkened minerals similar to terrestrial carbonaceous chondrites or basalts. This satellite of Mars has no atmosphere and also has a lot of craters, but the size is significantly smaller compared to Phobos.
Deimos is covered in a thick layer of dust, so its surface from space looks much smoother than Phobos. And when examined closely, astronomers noted that Deimos was subjected to less bombardment by meteorites compared to Phobos. The last large crater was formed, according to scientists, 100 million years ago.
Now you know exactly how many natural satellites Mars has, and what they are, these mysterious “Fear” and “Horror”.
Phobos is likely to be destroyed by the tidal forces of Mars within the next 100 million years. Credits: HiRISE, MRO, LPL (U. Arizona), NASA.
Mars and Earth have several similarities. Like Earth, Mars is a terrestrial planet (that is, this planet consists of silicate rocks and minerals). It also has polar ice caps, a tilted axis, and in the past likely had oceans filled with liquid water. In addition, Mars and Earth are the only terrestrial planets that have natural satellites.
In fact, Mars has two satellites - Phobos and Deimos (they are named after the Greek gods of horror and terror, respectively). Deimos is smaller than the Moon, and its orbit is located at a greater distance from the planet. This satellite has the characteristics of an asteroid, which may be an argument in determining its place of origin.
Deimos was discovered in 1877 by American astronomer Asaph Hall, who purposefully searched for Martian moons while at the United States Naval Observatory (USNO). The name of the satellite was given by Henry Madan shortly after its discovery.
Since Deimos is not a round body, its average radius lies between 6 and 6.38 km. (3.73-3.96 miles) or about 15 x 12.2 x 11 kilometers (9.32 x 7.58 x 6.835 miles), making it almost half the size of Phobos. Due to its small mass - about 1.4762 * 10 15 kilograms, the surface gravity of Deimos is weak (the acceleration of gravity is only 0.0039 m/s 2).
The orbit of Deimos is close to circular, the perihelion is located at 23455.5 km, and the apogee is at 23470.9 km, so the average distance is about 23463.2 km. With an average orbital speed of 1.3513 km/s, Deimos takes 30 hours, 18 minutes and 43.2 seconds to complete one revolution around Mars.
Both Deimos and Phobos are composed of silicate rocks rich in carbon, which makes them similar to main belt asteroids. Although the moon's surface is cratered, it is significantly smoother than the surface of Phobos.
Only two geological features on Deimos are given proper names: the Voltaire and Swift craters, named after famous 17th- and 18th-century publicists who speculated about the existence of two Martian moons before they were discovered.
Researchers believe that Mars' moon Phobos may someday turn into a ring around the Red Planet. Credit: Celestia.
The origin of the satellites of Mars still remains uncertain, but there are a number of hypotheses among researchers. The most widely accepted theory is that the Red Planet's moons are objects ejected from the asteroid belt by Jupiter's gravity and captured by Mars.
However, this theory remains controversial since it is unlikely that Mars would be able to slow down bodies of this size enough for them to reach their current orbits. A modified version of this hypothesis is that Phobos and Deimos were once a binary asteroid that was then captured by Mars.
Other popular hypotheses suggest that the moons formed in their current orbits as a result of accretion processes, or perhaps that in the distant past Mars was surrounded by many large asteroids that were thrown into its orbit after collisions with planetesimals. Over time, most of them fell to the surface.