Having explored the structure of the Solar System and dwarf planets in one of the previous ones, this article includes the natural satellites of the Solar System. This is one of the most interesting areas in research astronomy, since there are moons larger than planets, and beneath their surface there are oceans and possibly life forms.
Let's start with the satellites of the terrestrial planets. Since Mercury and Venus do not have natural satellites, acquaintance with the satellites of the Solar system should begin with the Earth.
Terrestrial planets: Mercury, Venus, Earth and Mars
Moon
As you know, our planet has only one satellite - the Moon. This is the most studied cosmic body, as well as the first one that man managed to visit. The Moon is the fifth largest natural satellite of a planet in the solar system.
Although the Moon is considered a satellite, it would technically be considered a planet if it had an orbit around the Sun. The diameter of the Moon is almost three and a half thousand kilometers (3476); for example, the diameter of Pluto is 2374 km.
The Moon is a full participant in the Earth-Moon gravitational system. We have already written about another such tandem in the Solar System - o. Although the mass of the Earth's satellite is not large and is a little more than a hundredth of the mass of the Earth, the Moon does not revolve around the Earth - they have a common center of mass.
Can the Earth-Moon system be considered a double planet? The differences between a binary planet and a planet-moon system are thought to lie in the location of the system's center of mass. If the center of mass is not located under the surface of one of the objects of the system, then it can be considered a double planet. It turns out that both bodies rotate around a point in space that is located between them. According to this definition, the Earth and Moon are a planet and a satellite, and Charon and Pluto are a double dwarf planet.
As the distance between the Earth and the Moon constantly increases (the Moon moves away from the Earth), the center of mass, which is now below the surface of the Earth, will eventually move and end up above the surface of our planet. But this happens quite slowly, and it will be possible to consider the Earth-Moon system as a double planet only after billions of years.
Earth-Moon system
Among cosmic bodies, the Moon influences the Earth almost the most strongly, except, perhaps, the Sun. The most obvious phenomena of the satellite's impact on the Earth are lunar tides, which regularly change the water level in the World Ocean.
Earth view from the pole (high tides, low tides)
Why is the surface of the Moon all covered with craters? Firstly, the Moon does not have an atmosphere that would protect its surface from meteorites. Secondly, there is no water or wind on the Moon, which could smooth out the places where meteorites fell. Therefore, over four billion years, a large number of craters have accumulated on the surface of the satellite.
The largest crater in the Solar System. South Pole - Aitken Basin (red - highlands, blue - lowlands)
Lunar crater Daedalus: diameter 93 km, depth 2.8 km (image from Apollo 11)
The Moon, as already mentioned, is the only satellite visited by man and the first celestial body, samples of which were delivered to Earth. The first person to set foot on the moon was Neil Armstrong on July 21, 1969. In total, twelve astronauts visited the Moon; The last time people landed on the moon was back in 1972.
The first photograph taken by Neil Armstrong after walking on the surface of the Moon
Edwin Aldrin on the Moon, July 1969 (NASA photo)
Before scientists obtained soil samples from the Moon, there were two fundamentally different theories about the origin of the Moon. Adherents of the first theory believed that the Earth and the Moon were formed at the same time from a cloud of gas and dust. Another theory was that the Moon was formed elsewhere and then captured by the Earth. The study of lunar samples has led to the emergence of a new theory about the “Giant Impact”: almost four and a half (4.36) billion years ago, the protoplanet Earth (Gaia) collided with the protoplanet Theia. The blow did not land in the center, but at an angle (almost tangentially). As a result, most of the substance of the impacted object and part of the substance of the earth's mantle were thrown into low-Earth orbit. From these debris the Moon was assembled. As a result of the impact, the Earth received a sharp increase in rotation speed (one revolution in five hours) and a noticeable tilt of the rotation axis. Although this theory also has shortcomings, it is currently considered the main one.
Formation of the Moon: The collision of Theia with the Earth, which is believed to have created the Moon
Moons of Mars
Mars has two small moons: Phobos and Deimos. They were discovered by Asaph Hall in 1877. It is noteworthy that, having become disillusioned with the search for Martian satellites, he already wanted to give up observation, but his wife Angelina was able to convince him. The next night he discovered Deimos. Six nights later - Phobos. On Phobos, he discovered a gigantic crater that reaches ten kilometers in width - almost half the width of the satellite itself! Hall gave him Angelina's maiden name, Stickney.
Image of the satellites of Mars with respect to scales and distances
Both satellites have a shape close to a triaxial ellipsoid. Because of their small size, gravity is not strong enough to compress them into a round shape.
Phobos. Stickney Crater can be seen to the right.
Interestingly, the tidal influence of Mars gradually slows down the movement of Phobos, thereby reducing its orbit, which will ultimately lead to its fall onto Mars. Every hundred years, Phobos becomes nine centimeters closer to Mars, and in about eleven million years it will collapse on its surface, if the same forces do not destroy it even earlier. Deimos, on the contrary, is moving away from Mars, and over time will be captured by the tidal forces of the Sun. As a result, Mars will be left without satellites.
There is practically no attraction on the “Martian” side of Phobos, or rather, almost none. This is caused 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 is different.
The satellites of Mars are always turned to the same side, since the period of revolution of each of them coincides with the corresponding period of revolution around Mars. In this regard, they are similar to the Moon, the far side of which is also never visible from the surface of the Earth.
The sizes of Deimos and Phobos are very small. For example, the radius of the Moon is 158 times greater than the radius of Phobos and approximately 290 times greater than the radius of Deimos.
The distances from the satellites to the planet are also insignificant: the Moon is at a distance of 384,000 km from the Earth, and Deimos and Phobos are 23,000 and 9,000 kilometers away from Mars, respectively.
The origin of Martian moons remains controversial. They could be asteroids captured by the gravitational field of Mars, but the difference in their structure from the objects of the group of asteroids of which they could be part speaks against this version. Others believe that they were formed as a result of the collapse of the satellite of Mars into two parts.
The next material will be devoted to the satellites of Jupiter, of which as many as 67 have been registered today! And perhaps there are life forms on some of them.
The solar system is the only planetary structure accessible to us for direct study. Information obtained from research in this area of space is used by scientists to understand the processes occurring in the Universe. They make it possible to understand how our system and those similar to it were born, and what future awaits us all.
Classification of planets of the solar system
Research by astrophysicists has made it possible to classify the planets of the solar system. They were divided into two types: earth-like and gas giants. The terrestrial planets include Mercury, Venus, Earth, and Mars. The gas giants are Jupiter, Saturn, Uranus and Neptune. Since 2006, Pluto has received the status of a dwarf planet and belongs to the Kuiper belt objects, which differ in their characteristics from representatives of both named groups.
Characteristics of terrestrial planets
Each type has a set of features associated with its internal structure and composition. High average density and the predominance of silicates and metals at all levels are the main characteristics that distinguish the terrestrial planets. Giants, in contrast, have a low density and consist primarily of gases.
All four planets have a similar internal structure: under the solid crust there is a viscous mantle that envelops the core. The central structure, in turn, is divided into two levels: the liquid and solid core. Its main components are nickel and iron. The mantle differs from the core in the predominance of manganese.
The sizes of the planets of the Solar system belonging to the terrestrial group are distributed in this way (from smallest to largest): Mercury, Mars, Venus, Earth.
Air envelope
Earth-like planets were surrounded by an atmosphere already in the first stages of their formation. Initially, its composition was dominated by changes in the atmosphere on Earth, which contributed to the emergence of life. Terrestrial planets thus include cosmic bodies surrounded by an atmosphere. However, among them there is one that has lost its air shell. This did not allow the preservation of the primary atmosphere.
Closest to the Sun
The smallest terrestrial planet is Mercury. Its study is complicated by its close location to the Sun. Data on Mercury were received only from two devices: Mariner 10 and Messenger. Based on them, it was possible to create a map of the planet and determine some of its features.
Mercury can indeed be recognized as the smallest planet of the terrestrial group: its radius is slightly less than 2.5 thousand kilometers. Its density is close to that of the earth. The relationship between this indicator and its size suggests that the planet is largely composed of metals.
The movement of Mercury has a number of features. Its orbit is highly elongated: at the most distant point the distance to the Sun is 1.5 times greater than at the nearest point. The planet makes one revolution around the star in approximately 88 Earth days. Moreover, in such a year, Mercury manages to turn around its axis only one and a half times. Such “behavior” is not typical for other planets in the solar system. Presumably the slowdown of the initially faster movement was caused by the tidal influence of the Sun.
Beautiful and terrible
The terrestrial planets include both identical and different cosmic bodies. Similar in structure, they all have features that make them impossible to confuse. Mercury, which is closest to the Sun, is not the hottest planet. There are even areas on it that are forever covered with ice. Venus, next in proximity to the star, is characterized by higher temperatures.
The planet, named after the goddess of love, has long been a candidate for habitable space objects. However, the very first flights to Venus refuted this hypothesis. The true essence of the planet is hidden by a dense atmosphere consisting of carbon dioxide and nitrogen. This air envelope contributes to the development of the greenhouse effect. As a result, the temperature on the planet’s surface reaches +475 ºС. Thus, there can be no life here.
The second largest and most distant planet from the Sun has a number of features. Venus is the brightest point in the night sky after the Moon. Its orbit is an almost perfect circle. Around its axis it moves from east to west. This direction is not typical for most planets. It makes a revolution around the Sun in 224.7 Earth days, and around its axis in 243, that is, a year here is shorter than a day.
Third planet from the Sun
The earth is unique in many ways. It is located in the so-called life zone, where the sun's rays are not able to turn the surface into a desert, but there is enough heat to prevent the planet from becoming covered with an ice crust. Slightly less than 80% of the surface is occupied by the World Ocean, which, together with rivers and lakes, forms a hydrosphere that is absent on the rest of the planets of the solar system.
The formation of a special atmosphere of the Earth, consisting mainly of nitrogen and oxygen, was facilitated by the development of life. As a result of the increase in oxygen concentration, the ozone layer was formed, which, together with the magnetic field, protects the planet from the harmful effects of solar radiation.
Earth's only satellite
The Moon has a fairly serious impact on the Earth. Our planet acquired a natural satellite almost immediately after its formation. remains a mystery for now, although there are several plausible hypotheses on this matter. The satellite has a stabilizing effect on the tilt of the earth's axis and also causes the planet to slow down. As a result, each new day becomes a little longer. The slowdown is a consequence of the moon's tidal influence, the same force that causes the ocean.
Red Planet
When asked which terrestrial planets are best studied after ours, there is always a clear answer: Mars. Due to their location and climate, Venus and Mercury have been studied to a much lesser extent.
If we compare the sizes of the planets of the solar system, Mars will be in seventh place on the list. Its diameter is 6800 km, and its mass is 10.7% of that of the Earth.
The red planet has a very thin atmosphere. Its surface is dotted with craters, and you can also see volcanoes, valleys and glacial polar caps. Mars has two satellites. The closest one to the planet - Phobos - is gradually declining and in the future will be torn apart by the gravity of Mars. Deimos, on the contrary, is characterized by slow removal.
The idea of the possibility of life on Mars has existed for more than a century. The latest research, carried out in 2012, discovered on the red planet. It was suggested that organic matter could have been brought to the surface by a rover from Earth. However, research has confirmed the origin of the substance: its source is the red planet itself. Nevertheless, an unambiguous conclusion about the possibility of life on Mars cannot be made without additional research.
Terrestrial planets include the space objects closest to us in location. That is why they are better studied today. Astronomers have already discovered several exoplanets that presumably also belong to this type. Of course, each such discovery increases the hope of finding life beyond the solar system.
For example, Vesta.
Main characteristics
Terrestrial planets are highly dense and composed primarily of silicates and metallic iron (as opposed to gas planets and rock-ice dwarf planets, Kuiper belt objects, and the Oort cloud). The largest terrestrial planet, Earth, is more than 14 times less massive than the least massive gas planet, Uranus, but is approximately 400 times more massive than the largest known Kuiper Belt object.
Terrestrial planets consist mainly of oxygen, silicon, iron, magnesium, aluminum and other heavy elements.
All terrestrial planets have the following structure:
- In the center is a core of iron mixed with nickel.
- The mantle is composed of silicates.
- Crust, formed as a result of partial melting of the mantle and also consisting of silicate rocks, but enriched in incompatible elements. Of the terrestrial planets, Mercury does not have a crust, which is explained by its destruction as a result of meteorite bombardment. The Earth differs from other terrestrial planets in the high degree of chemical differentiation of matter and the wide distribution of granites in the crust.
Two of the terrestrial planets (the farthest from the Sun - Earth and Mars) have satellites. None of them (unlike all giant planets) have rings.
Terrestrial exoplanets
It is believed that Earth-like planets are the most favorable for the emergence of life, so their search attracts close public attention. So in December 2005, scientists from the Space Science Institute (Pasadena, California) reported the discovery of a Sun-like star around which rocky planets are believed to be forming. Subsequently, planets were discovered that were only several times more massive than the Earth and would probably have a solid surface.
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An excerpt characterizing the Terrestrial Planets-What division are you in? - the adjutant shouted as he drove up.- Eighteenth. - So why are you here? You should have been ahead a long time ago, now you won’t make it until evening. - Those orders are stupid; “They don’t know what they’re doing,” the officer said and drove off. Then a general drove by and shouted something angrily, not in Russian. “Tafa lafa, you can’t understand what he’s muttering,” said the soldier, mimicking the departed general. - I would shoot them, scoundrels! “We were told to be there at nine o’clock, but we weren’t even halfway through.” These are the orders! - repeated from different sides. And the feeling of energy with which the troops went into action began to turn into annoyance and anger at the stupid orders and at the Germans. The reason for the confusion was that while the Austrian cavalry was moving on the left flank, the higher authorities found that our center was too far from the right flank, and the entire cavalry was ordered to move to the right side. Several thousand cavalry advanced ahead of the infantry, and the infantry had to wait. Ahead there was a clash between the Austrian column leader and the Russian general. The Russian general shouted, demanding that the cavalry be stopped; the Austrian argued that it was not he who was to blame, but the higher authorities. Meanwhile, the troops stood, bored and discouraged. After an hour's delay, the troops finally moved further and began to descend down the mountain. The fog that dispersed on the mountain only spread thicker in the lower areas where the troops descended. Ahead, in the fog, one shot was heard, then another, at first awkwardly at different intervals: draft... tat, and then more and more smoothly and more often, and the matter began over the Goldbach River. Not expecting to meet the enemy below the river and accidentally stumbling upon him in the fog, not hearing a word of inspiration from the highest commanders, with the consciousness spreading throughout the troops that it was too late, and, most importantly, in the thick fog not seeing anything ahead and around them, the Russians lazily and slowly exchanged fire with the enemy, moved forward and stopped again, not receiving orders from the commanders and adjutants, who were wandering through the fog in an unfamiliar area, not finding their units of troops. Thus began the case for the first, second and third columns that went down. The fourth column, with Kutuzov himself, stood on the Pratsen Heights. At the bottom, where the matter began, there was still a thick fog, at the top it had cleared, but nothing was visible from what was happening ahead. Whether all the enemy forces, as we assumed, were ten miles away from us or whether he was here, in this line of fog, no one knew until the ninth hour. It was 9 o'clock in the morning. The fog spread like a continuous sea along the bottom, but near the village of Šlapanice, at the height at which Napoleon stood, surrounded by his marshals, it was completely light. Above him was a clear, blue sky, and a huge ball of the sun, like a huge hollow crimson float, swayed on the surface of a milky sea of fog. Not only all the French troops, but Napoleon himself and his headquarters were located on the wrong side of the streams and the bottoms of the villages of Sokolnitz and Shlapanitz, behind which we intended to take a position and begin business, but on this side, so close to our troops that Napoleon could in our army to distinguish horse from foot. Napoleon stood somewhat ahead of his marshals on a small gray Arabian horse, wearing a blue overcoat, the same one in which he fought the Italian campaign. He silently peered into the hills, which seemed to protrude from a sea of fog, and along which Russian troops were moving in the distance, and listened to the sounds of shooting in the ravine. At that time, his still thin face did not move a single muscle; the shining eyes were motionless fixed on one place. His assumptions turned out to be correct. Some of the Russian troops had already descended into the ravine to the ponds and lakes, and some were clearing those Pratsen heights, which he intended to attack and considered the key to the position. He saw, in the midst of the fog, how, in a depression made up of two mountains near the village of Prats, Russian columns, all moving in one direction towards the hollows, bayonets shining, disappeared one after another into the sea of fog. According to the information he received in the evening, from the sounds of wheels and footsteps heard at night at the outposts, from the disorderly movement of the Russian columns, from all assumptions, he clearly saw that the allies considered him far ahead of them, that the columns moving near Pratzen formed the center of the Russian army, and that the center is already weakened enough to attack it successfully. But he still hadn't started the business. | |||||||||||||||||||||||||
Dwarf planets like Ceres and Pluto, as well as other large asteroids, are similar to terrestrial planets in that they have a rocky surface. However, they consist more of ice materials than stone.
Terrestrial exoplanets
Most of the planets discovered outside the solar system have been gas giants because they are the easiest to detect. But since 2005, hundreds of potential terrestrial exoplanets have been discovered, thanks in large part to the Kepler space mission. Most planets became known as "super-Earths" (that is, planets with masses between Earth and Neptune).
Examples of terrestrial exoplanets, a planet with a mass of 7-9 terrestrials. This planet orbits the red dwarf star Gliese 876, located 15 light-years from Earth. The existence of three (or four) terrestrial exoplanets was also confirmed between 2007 and 2010 in the system of Gliese 581, another red dwarf approximately 20 light-years from Earth.
The smallest of them, Gliese 581 e, is only 1.9 Earth's mass, but orbits too close to the star. The other two, Gliese 581 c and Gliese 581 d, as well as the proposed fourth planet Gliese 581 g, are more massive and orbit within the star. If this information is confirmed, the system will become interesting for the presence of potentially habitable terrestrial planets.
The first confirmed terrestrial exoplanet, Kepler-10b, a 3-4 Earth mass planet located 460 light-years from Earth, was discovered in 2011 by the Kepler mission. That same year, the Kepler Space Observatory released a list of 1,235 exoplanetary candidates, including six “super-Earths” located within the potentially habitable zone of their star.
Since then, Kepler has discovered hundreds of planets ranging in size from the Moon to large Earth, and even more candidates beyond those sizes.
Scientists have proposed several categories for classifying terrestrial planets. Silicate planets- This is the standard type of terrestrial planet in the Solar System, consisting primarily of a silicate solid mantle and a metallic (iron) core.
Iron planets is a theoretical type of terrestrial planet that is composed almost entirely of iron, and is therefore denser and has a smaller radius than other planets of comparable mass. These types of planets are thought to form in high-temperature regions close to the star, where the protoplanetary disk is rich in iron. Mercury may be an example of such a group: it formed close to the Sun and has a metallic core that is equivalent to 60-70% of the planetary mass.
Planets without a core- another theoretical type of terrestrial planets: they are composed of silicate rocks, but do not have a metallic core. In other words, planets without a core are the opposite of an iron planet. Planets without cores are thought to form further from the star, where the volatile oxidizer is more abundant. And although we do not have such planets, there are a lot of chondrites - asteroids.
Finally there is carbon planets(so-called "diamond planets"), a theoretical class of planets that consist of a metallic core surrounded primarily by carbon-based minerals. Again, there are no such planets in the Solar System, but there are an abundance of carbon-rich asteroids.
Until recently, everything scientists knew about planets - including how they formed and what types there were - came from studying our own solar system. But with the development of exoplanet research, which has seen a huge surge in the last ten years, our knowledge of the planets has increased significantly.
On the one hand, we have come to understand that the size and scale of planets is much larger than previously thought. Moreover, this is the first time we have seen many Earth-like planets (which may also be habitable) existing in other solar systems.
Who knows what we will find when we are able to send probes and manned missions to other terrestrial planets?
Chapter 8. Terrestrial planets: Mercury, Venus, Earth
Planet formation
One way or another, at the moment there are 8 planets known in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and several plutonoids, including Pluto, which until recently was listed among the planets. All planets move in orbits in the same direction and in the same plane and in almost circular orbits (with the exception of plutonoids). From the center to the outskirts of the solar system (to Pluto) 5.5 light hours. The distance from the Sun to the Earth is 149 million km, which is 107 of its diameters. The first planets from the Sun are strikingly different in size from the latter and, unlike them, are called terrestrial planets, and the distant ones are called giant planets.
Mercury
The planet closest to the Sun, Mercury, is named after the Roman god of trade, travelers and thieves. This small planet moves quickly in orbit and rotates very slowly around its axis. Mercury has been known since ancient times, but astronomers did not immediately realize that it was a planet, and that in the morning and evening they saw the same star.
Mercury is located at a distance of about 0.387 AU from the Sun. (1 AU is equal to the average radius of the Earth’s orbit), and the distance from Mercury to the Earth, as it and the Earth move in their orbits, changes from 82 to 217 million km. The inclination of the plane of Mercury's orbit to the plane of the ecliptic (plane of the solar system) is 7°. Mercury's axis is almost perpendicular to the plane of its orbit, and its orbit is elongated. Thus, there are no seasons on Mercury, and the changes of day and night occur very rarely, approximately once every two Mercury years. One side of it, facing the Sun for a long time, is very hot, and the other, turned away from the Sun for a long time, is in terrible cold. Mercury moves around the Sun at a speed of 47.9 km/s. The weight of Mercury is almost 20 times less than the weight of the Earth (0.055M), and its density is almost the same as that of the Earth (5.43 g/cm3). The radius of the planet Mercury is 0.38R (radius of the Earth, 2440 km).
Due to its proximity to the Sun, under the influence of gravity, powerful tidal forces arose in the body of Mercury, which slowed down its rotation around its axis. In the end, Mercury found itself in a resonant trap. The period of its revolution around the Sun, measured in 1965, was 87.95 Earth days, and the period of rotation around its axis was 58.65 Earth days. Mercury completes three full revolutions around its axis in 176 days. During the same period, the planet makes two revolutions around the Sun. In the future, tidal braking of Mercury should lead to equality of its revolution around its axis and revolution around the Sun. Then it will always face the Sun in one direction, just as the Moon faces the Earth.
Mercury has no satellites. Perhaps, once upon a time, Mercury itself was a satellite of Venus, but due to solar gravity it was “taken away” from Venus and became an independent planet. The planet is actually spherical in shape. The acceleration of free fall on its surface is almost 3 times less than that on Earth (g = 3.72 m/s 2 ).
Its proximity to the Sun makes observing Mercury difficult. In the sky, it does not move far from the Sun - a maximum of 29°; from the Earth it is visible either before sunrise (morning visibility) or after sunset (evening visibility).
In its physical characteristics, Mercury resembles the Moon; there are many craters on its surface. Mercury has a very thin atmosphere. The planet has a large iron core, which is a source of gravity and a magnetic field, the strength of which is 0.1 of the strength of the Earth's magnetic field. Mercury's core makes up 70% of the planet's volume. The surface temperature ranges from 90° to 700° K (–180° to +430° C). The sun's equatorial side heats up much more than the polar regions. Different degrees of surface heating create a difference in the temperature of the rarefied atmosphere, which should cause its movement - wind.
