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Planets of the solar system

According to the official position of the International Astronomical Union (IAU), an organization that assigns names to astronomical objects, there are only 8 planets.

Pluto was removed from the category of planets in 2006. because in the Kuiper belt are objects that are larger / or equal in size to Pluto. Therefore, even if it is taken as a full-fledged celestial body, then it is necessary to add Eris to this category, which has almost the same size with Pluto.

As defined by MAC, there are 8 known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

All planets are divided into two categories depending on their physical characteristics: terrestrial and gas giants.

Schematic representation of the location of the planets

terrestrial planets

Mercury

The smallest planet in the solar system has a radius of only 2440 km. The period of revolution around the Sun, for ease of understanding, equated to the earth's year, is 88 days, while Mercury has time to complete a revolution around its own axis only one and a half times. Thus, its day lasts approximately 59 Earth days. For a long time it was believed that this planet is always turned to the Sun by the same side, since the periods of its visibility from the Earth were repeated with a frequency approximately equal to four Mercury days. This misconception was dispelled with the advent of the possibility of using radar research and conducting continuous observations using space stations. The orbit of Mercury is one of the most unstable; not only the speed of movement and its distance from the Sun change, but also the position itself. Anyone interested can observe this effect.

Mercury in color, as seen by the MESSENGER spacecraft

Mercury's proximity to the Sun has caused it to experience the largest temperature fluctuations of any of the planets in our system. The average daytime temperature is about 350 degrees Celsius, and the nighttime temperature is -170 °C. Sodium, oxygen, helium, potassium, hydrogen and argon have been identified in the atmosphere. There is a theory that it was previously a satellite of Venus, but so far this remains unproven. It has no satellites of its own.

Venus

The second planet from the Sun, the atmosphere of which is almost entirely composed of carbon dioxide. It is often called the Morning Star and the Evening Star, because it is the first of the stars to become visible after sunset, just as before dawn it continues to be visible even when all other stars have disappeared from view. The percentage of carbon dioxide in the atmosphere is 96%, there is relatively little nitrogen in it - almost 4%, and water vapor and oxygen are present in very small amounts.

Venus in the UV spectrum

Such an atmosphere creates a greenhouse effect, the temperature on the surface because of this is even higher than that of Mercury and reaches 475 ° C. Considered the slowest, the Venusian day lasts 243 Earth days, which is almost equal to a year on Venus - 225 Earth days. Many call it the sister of the Earth because of the mass and radius, the values ​​​​of which are very close to the earth's indicators. The radius of Venus is 6052 km (0.85% of the earth). There are no satellites, like Mercury.

The third planet from the Sun and the only one in our system where there is liquid water on the surface, without which life on the planet could not develop. At least life as we know it. The radius of the Earth is 6371 km and, unlike the rest of the celestial bodies in our system, more than 70% of its surface is covered with water. The rest of the space is occupied by the continents. Another feature of the Earth is the tectonic plates hidden under the planet's mantle. At the same time, they are able to move, albeit at a very low speed, which over time causes a change in the landscape. The speed of the planet moving along it is 29-30 km / s.

Our planet from space

One rotation around its axis takes almost 24 hours, and a complete orbit lasts 365 days, which is much longer in comparison with the nearest neighboring planets. The Earth day and year are also taken as a standard, but this is done only for the convenience of perceiving time intervals on other planets. The Earth has one natural satellite, the Moon.

Mars

The fourth planet from the Sun, known for its rarefied atmosphere. Since 1960, Mars has been actively explored by scientists from several countries, including the USSR and the USA. Not all research programs have been successful, but water found in some areas suggests that primitive life exists on Mars, or existed in the past.

The brightness of this planet allows you to see it from Earth without any instruments. Moreover, once every 15-17 years, during the Opposition, it becomes the brightest object in the sky, eclipsing even Jupiter and Venus.

The radius is almost half that of the earth and is 3390 km, but the year is much longer - 687 days. He has 2 satellites - Phobos and Deimos .

Visual model of the solar system

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• Sun

  The sun is a star, which is a hot ball of hot gases at the center of our solar system. Its influence extends far beyond the orbits of Neptune and Pluto. Without the Sun and its intense energy and heat, there would be no life on Earth. There are billions of stars, like our Sun, scattered throughout the Milky Way galaxy.

• Mercury

  Sun-scorched Mercury is only slightly larger than Earth's moon. Like the Moon, Mercury is practically devoid of an atmosphere and cannot smooth out the traces of impact from the fall of meteorites, therefore, like the Moon, it is covered with craters. The day side of Mercury is very hot on the Sun, and on the night side the temperature drops hundreds of degrees below zero. In the craters of Mercury, which are located at the poles, there is ice. Mercury makes one revolution around the Sun in 88 days.

• Venus

  Venus is a world of monstrous heat (even more than on Mercury) and volcanic activity. Similar in structure and size to Earth, Venus is covered in a thick and toxic atmosphere that creates a strong greenhouse effect. This scorched world is hot enough to melt lead. Radar images through the mighty atmosphere revealed volcanoes and deformed mountains. Venus rotates in the opposite direction from the rotation of most planets.

• Earth is an ocean planet. Our home, with its abundance of water and life, makes it unique in our solar system. Other planets, including several moons, also have ice deposits, atmospheres, seasons, and even weather, but only on Earth did all these components come together in such a way that life became possible.

• Mars

  Although details of the surface of Mars are difficult to see from Earth, telescope observations show that Mars has seasons and white spots at the poles. For decades, people have assumed that the bright and dark areas on Mars are patches of vegetation and that Mars might be a suitable place for life, and that water exists in the polar caps. When the Mariner 4 spacecraft flew by Mars in 1965, many of the scientists were shocked to see pictures of the bleak, cratered planet. Mars turned out to be a dead planet. More recent missions, however, have revealed that Mars holds many mysteries that have yet to be solved.

• Jupiter

  Jupiter is the most massive planet in our solar system, has four large moons and many small moons. Jupiter forms a kind of miniature solar system. To turn into a full-fledged star, Jupiter had to become 80 times more massive.

• Saturn

  Saturn is the most distant of the five planets that were known before the invention of the telescope. Like Jupiter, Saturn is made up mostly of hydrogen and helium. Its volume is 755 times that of the Earth. Winds in its atmosphere reach speeds of 500 meters per second. These fast winds, combined with heat rising from the planet's interior, cause the yellow and golden streaks we see in the atmosphere.

• Uranus

  The first planet found with a telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh planet is so far from the Sun that one revolution around the Sun takes 84 years.

• Neptune

  Nearly 4.5 billion kilometers from the Sun, distant Neptune rotates. It takes 165 years to complete one revolution around the Sun. It is invisible to the naked eye due to its vast distance from Earth. Interestingly, its unusual elliptical orbit intersects with the orbit of the dwarf planet Pluto, which is why Pluto is inside Neptune's orbit for about 20 out of 248 years during which it makes one revolution around the Sun.

• Pluto

  Tiny, cold and incredibly distant, Pluto was discovered in 1930 and has long been considered the ninth planet. But after the discovery of Pluto-like worlds even further away, Pluto was reclassified as a dwarf planet in 2006.

The planets are giants

There are four gas giants located beyond the orbit of Mars: Jupiter, Saturn, Uranus, Neptune. They are in the outer solar system. They differ in their massiveness and gas composition.

Planets of the solar system, not to scale

Jupiter

The fifth planet from the Sun and the largest planet in our system. Its radius is 69912 km, it is 19 times larger than the Earth and only 10 times smaller than the Sun. A year on Jupiter is not the longest in the solar system, lasting 4333 Earth days (incomplete 12 years). His own day has a duration of about 10 Earth hours. The exact composition of the planet's surface has not yet been determined, but it is known that krypton, argon and xenon are present on Jupiter in much larger quantities than on the Sun.

There is an opinion that one of the four gas giants is actually a failed star. This theory is also supported by the largest number of satellites, of which Jupiter has many - as many as 67. To imagine their behavior in the orbit of the planet, a fairly accurate and clear model of the solar system is needed. The largest of them are Callisto, Ganymede, Io and Europa. At the same time, Ganymede is the largest satellite of the planets in the entire solar system, its radius is 2634 km, which is 8% larger than the size of Mercury, the smallest planet in our system. Io has the distinction of being one of only three moons with an atmosphere.

Saturn

The second largest planet and the sixth largest in the solar system. In comparison with other planets, the composition of chemical elements is most similar to the Sun. The surface radius is 57,350 km, the year is 10,759 days (almost 30 Earth years). A day here lasts a little longer than on Jupiter - 10.5 Earth hours. By the number of satellites, it is not far behind its neighbor - 62 versus 67. The largest satellite of Saturn is Titan, just like Io, which is distinguished by the presence of an atmosphere. Slightly smaller than it, but no less famous for this - Enceladus, Rhea, Dione, Tethys, Iapetus and Mimas. It is these satellites that are the objects for the most frequent observation, and therefore we can say that they are the most studied in comparison with the rest.

For a long time, the rings on Saturn were considered a unique phenomenon, inherent only to him. Only recently it was found that all gas giants have rings, but the rest are not so clearly visible. Their origin has not yet been established, although there are several hypotheses about how they appeared. In addition, it was recently discovered that Rhea, one of the satellites of the sixth planet, also has some kind of rings.

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SOLAR SYSTEM, The sun and the celestial bodies revolving around it - 8 planets (Pluto was recognized as a dwarf planet in 2006 at the 26th Assembly of the International Astronomical Union.), more than 63 satellites, four systems of rings in giant planets, tens of thousands of asteroids, a myriad of meteoroids ranging in size from boulders to dust particles , as well as millions of comets. In the space between them moving particles of the solar wind - electrons and protons. The entire solar system has not yet been explored: for example, most of the planets and their satellites have only been briefly examined from flyby trajectories, only one hemisphere of Mercury has been photographed, and there have not yet been expeditions to Pluto. But still, with the help of telescopes and space probes, a lot of important data has already been collected.

Almost the entire mass of the solar system (99.87%) is concentrated in the Sun. The size of the Sun also greatly exceeds any planet in its system: even Jupiter, which is 11 times larger than the Earth, has a radius 10 times smaller than the sun. The sun is an ordinary star that shines on its own due to the high surface temperature. The planets, on the other hand, shine by reflected sunlight (albedo) because they themselves are quite cold. They are in this order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and the dwarf planet Pluto. Distances in the solar system are usually measured in units of the average distance of the Earth from the Sun, called the astronomical unit (1 AU = 149.6 million km). For example, the average distance of Pluto from the Sun is 39 AU, but sometimes it is removed by 49 AU. Comets are known to fly away at 50,000 AU. Distance from Earth to nearest star a Centaur 272,000 AU, or 4.3 light years (i.e., light moving at a speed of 299,793 km / s travels this distance in 4.3 years). For comparison, light travels from the Sun to the Earth in 8 minutes, and to Pluto in 6 hours.

The planets revolve around the Sun in almost circular orbits lying approximately in the same plane, in a counterclockwise direction, as viewed from the north pole of the Earth. The plane of the Earth's orbit (the plane of the ecliptic) lies close to the median plane of the orbits of the planets. Therefore, the visible paths of the planets, the Sun and the Moon in the sky pass near the line of the ecliptic, and they themselves are always visible against the background of the constellations of the Zodiac. Orbital inclinations are measured from the plane of the ecliptic. Tilt angles less than 90° correspond to forward orbital motion (counterclockwise), and angles greater than 90° correspond to reverse motion. All the planets in the solar system move in the forward direction; Pluto has the highest orbital inclination (17°). Many comets move in the opposite direction, for example, the orbital inclination of Halley's Comet is 162°.

From the point of view of an earthly observer, the planets of the solar system are divided into two groups. Mercury and Venus, which are closer to the Sun than the Earth, are called the lower (inner) planets, and the more distant ones (from Mars to Pluto) are called the upper (external). The lower planets have a limiting angle of removal from the Sun: 28 ° for Mercury and 47 ° for Venus. When such a planet is as far as possible west (east) of the Sun, it is said to be at its greatest western (eastern) elongation. When an inferior planet is seen directly in front of the Sun, it is said to be in inferior conjunction; when directly behind the Sun - in superior conjunction. Like the Moon, these planets go through all phases of illumination by the Sun during the synodic period. Ps- the time for which the planet returns to its original position relative to the Sun from the point of view of an earthly observer. The true orbital period of the planet ( P) is called sidereal. For the lower planets, these periods are related by the ratio:

1/Ps = 1/P – 1/P o

Where P o is the orbital period of the Earth. For the upper planets, this ratio has a different form:

1/P s= 1/P o– 1/P

The upper planets are characterized by a limited range of phases. The maximum phase angle (Sun–planet–Earth) is 47° for Mars, 12° for Jupiter, and 6° for Saturn. When the upper planet is visible behind the Sun, it is in conjunction, and when in the opposite direction to the Sun, it is in opposition. A planet observed at an angular distance of 90° from the Sun is in quadrature (east or west).

The asteroid belt, passing between the orbits of Mars and Jupiter, divides the planetary system of the Sun into two groups. Inside it are the terrestrial planets (Mercury, Venus, Earth and Mars), similar in that they are small, rocky and rather dense bodies: their average density is from 3.9 to 5.5 g / cm 3. They rotate relatively slowly around their axes, lack rings and have few natural satellites: the Earth's Moon and the Martian Phobos and Deimos. Outside the asteroid belt are the giant planets: Jupiter, Saturn, Uranus and Neptune. They are characterized by large radii, low density (0.7–1.8 g/cm3), and deep atmospheres rich in hydrogen and helium. Jupiter, Saturn and possibly other giants do not have a solid surface. All of them rotate rapidly, have many satellites and are surrounded by rings. The distant little Pluto and the large satellites of the giant planets are in many ways similar to the terrestrial planets.

Ancient people knew the planets visible to the naked eye, i.e. all internal and external up to Saturn. V. Herschel discovered Uranus in 1781. The first asteroid was discovered by J. Piazzi in 1801. Analyzing deviations in the motion of Uranus, W. Le Verrier and J. Adams theoretically discovered Neptune; at the calculated place it was discovered by I. Galle in 1846. The most distant Pluto was discovered in 1930 by K. Tombo as a result of a long search for a non-Neptunian planet organized by P. Lovell. Four large satellites of Jupiter were discovered by Galileo in 1610. Since then, with the help of telescopes and space probes, numerous satellites have been found for all outer planets. H. Huygens in 1656 established that Saturn is surrounded by a ring. The dark rings of Uranus were discovered from Earth in 1977 when observing the occultation of a star. The transparent stone rings of Jupiter were discovered in 1979 by the Voyager 1 interplanetary probe. Since 1983, at the moments of the occultation of the stars, signs of inhomogeneous rings have been noted near Neptune; in 1989 an image of these rings was transmitted by Voyager 2 to ZODIAC; SPACE PROBE; HEAVENLY SPHERE).

SUN

The Sun is located in the center of the solar system - a typical single star with a radius of about 700,000 km and a mass of 2×10 30 kg. The temperature of the visible surface of the Sun - the photosphere - approx. 5800 K. The density of gas in the photosphere is thousands of times less than the density of air near the Earth's surface. Inside the Sun, temperature, density and pressure increase with depth, reaching 16 million K, 160 g/cm 3 and 3.5×10 11 bar in the center, respectively (the air pressure in the room is about 1 bar). Under the influence of high temperature in the core of the Sun, hydrogen is converted into helium with the release of a large amount of heat; this keeps the Sun from collapsing under its own gravity. The energy released in the core leaves the Sun mainly in the form of radiation from the photosphere with a power of 3.86 x 10 26 W. With such intensity, the Sun has been emitting for 4.6 billion years, having converted 4% of its hydrogen into helium during this time; at the same time, 0.03% of the mass of the Sun turned into energy. Models of stellar evolution indicate that the Sun is now in the middle of its life.

To determine the abundance of various chemical elements on the Sun, astronomers study the absorption and emission lines in the spectrum of sunlight. Absorption lines are dark gaps in the spectrum, indicating the absence of photons of a given frequency in it, absorbed by a certain chemical element. Emission lines, or emission lines, are the brighter parts of the spectrum, indicating an excess of photons emitted by a chemical element. The frequency (wavelength) of a spectral line indicates which atom or molecule is responsible for its occurrence; the contrast of the line indicates the amount of light emitting or absorbing substance; the width of the line makes it possible to judge its temperature and pressure.

The study of the thin (500 km) photosphere of the Sun makes it possible to estimate the chemical composition of its interior, since the outer regions of the Sun are well mixed by convection, the spectra of the Sun are of high quality, and the physical processes responsible for them are quite clear. However, it should be noted that only half of the lines in the solar spectrum have been identified so far.

The composition of the Sun is dominated by hydrogen. In second place is helium, whose name ("helios" in Greek for "Sun") recalls that it was discovered spectroscopically on the Sun earlier (1899) than on Earth. Since helium is an inert gas, it is extremely reluctant to react with other atoms and is also reluctant to show itself in the optical spectrum of the Sun - just one line, although many less abundant elements are represented in the spectrum of the Sun by numerous lines. Here is the composition of the "solar" substance: for 1 million hydrogen atoms there are 98,000 helium atoms, 851 oxygen, 398 carbon, 123 neon, 100 nitrogen, 47 iron, 38 magnesium, 35 silicon, 16 sulfur, 4 argon, 3 aluminum, according to 2 atoms of nickel, sodium and calcium, as well as a little bit of all other elements. Thus, by mass, the Sun is about 71% hydrogen and 28% helium; the remaining elements account for slightly more than 1%. From the point of view of planetology, it is noteworthy that some objects of the solar system have almost the same composition as the Sun ( see below section on meteorites).

Just as weather events change the appearance of planetary atmospheres, the appearance of the sun's surface also changes with characteristic times ranging from hours to decades. However, there is an important difference between the atmospheres of the planets and the Sun, which is that the movement of gases on the Sun is controlled by its powerful magnetic field. Sunspots are those areas of the luminary's surface where the vertical magnetic field is so strong (200–3000 gauss) that it prevents the horizontal movement of gas and thereby suppresses convection. As a result, the temperature in this region drops by about 1000 K, and a dark central part of the spot appears - the "shadow", surrounded by a hotter transition region - the "penumbra". The size of a typical sunspot is slightly larger than the Earth's diameter; there is such a spot for several weeks. The number of spots on the Sun either increases or decreases with the cycle duration from 7 to 17 years, averaging 11.1 years. Usually, the more spots appear in a cycle, the shorter the cycle itself. The direction of the magnetic polarity of the spots reverses from cycle to cycle, so the true cycle of sunspot activity is 22.2 years. At the beginning of each cycle, the first spots appear at high latitudes, ca. 40 °, and gradually the zone of their birth shifts to the equator to a latitude of approx. 5°. SUN.

There are 5 huge rotating hydrogen-helium balls in the solar system: the Sun, Jupiter, Saturn, Uranus and Neptune. In the depths of these gigantic celestial bodies, inaccessible to direct research, almost all the matter of the solar system is concentrated. The Earth's interior is also inaccessible to us, but by measuring the propagation time of seismic waves (long-wavelength sound waves) excited in the body of the planet by earthquakes, seismologists compiled a detailed map of the Earth's interior: they learned the dimensions and densities of the Earth's core and its mantle, and also obtained three-dimensional images using seismic tomography. images of moving plates of its crust. Similar methods can be applied to the Sun, since there are waves on its surface with a period of approx. 5 minutes, caused by many seismic vibrations propagating in its bowels. These processes are studied by helioseismology. Unlike earthquakes, which produce short bursts of waves, vigorous convection in the interior of the Sun creates constant seismic noise. Helioseismologists have found that under the convective zone, which occupies the outer 14% of the Sun's radius, matter rotates synchronously with a period of 27 days (nothing is known about the rotation of the solar core yet). Above, in the convective zone itself, rotation occurs synchronously only along cones of equal latitude and the farther from the equator, the slower: the equatorial regions rotate with a period of 25 days (ahead of the average rotation of the Sun), and the polar regions - with a period of 36 days (lag behind the average rotation) . Recent attempts to apply seismological methods to gas giant planets have not yielded results, since instruments are not yet able to fix the resulting oscillations.

Above the photosphere of the Sun is a thin hot layer of the atmosphere, which can be seen only in rare moments of solar eclipses. It is a chromosphere several thousand kilometers thick, so named for its red color due to the hydrogen H a emission line. The temperature almost doubles from the photosphere to the upper chromosphere, from which, for some unknown reason, the energy leaving the Sun is released as heat. Above the chromosphere, the gas is heated to 1 million K. This region, called the corona, extends for about 1 radius of the Sun. The gas density in the corona is very low, but the temperature is so high that the corona is a powerful source of X-rays.

Sometimes giant formations appear in the atmosphere of the Sun - eruptive prominences. They look like arches rising from the photosphere to a height of up to half the solar radius. Observations clearly indicate that the shape of the prominences is determined by the magnetic field lines. Another interesting and extremely active phenomenon is solar flares, powerful ejections of energy and particles lasting up to two hours. The flow of photons generated by such a solar flare reaches the Earth at the speed of light in 8 minutes, and the flow of electrons and protons in several days. Solar flares occur in places where the direction of the magnetic field changes sharply, caused by the movement of matter in sunspots. The maximum flare activity of the Sun usually occurs a year before the maximum of the sunspot cycle. Such predictability is very important, because a flurry of charged particles born from a powerful solar flare can damage even ground-based communications and energy networks, not to mention astronauts and space technology.

Under the pressure of the solar wind in the interstellar medium around the Sun, a giant cavern was formed - the heliosphere. At its boundary - the heliopause - there should be a shock wave in which the solar wind and interstellar gas collide and condense, exerting equal pressure on each other. Four space probes are now approaching the heliopause: Pioneer 10 and 11, Voyager 1 and 2. None of them met her at a distance of 75 AU. from the sun. It's a very dramatic race against time: Pioneer 10 stopped operating in 1998, and the others are trying to reach the heliopause before their batteries run out of power. According to the calculations, Voyager 1 is flying in exactly the direction from which the interstellar wind is blowing, and therefore will be the first to reach the heliopause.

PLANETS: DESCRIPTION

Mercury.

It is difficult to observe Mercury from the Earth with a telescope: it does not move away from the Sun at an angle of more than 28 °. It was studied using radar from Earth, and the Mariner 10 interplanetary probe photographed half of its surface. Mercury revolves around the Sun in 88 Earth days in a rather elongated orbit with a distance from the Sun at perihelion of 0.31 AU. and at aphelion 0.47 a.u. It rotates around the axis with a period of 58.6 days, exactly equal to 2/3 of the orbital period, so each point on its surface rotates towards the Sun only once in 2 Mercury years, i.e. a sunny day there lasts 2 years!

Of the major planets, only Pluto is smaller than Mercury. But in terms of average density, Mercury is in second place after Earth. It probably has a large metallic core, which is 75% of the radius of the planet (it occupies 50% of the radius of the Earth). The surface of Mercury is similar to that of the moon: dark, completely dry and covered with craters. The average light reflectance (albedo) of the surface of Mercury is about 10%, about the same as that of the Moon. Probably, its surface is also covered with regolith - sintered crushed material. The largest impact formation on Mercury is the Caloris basin, 2000 km in size, resembling lunar seas. However, unlike the Moon, there are peculiar structures on Mercury - ledges several kilometers high that stretch for hundreds of kilometers. Perhaps they were formed as a result of the compression of the planet during the cooling of its large metal core or under the influence of powerful solar tides. The surface temperature of the planet during the day is about 700 K, and at night about 100 K. According to radar data, ice may lie at the bottom of polar craters in conditions of eternal darkness and cold.

Mercury has practically no atmosphere - only an extremely rarefied helium shell with the density of the earth's atmosphere at an altitude of 200 km. Probably, helium is formed during the decay of radioactive elements in the bowels of the planet. Mercury has a weak magnetic field and no satellites.

Venus.

This is the second planet from the Sun and the closest planet to the Earth - the brightest "star" in our sky; sometimes it is visible even during the day. Venus is similar to the Earth in many ways: its size and density are only 5% less than that of the Earth; probably, the bowels of Venus are similar to those of the earth. The surface of Venus is always covered with a thick layer of yellowish-white clouds, but with the help of radars it has been studied in some detail. Around the axis, Venus rotates in the opposite direction (clockwise, when viewed from the north pole) with a period of 243 Earth days. Its orbital period is 225 days; therefore, a Venusian day (from sunrise to the next sunrise) lasts 116 Earth days.

The atmosphere of Venus is composed primarily of carbon dioxide (CO 2 ) with small amounts of nitrogen (N 2 ) and water vapor (H 2 O ). Hydrochloric acid (HCl) and hydrofluoric acid (HF) were found as small impurities. The pressure at the surface is 90 bar (as in the earth's seas at a depth of 900 m); the temperature is about 750 K over the entire surface both day and night. The reason for such a high temperature near the surface of Venus is what is not quite accurately called the "greenhouse effect": the sun's rays relatively easily pass through the clouds of its atmosphere and heat the surface of the planet, but thermal infrared radiation from the surface itself escapes through the atmosphere back into space with great difficulty.

The clouds of Venus are made up of microscopic droplets of concentrated sulfuric acid (H 2 SO 4). The upper layer of clouds is 90 km away from the surface, the temperature there is approx. 200 K; the lower layer - for 30 km, the temperature is approx. 430 K. Even lower it is so hot that there are no clouds. Of course, there is no liquid water on the surface of Venus. The atmosphere of Venus at the level of the upper cloud layer rotates in the same direction as the surface of the planet, but much faster, making a revolution in 4 days; this phenomenon is called superrotation, and no explanation has yet been found for it.

Automatic stations descended on the day and night sides of Venus. During the day, the surface of the planet is illuminated by scattered sunlight with about the same intensity as on an overcast day on Earth. A lot of lightning has been seen on Venus at night. The Venera stations transmitted images of small areas at the landing sites, where rocky ground is visible. In general, the topography of Venus has been studied from radar images transmitted by the Pioneer-Venera (1979), Venera-15 and -16 (1983), and Magellan (1990) orbiters. The smallest details on the best of them have a size of about 100 m.

Unlike Earth, there are no distinct continental plates on Venus, but there are several global elevations, such as the land of Ishtar the size of Australia. On the surface of Venus, there are many meteorite craters and volcanic domes. Obviously, the crust of Venus is thin, so that the molten lava comes close to the surface and easily pours out on it after the fall of meteorites. Since there is no rain or strong winds near the surface of Venus, surface erosion occurs very slowly, and geological structures remain visible from space for hundreds of millions of years. Little is known about the interior of Venus. It probably has a metal core taking up 50% of its radius. But the planet does not have a magnetic field due to its very slow rotation. Venus has no satellites.

Earth.

Our planet is the only one in which most of the surface (75%) is covered with liquid water. Earth is an active planet, and perhaps the only one whose surface renewal is due to plate tectonics, manifesting itself as mid-ocean ridges, island arcs, and folded mountain belts. The distribution of the heights of the solid surface of the Earth is bimodal: the average level of the ocean floor is 3900 m below sea level, and the continents, on average, rise above it by 860 m.

Seismic data indicate the following structure of the earth's interior: crust (30 km), mantle (up to a depth of 2900 km), metallic core. Part of the core is melted; the terrestrial magnetic field is generated there, which traps charged particles of the solar wind (protons and electrons) and forms around the Earth two toroidal regions filled with them - radiation belts (Van Allen belts), localized at altitudes of 4000 and 17 000 km from the Earth's surface (GEOMAGNETISM).

There are indications that the Earth's climate is changing in the short (10,000 years) and long (100 million years) scales. The reason for this may be changes in the orbital motion of the Earth, the tilt of the axis of rotation, the frequency of volcanic eruptions. Fluctuations in the intensity of solar radiation are not excluded. In our era, climate is also affected by human activities: emissions of gases and dust into the atmosphere AIR POLLUTION; WATER POLLUTION; ENVIRONMENTAL DEGRADATION). The Earth has a satellite - the Moon, the origin of which has not yet been unraveled.

Moon.

One of the largest satellites, the Moon is in second place after Charon (Pluto's satellite) in relation to the masses of the satellite and the planet. Its radius is 3.7, and its mass is 81 times less than that of the Earth. The average density of the Moon is 3.34 g/cm 3 , which indicates that it does not have a significant metallic core. The force of gravity on the lunar surface is 6 times less than that of the earth.

The Moon revolves around the Earth in an orbit with an eccentricity of 0.055. The inclination of the plane of its orbit to the plane of the earth's equator varies from 18.3° to 28.6°, and with respect to the ecliptic, from 4°59° to 5°19°. The daily rotation and orbital circulation of the Moon are synchronized, so we always see only one of its hemispheres. True, small wiggles (librations) of the Moon make it possible to see about 60% of its surface within a month. The main reason for librations is that the daily rotation of the Moon occurs at a constant speed, while the orbital circulation is variable (due to the eccentricity of the orbit).

The parts of the lunar surface have long been conditionally divided into "marine" and "continental". The surface of the seas looks darker, lies lower and is much less covered with meteorite craters than the continental surface. The seas are flooded with basaltic lavas, and the continents are composed of anorthositic rocks rich in feldspars. Judging by the large number of craters, the continental surfaces are much older than the sea ones. Intense meteorite bombardment made the upper layer of the lunar crust finely fragmented, and turned the outer few meters into a powder called regolith.

Astronauts and robotic probes have brought back samples of rocky soil and regolith from the Moon. The analysis showed that the age of the sea surface is about 4 billion years. Consequently, the period of intense meteorite bombardment falls on the first 0.5 billion years after the formation of the Moon 4.6 billion years ago. Then the frequency of meteorite impacts and crater formation remained virtually unchanged and still amounts to one crater 1 km in diameter in 10 5 years.

Lunar rocks are poor in volatile elements (H 2 O, Na, K, etc.) and iron, but rich in refractory elements (Ti, Ca, etc.). Only at the bottom of the lunar polar craters can there be deposits of ice, such as on Mercury. The moon has virtually no atmosphere and there is no evidence that the lunar soil has ever been exposed to liquid water. There is no organic matter in it either - only traces of carbonaceous chondrites that fell with meteorites. The absence of water and air, as well as strong fluctuations in surface temperature (390 K during the day and 120 K at night), make the Moon uninhabitable.

The seismometers delivered to the Moon made it possible to learn something about the lunar interior. Weak "moonquakes" often occur there, probably due to the tidal influence of the Earth. The moon is fairly homogeneous, has a small dense core and a crust about 65 km thick made of lighter materials, with the upper 10 km of the crust crushed by meteorites as early as 4 billion years ago. Large impact basins are evenly distributed over the lunar surface, but the thickness of the crust on the visible side of the Moon is less, so 70% of the sea surface is concentrated on it.

The history of the lunar surface is generally known: after the end of the stage of intense meteorite bombardment 4 billion years ago, the bowels were still hot enough for about 1 billion years, and basaltic lava poured into the seas. Then only a rare fall of meteorites changed the face of our satellite. But the origin of the moon is still debated. It could form on its own and then be captured by the Earth; could have formed along with the Earth as its satellite; finally, it could separate from the Earth during the formation period. The second possibility was popular until recently, but in recent years the hypothesis of the formation of the Moon from the material ejected by the proto-Earth during a collision with a large celestial body has been seriously considered.

Mars.

Mars is similar to Earth, but almost half its size and has a slightly lower average density. The period of daily rotation (24 h 37 min) and the tilt of the axis (24°) almost do not differ from those on Earth.

To an earthly observer, Mars appears as a reddish star, the brightness of which changes noticeably; it is maximum during periods of confrontations that repeat in a little over two years (for example, in April 1999 and June 2001). Mars is especially close and bright during periods of great opposition that occurs if it passes near perihelion at the time of opposition; this happens every 15–17 years (the next one is in August 2003).

A telescope on Mars shows bright orange regions and darker regions that change in tone with the seasons. Bright white snow caps lie at the poles. The reddish color of the planet is associated with a large amount of iron oxides (rust) in its soil. The composition of the dark regions probably resembles terrestrial basalts, while the light regions are composed of finely dispersed material.

Basically, our knowledge about Mars is obtained by automatic stations. The most productive were two orbiters and two landers of the Viking expedition, which landed on Mars on July 20 and September 3, 1976 in the areas of Chris (22 ° N, 48 ° W) and Utopia (48 ° N). ., 226° W), with Viking 1 operating until November 1982. Both of them landed in classic bright areas and ended up in a reddish sandy desert strewn with dark stones. On July 4, 1997, the Mars Pathfinder probe (USA) delivered the first automatic self-propelled vehicle to the Ares Valley (19 ° N, 34 ° W) that discovered mixed rocks and, possibly, turned by water and mixed with sand and clay pebbles, indicating strong changes in the Martian climate and the presence of a large amount of water in the past.

The rarefied atmosphere of Mars consists of 95% carbon dioxide and 3% nitrogen. Small amounts of water vapor, oxygen and argon are present. The average pressure at the surface is 6 mbar (i.e., 0.6% of the earth). At such a low pressure, there can be no liquid water. The average daily temperature is 240 K, and the maximum in summer at the equator reaches 290 K. Daily temperature fluctuations are about 100 K. Thus, the climate of Mars is the climate of a cold, dehydrated high-altitude desert.

At the high latitudes of Mars, temperatures drop below 150 K in winter and atmospheric carbon dioxide (CO 2 ) freezes and falls to the surface as white snow, forming the polar cap. Periodic condensation and sublimation of the polar caps causes seasonal fluctuations in atmospheric pressure by 30%. By the end of winter, the boundary of the polar cap drops to 45°–50° latitude, and in summer it leaves a small area (300 km in diameter at the south pole and 1000 km at the north), probably consisting of water ice, the thickness of which can reach 1–2 km.

Sometimes strong winds blow on Mars, lifting clouds of fine sand into the air. Especially powerful dust storms occur at the end of spring in the southern hemisphere, when Mars passes through the perihelion of the orbit and the solar heat is especially high. For weeks and even months, the atmosphere becomes opaque with yellow dust. Orbiters of the Vikings transmitted images of powerful sand dunes at the bottom of large craters. Dust deposits change the appearance of the Martian surface from season to season so much that it is noticeable even from Earth when viewed through a telescope. In the past, these seasonal changes in surface color were thought by some astronomers to be signs of vegetation on Mars.

The geology of Mars is very diverse. Large expanses of the southern hemisphere are covered with old craters left from the era of ancient meteorite bombardment (4 billion years ago). Much of the northern hemisphere is covered by younger lava flows. Particularly interesting is the Tharsis Upland (10° N, 110° W), on which several giant volcanic mountains are located. The highest among them - Mount Olympus - has a diameter at the base of 600 km and a height of 25 km. Although there are no signs of volcanic activity now, the age of the lava flows does not exceed 100 million years, which is small compared to the age of the planet 4.6 billion years.

Although ancient volcanoes point to the once powerful activity of the Martian interior, there are no signs of plate tectonics: there are no folded mountain belts and other indicators of crustal compression. However, there are powerful rift faults, the largest of which, the Mariner Valley, stretches from Tharsis to the east for 4000 km with a maximum width of 700 km and a depth of 6 km.

One of the most interesting geological discoveries made on the basis of photographs from spacecraft was the branched winding valleys hundreds of kilometers long, reminiscent of the dried-up channels of earthly rivers. This suggests a more favorable climate in the past, when temperatures and pressures may have been higher and rivers flowed across the surface of Mars. True, the location of the valleys in the southern, heavily cratered regions of Mars indicates that there were rivers on Mars a very long time ago, probably in the first 0.5 billion years of its evolution. Water now lies on the surface as ice at the polar caps and possibly below the surface as a layer of permafrost.

The internal structure of Mars is poorly understood. Its low average density indicates the absence of a significant metallic core; in any case, it is not melted, which follows from the absence of a magnetic field on Mars. The seismometer on the landing block of the Viking-2 apparatus did not record the seismic activity of the planet for 2 years of operation (the seismometer did not operate on the Viking-1).

Mars has two small moons, Phobos and Deimos. Both are irregularly shaped, covered in meteorite craters, and are likely asteroids captured by the planet in the distant past. Phobos revolves around the planet in a very low orbit and continues to approach Mars under the influence of the tides; it would later be destroyed by the planet's gravity.

Jupiter.

The largest planet in the solar system, Jupiter, is 11 times larger than the Earth and 318 times more massive than it. Its low average density (1.3 g/cm 3 ) indicates a composition close to that of the sun: it is mainly hydrogen and helium. The rapid rotation of Jupiter around its axis causes its polar compression by 6.4%.

A telescope on Jupiter shows cloud bands parallel to the equator; light zones in them are interspersed with reddish belts. It is likely that the light zones are areas of updrafts where the tops of ammonia clouds are visible; reddish belts are associated with downdrafts, the bright color of which is determined by ammonium hydrosulfate, as well as compounds of red phosphorus, sulfur and organic polymers. In addition to hydrogen and helium, CH 4 , NH 3 , H 2 O, C 2 H 2 , C 2 H 6 , HCN, CO, CO 2 , PH 3 and GeH 4 have been spectroscopically detected in Jupiter's atmosphere. The temperature at the tops of the ammonia clouds is 125 K, but it increases by 2.5 K/km with depth. At a depth of 60 km there should be a layer of water clouds.

The speeds of cloud movement in the zones and neighboring belts differ significantly: for example, in the equatorial belt, clouds move eastward 100 m/s faster than in neighboring zones. The difference in speeds causes strong turbulence at the boundaries of zones and belts, which makes their shape very intricate. One of the manifestations of this is oval rotating spots, the largest of which - the Great Red Spot - was discovered more than 300 years ago by Cassini. This spot (25,000-15,000 km) is larger than the Earth's disk; it has a spiral cyclonic structure and makes one rotation around its axis in 6 days. The rest of the spots are smaller and for some reason all white.

Jupiter does not have a solid surface. The upper layer of the planet with a length of 25% of the radius consists of liquid hydrogen and helium. Below, where the pressure exceeds 3 million bar and the temperature is 10,000 K, hydrogen passes into the metallic state. It is possible that near the center of the planet there is a liquid core of heavier elements with a total mass of about 10 Earth masses. In the center, the pressure is about 100 million bar and the temperature is 20–30 thousand K.

The liquid metal interior and the rapid rotation of the planet caused its powerful magnetic field, which is 15 times stronger than the earth's. Jupiter's huge magnetosphere, with powerful radiation belts, extends beyond the orbits of its four large satellites.

The temperature in the center of Jupiter has always been lower than necessary for the occurrence of thermonuclear reactions. But Jupiter's internal reserves of heat, which have remained from the epoch of formation, are large. Even now, 4.6 billion years later, it emits about the same amount of heat as it receives from the Sun; in the first million years of evolution, the radiation power of Jupiter was 10 4 times higher. Since this was the era of the formation of large satellites of the planet, it is not surprising that their composition depends on the distance to Jupiter: the two closest to it - Io and Europa - have a rather high density (3.5 and 3.0 g / cm 3), and more distant ones - Ganymede and Callisto - contain a lot of water ice and therefore are less dense (1.9 and 1.8 g / cm 3).

Satellites.

Jupiter has at least 16 satellites and a weak ring: it is 53,000 km away from the upper cloud layer, has a width of 6,000 km, and apparently consists of small and very dark solid particles. The four largest moons of Jupiter are called Galilean because they were discovered by Galileo in 1610; independently of him, in the same year, they were discovered by the German astronomer Marius, who gave them their current names - Io, Europa, Ganymede and Callisto. The smallest of the satellites - Europa - is slightly smaller than the Moon, and Ganymede is larger than Mercury. All of them are visible through binoculars.

On the surface of Io, the Voyagers discovered several active volcanoes, ejecting material hundreds of kilometers into the air. The surface of Io is covered with reddish sulfur deposits and light spots of sulfur dioxide - products of volcanic eruptions. In the form of a gas, sulfur dioxide forms an extremely rarefied atmosphere of Io. The energy of volcanic activity is drawn from the tidal influence of the planet on the satellite. Io's orbit passes through Jupiter's radiation belts, and it has long been established that the satellite interacts strongly with the magnetosphere, causing radio bursts in it. In 1973, a torus of luminous sodium atoms was discovered along the orbit of Io; later sulfur, potassium and oxygen ions were found there. These substances are knocked out by energetic protons of the radiation belts either directly from the surface of Io, or from the gaseous plumes of volcanoes.

Although Jupiter's tidal influence on Europa is weaker than on Io, its interior may also be partially melted. Spectral studies show that Europa has water ice on its surface, and its reddish hue is likely due to sulfur pollution from Io. The almost complete absence of impact craters indicates the geological youth of the surface. The folds and faults of the ice surface of Europa resemble the ice fields of the earth's polar seas; probably, on Europa, there is liquid water under a layer of ice.

Ganymede is the largest moon in the solar system. Its density is low; it is probably half rock and half ice. Its surface looks strange and shows signs of crustal expansion, possibly accompanying the process of subsurface differentiation. The areas of the ancient cratered surface are separated by younger trenches, hundreds of kilometers long and 1–2 km wide, lying at a distance of 10–20 km from each other. It is likely that this is younger ice, formed by the outpouring of water through cracks immediately after differentiation about 4 billion years ago.

Callisto is similar to Ganymede, but there are no signs of faults on its surface; all of it is very old and heavily cratered. The surface of both satellites is covered with ice interspersed with regolith-type rocks. But if on Ganymede the ice is about 50%, then on Callisto it is less than 20%. The composition of the rocks of Ganymede and Callisto is probably similar to that of carbonaceous meteorites.

Jupiter's moons have no atmosphere, except for the rarefied SO 2 volcanic gas on Io.

Of Jupiter's dozen minor moons, four are closer to the planet than the Galilean ones; the largest of them, Amalthea, is a cratered object of irregular shape (dimensions 270-166-150 km). Its dark surface—very red—possibly covered in gray from Io. The outer small satellites of Jupiter are divided into two groups in accordance with their orbits: 4 closer to the planet turn in the forward (relative to the rotation of the planet) direction, and 4 more distant - in the opposite direction. They are all small and dark; they are probably captured by Jupiter from among the asteroids of the Trojan group ( cm. ASTEROID).

Saturn.

The second largest giant planet. This is a hydrogen-helium planet, but the relative abundance of helium in Saturn is less than that of Jupiter; below and its average density. The rapid rotation of Saturn leads to its large oblateness (11%).

In a telescope, the disk of Saturn does not look as spectacular as Jupiter: it has a brownish-orange color and weakly pronounced belts and zones. The reason is that the upper regions of its atmosphere are filled with light-scattering ammonia (NH 3) fog. Saturn is further from the Sun, so the temperature of its upper atmosphere (90 K) is 35 K lower than that of Jupiter, and ammonia is in a condensed state. With depth, the temperature of the atmosphere increases by 1.2 K/km, so the cloud structure resembles that of Jupiter: there is a layer of water clouds under the ammonium hydrosulfate cloud layer. In addition to hydrogen and helium, CH 4 , NH 3 , C 2 H 2 , C 2 H 6 , C 3 H 4 , C 3 H 8 and PH 3 have been spectroscopically detected in Saturn's atmosphere.

In terms of internal structure, Saturn also resembles Jupiter, although due to its smaller mass it has lower pressure and temperature in the center (75 million bar and 10,500 K). Saturn's magnetic field is comparable to Earth's.

Like Jupiter, Saturn generates internal heat, twice as much as it receives from the Sun. True, this ratio is greater than that of Jupiter, because Saturn, located twice as far away, receives four times less heat from the Sun.

Rings of Saturn.

Saturn is surrounded by a uniquely powerful system of rings up to a distance of 2.3 planetary radii. They are easily distinguishable when viewed through a telescope, and when studied from a close distance, they show an exceptional variety: from a massive ring B to a narrow ring F, from spiral density waves to the completely unexpected radially elongated "spokes" discovered by Voyagers.

The particles that fill the rings of Saturn reflect light much better than the material of the dark rings of Uranus and Neptune; their study in different spectral ranges shows that these are "dirty snowballs" with dimensions of the order of a meter. The three classical rings of Saturn, in order from outer to inner, are denoted by letters A, B And C. Ring B quite dense: the radio signals from Voyager had difficulty passing through it. Gap of 4000 km between the rings A And B, called the division (or gap) of Cassini, is not really empty, but is comparable in density to a pale ring C, which was formerly called the crepe ring. Near the outer edge of the ring A there is a less visible Encke fissure.

In 1859 Maxwell concluded that Saturn's rings must be composed of individual particles orbiting the planet. At the end of the 19th century this was confirmed by spectral observations, which showed that the inner parts of the rings rotate faster than the outer ones. Since the rings lie in the plane of the planet's equator, which means they are inclined to the orbital plane by 27 °, the Earth falls into the plane of the rings twice in 29.5 years, and we observe them edge-on. At this moment, the rings "disappear", which proves their very small thickness - no more than a few kilometers.

Detailed images of the rings taken by Pioneer 11 (1979) and Voyagers (1980 and 1981) showed a much more complex structure than expected. The rings are divided into hundreds of individual ringlets with a typical width of several hundred kilometers. Even in the Cassini gap there were at least five rings. A detailed analysis showed that the rings are inhomogeneous both in size and, possibly, in particle composition. The complex structure of the rings is probably due to the gravitational influence of small satellites close to them, which were not previously suspected.

Probably the most unusual is the thinnest ring F, discovered in 1979 by Pioneer at a distance of 4000 km from the outer edge of the ring A. Voyager 1 discovered that the ring F twisted and braided like a braid, but flying for 9 months. later, Voyager 2 found the structure of the ring F much simpler: the “strands” of the substance were no longer intertwined with each other. This structure and its rapid evolution is partly due to the influence of two small satellites (Prometheus and Pandora) moving at the outer and inner edges of this ring; they are called "watchdogs". It is not excluded, however, the presence of even smaller bodies or temporary accumulations of matter inside the ring itself. F.

Satellites.

Saturn has at least 18 moons. Most of them are probably icy. Some have very interesting orbits. For example, Janus and Epimetheus have almost the same orbital radii. In the orbit of Dione, 60 ° ahead of her (this position is called the leading Lagrange point), the smaller satellite Helena moves. Tethys is accompanied by two small moons, Telesto and Calypso, at the leading and trailing Lagrangian points of its orbit.

The radii and masses of seven satellites of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Iapetus) have been measured with good accuracy. All of them are mostly icy. The smaller ones have densities of 1–1.4 g/cm 3 , which is close to the density of water ice with more or less admixture of rocks. Whether they contain methane and ammonia ice is not yet clear. The higher density of Titanium (1.9 g/cm 3 ) is the result of its large mass, which causes compression of the interior. In diameter and density, Titan is very similar to Ganymede; they probably have the same internal structure. Titan is the second largest moon in the solar system, and is unique in that it has a constant powerful atmosphere, consisting mainly of nitrogen and a small amount of methane. The pressure at its surface is 1.6 bar, the temperature is 90 K. Under such conditions, liquid methane can be on the surface of Titan. The upper layers of the atmosphere up to altitudes of 240 km are filled with orange clouds, probably consisting of particles of organic polymers synthesized under the influence of the ultraviolet rays of the Sun.

The rest of Saturn's moons are too small to have an atmosphere. Their surfaces are covered with ice and heavily cratered. Only on the surface of Enceladus are there significantly fewer craters. Probably, the tidal influence of Saturn keeps its bowels in a molten state, and meteorite impacts lead to an outpouring of water and filling the craters. Some astronomers believe that particles from the surface of Enceladus formed a wide ring. E extending along its orbit.

The satellite Iapetus is very interesting, in which the rear (relative to the direction of orbital motion) hemisphere is covered with ice and reflects 50% of the incident light, and the front hemisphere is so dark that it reflects only 5% of the light; it is covered with something like the substance of carbonaceous meteorites. It is possible that the material ejected under the influence of meteorite impacts from the surface of Saturn's outer satellite Phoebe falls on the forward hemisphere of Iapetus. In principle, this is possible, since Phoebe moves in the orbit in the opposite direction. In addition, the surface of Phoebe is quite dark, but there is no exact data on it yet.

Uranus.

Uranus is aquamarine and looks featureless because its upper atmosphere is filled with fog, through which the Voyager 2 probe that flew near it in 1986 could hardly see a few clouds. The axis of the planet is inclined to the orbital axis by 98.5°, i.e. lies almost in the plane of the orbit. Therefore, each of the poles is turned directly to the Sun for some time, and then goes into the shadow for half a year (42 Earth years).

The atmosphere of Uranus contains mostly hydrogen, 12–15% helium, and a few other gases. The temperature of the atmosphere is about 50 K, although in the upper rarefied layers it rises to 750 K during the day and 100 K at night. The magnetic field of Uranus is slightly weaker than the earth's in strength at the surface, and its axis is inclined to the axis of rotation of the planet by 55 °. Little is known about the internal structure of the planet. The cloud layer probably extends to a depth of 11,000 km, followed by a hot water ocean 8,000 km deep, and below it a molten stone core with a radius of 7,000 km.

Rings.

In 1976, unique rings of Uranus were discovered, consisting of separate thin rings, the widest of which has a thickness of 100 km. The rings are located in the range of distances from 1.5 to 2.0 radii of the planet from its center. Unlike the rings of Saturn, the rings of Uranus are made up of large dark rocks. It is believed that a small satellite, or even two satellites, moves in each ring, as in a ring. F Saturn.

Satellites.

20 moons of Uranus have been discovered. The largest - Titania and Oberon - with a diameter of 1500 km. There are 3 more large ones, more than 500 km in size, the rest are very small. The surface spectra of five large satellites indicate a large amount of water ice. The surfaces of all satellites are covered with meteorite craters.

Neptune.

Externally, Neptune is similar to Uranus; its spectrum is also dominated by methane and hydrogen bands. The flow of heat from Neptune significantly exceeds the power of the solar heat incident on it, which indicates the existence of an internal source of energy. Perhaps much of the internal heat is released as a result of tides caused by the massive moon Triton, which is orbiting in the opposite direction at a distance of 14.5 planetary radii. Voyager 2, flying in 1989 at a distance of 5000 km from the cloud layer, discovered 6 more satellites and 5 rings near Neptune. The Great Dark Spot and a complex system of eddy currents were discovered in the atmosphere. The pinkish surface of Triton revealed amazing geological details, including powerful geysers. The satellite Proteus discovered by Voyager turned out to be larger than Nereid, discovered from Earth back in 1949.

Pluto.

Pluto has a highly elongated and tilted orbit; at perihelion it approaches the Sun at 29.6 AU. and is removed at aphelion at 49.3 AU. Pluto passed perihelion in 1989; from 1979 to 1999 it was closer to the Sun than Neptune. However, due to the large inclination of Pluto's orbit, its path never crosses with Neptune. The average surface temperature of Pluto is 50 K, it changes from aphelion to perihelion by 15 K, which is quite noticeable at such low temperatures. In particular, this leads to the appearance of a rarefied methane atmosphere during the period of the planet's passage of perihelion, but its pressure is 100,000 times less than the pressure of the earth's atmosphere. Pluto cannot hold an atmosphere for long because it is smaller than the Moon.

Pluto's moon Charon takes 6.4 days to orbit close to the planet. Its orbit is very strongly inclined to the ecliptic, so that eclipses occur only in rare epochs of the Earth's passage through the plane of Charon's orbit. The brightness of Pluto changes regularly with a period of 6.4 days. Therefore, Pluto rotates synchronously with Charon and has large spots on the surface. In relation to the size of the planet, Charon is very large. Pluto-Charon is often referred to as a "double planet". At one time, Pluto was considered an "escaped" satellite of Neptune, but after the discovery of Charon, this looks unlikely.

PLANETS: COMPARATIVE ANALYSIS

Internal structure.

The objects of the solar system in terms of their internal structure can be divided into 4 categories: 1) comets, 2) small bodies, 3) terrestrial planets, 4) gas giants. Comets are simple icy bodies with a special composition and history. The category of small bodies includes all other celestial objects with radii less than 200 km: interplanetary dust grains, particles of planetary rings, small satellites and most asteroids. During the evolution of the solar system, they all lost the heat released during primary accretion and cooled down, not having sufficient size to heat up due to the radioactive decay taking place in them. Earth-type planets are very diverse: from the "iron" Mercury to the mysterious ice system Pluto-Charon. In addition to the largest planets, the Sun is sometimes classified as a gas giant.

The most important parameter that determines the composition of the planet is the average density (total mass divided by total volume). Its value immediately indicates what the planet is - "stone" (silicates, metals), "ice" (water, ammonia, methane) or "gas" (hydrogen, helium). Although the surfaces of Mercury and the Moon are strikingly similar, their internal composition is completely different, since the average density of Mercury is 1.6 times higher than that of the Moon. At the same time, the mass of Mercury is small, which means that its high density is mainly due not to the compression of matter under the action of gravity, but to a special chemical composition: Mercury contains 60–70% of metals and 30–40% of silicates by mass. The metal content per unit mass of Mercury is significantly higher than that of any other planet.

Venus rotates so slowly that its equatorial swelling is measured only in fractions of a meter (at the Earth - 21 km) and cannot at all tell anything about the internal structure of the planet. Its gravitational field correlates with the topography of the surface, in contrast to the Earth, where the continents "float". It is possible that the continents of Venus are fixed by the rigidity of the mantle, but it is possible that the topography of Venus is dynamically maintained by vigorous convection in its mantle.

The surface of the Earth is much younger than the surfaces of other bodies in the solar system. The reason for this is mainly the intensive processing of the crust material as a result of plate tectonics. Erosion under the action of liquid water also has a noticeable effect. The surfaces of most planets and moons are dominated by ring structures associated with impact craters or volcanoes; on Earth, plate tectonics has caused its major uplands and lowlands to be linear. An example is mountain ranges that rise where two plates collide; oceanic trenches that mark places where one plate goes under another (subduction zones); as well as mid-ocean ridges in those places where two plates diverge under the action of young crust emerging from the mantle (spreading zone). Thus, the relief of the earth's surface reflects the dynamics of its interior.

Small samples of the Earth's upper mantle become available for laboratory study when they rise to the surface as part of igneous rocks. Ultramafic inclusions are known (ultrabasic, poor in silicates and rich in Mg and Fe), containing minerals that form only at high pressure (for example, diamond), as well as paired minerals that can coexist only if they were formed at high pressure. These inclusions made it possible to estimate with sufficient accuracy the composition of the upper mantle down to a depth of approx. 200 km. The mineralogical composition of the deep mantle is not well known, since there are no accurate data on the temperature distribution with depth yet, and the main phases of deep minerals have not been reproduced in the laboratory. The Earth's core is divided into outer and inner. The outer core does not transmit transverse seismic waves, therefore, it is liquid. However, at a depth of 5200 km, the core matter again begins to conduct transverse waves, but at a low speed; this means that the inner core is partially "frozen". The density of the core is lower than that of a pure iron-nickel liquid, probably due to the admixture of sulfur.

A quarter of the Martian surface is occupied by the Tharsis Hill, which has risen by 7 km relative to the average radius of the planet. It is on it that most volcanoes are located, during the formation of which lava spread over a long distance, which is typical for molten rocks rich in iron. One of the reasons for the huge size of Martian volcanoes (the largest in the solar system) is that, unlike Earth, Mars does not have plates moving relative to hot pockets in the mantle, so volcanoes take a long time to grow in one place. Mars has no magnetic field and no seismic activity has been detected. There were many iron oxides in its soil, which indicates a weak differentiation of the interior.

Internal warmth.

Many planets radiate more heat than they receive from the sun. The amount of heat generated and stored in the bowels of the planet depends on its history. For an emerging planet, meteorite bombardment is the main source of heat; then heat is released during the differentiation of the interior, when the densest components, such as iron and nickel, settle towards the center and form the core. Jupiter, Saturn and Neptune (but not Uranus for some reason) are still radiating the heat they stored up when they formed 4.6 billion years ago. For terrestrial planets, an important source of heating in the present era is the decay of radioactive elements - uranium, thorium and potassium - which were included in a small amount in the original chondrite (solar) composition. The dissipation of motion energy in tidal deformations - the so-called "tidal dissipation" - is the main source of heating of Io and plays a significant role in the evolution of some planets, the rotation of which (for example, Mercury) was slowed down by tides.

Convection in the mantle.

If the liquid is heated strongly enough, convection develops in it, since thermal conductivity and radiation cannot cope with the heat flux supplied locally. It may seem strange to say that the interiors of terrestrial planets are covered by convection, like a liquid. Don't we know that, according to seismological data, transverse waves propagate in the earth's mantle and, consequently, the mantle does not consist of liquid, but of solid rocks? But let's take ordinary glass putty: with slow pressure it behaves like a viscous liquid, with sharp pressure it behaves like an elastic body, and with impact it behaves like a stone. This means that in order to understand how matter behaves, we must take into account on what time scale processes occur. Transverse seismic waves pass through the bowels of the earth in minutes. On a geologic time scale measured in millions of years, rocks deform plastically if significant stress is constantly applied to them.

It is amazing that the earth's crust is still straightening, returning to its former form, which it had before the last glaciation, which ended 10,000 years ago. Having studied the age of the uplifted shores of Scandinavia, N. Haskel calculated in 1935 that the viscosity of the earth's mantle is 10 23 times greater than the viscosity of liquid water. But even at the same time, mathematical analysis shows that the earth's mantle is in a state of intense convection (such a movement of the earth's interior could be seen in an accelerated movie, where a million years pass in a second). Similar calculations show that Venus, Mars and, to a lesser extent, Mercury and the Moon also probably have convective mantles.

We are just beginning to unravel the nature of convection in gas giant planets. It is known that convective motions are strongly influenced by the rapid rotation that exists in giant planets, but it is very difficult to experimentally study convection in a rotating sphere with a central attraction. So far, the most accurate experiments of this kind have been carried out in microgravity in near-Earth orbit. These experiments, together with theoretical calculations and numerical models, showed that convection occurs in tubes stretched along the axis of rotation of the planet and bent in accordance with its sphericity. Such convective cells are called "bananas" because of their shape.

The pressure of the gas giant planets varies from 1 bar at the level of the cloud tops to about 50 Mbar in the center. Therefore, their main component - hydrogen - resides at different levels in different phases. At pressures above 3 Mbar, ordinary molecular hydrogen becomes a liquid metal similar to lithium. Calculations show that Jupiter is mainly composed of metallic hydrogen. And Uranus and Neptune, apparently, have an extended mantle of liquid water, which is also a good conductor.

A magnetic field.

The external magnetic field of the planet carries important information about the movement of its interior. It is the magnetic field that sets the reference frame in which the wind speed is measured in the cloudy atmosphere of the giant planet; it indicates that powerful flows exist in the liquid metal core of the Earth, and active mixing takes place in the water mantles of Uranus and Neptune. On the contrary, the absence of a strong magnetic field in Venus and Mars imposes restrictions on their internal dynamics. Among the terrestrial planets, the Earth's magnetic field has an outstanding intensity, indicating an active dynamo effect. The absence of a strong magnetic field at Venus does not mean that its core has solidified: most likely, the slow rotation of the planet prevents the dynamo effect.

Uranus and Neptune have the same magnetic dipoles with a large inclination to the axes of the planets and a shift relative to their centers; this indicates that their magnetism originates in the mantles and not in the cores. Jupiter's moons Io, Europa and Ganymede have their own magnetic fields, while Callisto does not. Remaining magnetism found in the moon.

Atmosphere.

The Sun, eight of the nine planets, and three of the sixty-three satellites have an atmosphere. Each atmosphere has its own special chemical composition and behavior called "weather". Atmospheres are divided into two groups: for terrestrial planets, the dense surface of the continents or the ocean determines the conditions at the lower boundary of the atmosphere, and for gas giants, the atmosphere is practically bottomless.

For terrestrial planets, a thin (0.1 km) layer of the atmosphere near the surface constantly experiences heating or cooling from it, and during movement - friction and turbulence (due to uneven terrain); this layer is called the surface or boundary layer. Near the surface, molecular viscosity seems to "glue" the atmosphere to the ground, so even a light breeze creates a strong vertical velocity gradient that can cause turbulence. The change in air temperature with height is controlled by convective instability, since from below the air is heated from a warm surface, becomes lighter and floats; as it rises into areas of low pressure, it expands and radiates heat into space, causing it to cool, become denser, and sink. As a result of convection, an adiabatic vertical temperature gradient is established in the lower layers of the atmosphere: for example, in the Earth's atmosphere, the air temperature decreases with height by 6.5 K/km. This situation exists up to the tropopause (Greek "tropo" - turn, "pause" - termination), limiting the lower layer of the atmosphere, called the troposphere. It is here that the changes that we call the weather occur. Near the Earth, the tropopause passes at altitudes of 8–18 km; at the equator it is 10 km higher than at the poles. Due to the exponential decrease in density with height, 80% of the mass of the Earth's atmosphere is enclosed in the troposphere. It also contains almost all the water vapor, and hence the clouds that create the weather.

On Venus, carbon dioxide and water vapor, along with sulfuric acid and sulfur dioxide, absorb nearly all infrared radiation emitted from the surface. This causes a strong greenhouse effect, i.e. leads to the fact that the surface temperature of Venus is 500 K higher than that which it would have in an atmosphere transparent to infrared radiation. The main "greenhouse" gases on Earth are water vapor and carbon dioxide, which raise the temperature by 30 K. On Mars, carbon dioxide and atmospheric dust cause a weak greenhouse effect of only 5 K. The hot surface of Venus prevents the release of sulfur from the atmosphere by binding it to the surface rocks. The lower atmosphere of Venus is enriched with sulfur dioxide, so there is a dense layer of sulfuric acid clouds in it at altitudes from 50 to 80 km. An insignificant amount of sulfur-containing substances is also found in the earth's atmosphere, especially after powerful volcanic eruptions. Sulfur has not been recorded in the atmosphere of Mars, therefore, its volcanoes are inactive in the current epoch.

On Earth, a stable decrease in temperature with height in the troposphere changes above the tropopause to an increase in temperature with height. Therefore, there is an extremely stable layer called the stratosphere (lat. stratum - layer, flooring). The existence of permanent thin aerosol layers and the long stay there of radioactive elements from nuclear explosions are direct evidence of the absence of mixing in the stratosphere. In the terrestrial stratosphere, the temperature continues to rise with height up to the stratopause, passing at an altitude of approx. 50 km. The source of heat in the stratosphere is the photochemical reactions of ozone, the concentration of which is maximum at an altitude of approx. 25 km. Ozone absorbs ultraviolet radiation, so below 75 km almost all of it is converted into heat. The chemistry of the stratosphere is complex. Ozone is mainly formed over the equatorial regions, but its highest concentration is found over the poles; this indicates that the ozone content is influenced not only by chemistry, but also by the dynamics of the atmosphere. Mars also has higher ozone concentrations over the poles, especially over the winter pole. The dry atmosphere of Mars has relatively few hydroxyl radicals (OH) that deplete ozone.

The temperature profiles of the atmospheres of the giant planets are determined from ground-based observations of planetary occultations of stars and from probe data, in particular, from the attenuation of radio signals when the probe enters the planet. Each planet has a tropopause and a stratosphere, above which lie the thermosphere, exosphere, and ionosphere. The temperature of the thermospheres of Jupiter, Saturn and Uranus, respectively, is approx. 1000, 420 and 800 K. The high temperature and relatively low gravity on Uranus allow the atmosphere to extend to the rings. This causes deceleration and rapid fall of dust particles. Since there are still dust lanes in the rings of Uranus, there must be a source of dust there.

Although the temperature structure of the troposphere and stratosphere in the atmospheres of different planets has much in common, their chemical composition is very different. The atmospheres of Venus and Mars are mostly carbon dioxide, but they represent two extreme examples of atmospheric evolution: Venus has a dense and hot atmosphere, while Mars has a cold and rarefied one. It is important to understand whether the earth's atmosphere will eventually come to one of these two types, and whether these three atmospheres have always been so different.

The fate of the original water on the planet can be determined by measuring the content of deuterium in relation to the light isotope of hydrogen: the D / H ratio imposes a limit on the amount of hydrogen leaving the planet. The mass of water in the atmosphere of Venus is now 10 -5 of the mass of the Earth's oceans. But the D/H ratio on Venus is 100 times higher than on Earth. If at first this ratio was the same on Earth and Venus and the water reserves on Venus were not replenished during its evolution, then a hundredfold increase in the D/H ratio on Venus means that once there was a hundred times more water on Venus than now. The explanation for this is usually sought in the framework of the "greenhouse volatilization" theory, which states that Venus was never cold enough for water to condense on its surface. If water always filled the atmosphere in the form of steam, then the photodissociation of water molecules led to the release of hydrogen, the light isotope of which escaped from the atmosphere into space, and the remaining water was enriched with deuterium.

Of great interest is the strong difference between the atmospheres of Earth and Venus. It is believed that the modern atmospheres of terrestrial planets were formed as a result of degassing of the bowels; in this case, water vapor and carbon dioxide were mainly released. On Earth, water was concentrated in the ocean, and carbon dioxide was bound in sedimentary rocks. But Venus is closer to the Sun, it is hot there and there is no life; so carbon dioxide remained in the atmosphere. Water vapor under the action of sunlight dissociated into hydrogen and oxygen; hydrogen escaped into space (the earth's atmosphere also quickly loses hydrogen), and oxygen turned out to be bound in rocks. True, the difference between these two atmospheres may turn out to be deeper: there is still no explanation for the fact that there is much more argon in the atmosphere of Venus than in the atmosphere of the Earth.

The surface of Mars is now a cold and dry desert. During the warmest part of the day, the temperature can be slightly above the normal freezing point of water, but the low atmospheric pressure does not allow the water on the surface of Mars to be in a liquid state: the ice immediately turns into steam. However, there are several canyons on Mars that resemble dry riverbeds. Some of them appear to be cut by short-term but catastrophically powerful water currents, while others show deep ravines and an extensive network of valleys, indicating the likely long-term existence of lowland rivers in the early periods of Mars' history. There are also morphological indications that the old craters of Mars are destroyed by erosion much more than the young ones, and this is possible only if the atmosphere of Mars was much denser than now.

In the early 1960s, the polar caps of Mars were thought to be composed of water ice. But in 1966, R. Leighton and B. Murray considered the heat balance of the planet and showed that carbon dioxide should condense in large quantities at the poles, and a balance of solid and gaseous carbon dioxide should be maintained between the polar caps and the atmosphere. It is curious that the seasonal growth and reduction of the polar caps lead to pressure fluctuations in the Martian atmosphere by 20% (for example, in the cabins of old jet liners, pressure drops during takeoff and landing were also about 20%). Space photographs of the Martian polar caps show amazing spiral patterns and stepped terraces that the Mars Polar Lander (1999) probe was supposed to explore, but it suffered a landing failure.

It is not known exactly why the pressure of the Martian atmosphere dropped so much, probably from a few bar in the first billion years to 7 mbar now. It is possible that the weathering of surface rocks removed carbon dioxide from the atmosphere, sequestering carbon in carbonate rocks, as happened on Earth. At a surface temperature of 273 K, this process could destroy the carbon dioxide atmosphere of Mars with a pressure of several bar in just 50 million years; it has obviously proved very difficult to maintain a warm and humid climate on Mars throughout the history of the solar system. A similar process also affects the carbon content in the earth's atmosphere. About 60 bar of carbon is now bound in the earth's carbonate rocks. Obviously, in the past, the earth's atmosphere contained much more carbon dioxide than now, and the temperature of the atmosphere was higher. The main difference between the evolution of the atmosphere of Earth and Mars is that on Earth, plate tectonics supports the carbon cycle, while on Mars it is "locked" in rocks and polar caps.

circumplanetary rings.

It is curious that each of the giant planets has ring systems, but not a single terrestrial planet has. Those who first look at Saturn through a telescope often exclaim: “Well, just like in the picture!”, Seeing its amazingly bright and clear rings. However, the rings of the remaining planets are almost invisible in a telescope. Jupiter's pale ring is experiencing a mysterious interaction with its magnetic field. Uranus and Neptune are surrounded by several thin rings each; the structure of these rings reflects their resonant interaction with nearby satellites. The three annular arcs of Neptune are especially intriguing to researchers, since they are clearly limited both in the radial and azimuthal directions.

A big surprise was the discovery of the narrow rings of Uranus during the observation of its coverage of a star in 1977. The fact is that there are many phenomena that could noticeably expand narrow rings in just a few decades: these are mutual collisions of particles, the Poynting-Robertson effect (radiative braking) and plasma braking. From a practical point of view, narrow rings, whose position can be measured with high accuracy, have turned out to be a very convenient indicator of the orbital motion of particles. The precession of Uranus' rings made it possible to elucidate the distribution of mass within the planet.

Those who have had to drive a car with a dusty windshield towards the rising or setting sun know that dust particles strongly scatter light in the direction it falls. That is why it is difficult to detect dust in planetary rings by observing them from the Earth, i.e. from the side of the sun. But every time the space probe flew past the outer planet and "looked" back, we got images of the rings in transmitted light. In such images of Uranus and Neptune, previously unknown dust rings were discovered, which are much wider than the narrow rings known for a long time.

Rotating disks are the most important topic of modern astrophysics. Many dynamical theories developed to explain the structure of galaxies can also be used to study planetary rings. Thus, the rings of Saturn have become an object for testing the theory of self-gravitating disks. The self-gravity property of these rings is indicated by the presence of both helical density waves and helical bending waves in them, which are visible in the detailed images. The wave packet found in Saturn's rings has been attributed to the planet's strong horizontal resonance with its moon Iapetus, which excites spiral density waves in the outer Cassini division.

Many conjectures have been made about the origin of the rings. It is important that they lie inside the Roche zone, i.e. at such a distance from the planet where the mutual attraction of particles is less than the difference in the forces of attraction between them by the planet. Inside the Roche zone, scattered particles cannot form a satellite of the planet. Perhaps the substance of the rings has remained “unclaimed” since the formation of the planet itself. But perhaps these are traces of a recent catastrophe - a collision of two satellites or the destruction of a satellite by the tidal forces of the planet. If you collect all the substance of the rings of Saturn, you get a body with a radius of approx. 200 km. In the rings of other planets, there is much less substance.

SMALL BODIES OF THE SOLAR SYSTEM

Asteroids.

Many small planets - asteroids - revolve around the Sun mainly between the orbits of Mars and Jupiter. Astronomers took the name "asteroid" because in a telescope they look like faint stars ( aster Greek for "star"). At first they thought that these were fragments of a large planet that once existed, but then it became clear that asteroids never formed a single body; most likely, this substance could not unite into a planet due to the influence of Jupiter. According to estimates, the total mass of all asteroids in our era is only 6% of the mass of the Moon; half of this mass is contained in the three largest - 1 Ceres, 2 Pallas and 4 Vesta. The number in the asteroid designation indicates the order in which it was discovered. Asteroids with precisely known orbits are assigned not only serial numbers, but also names: 3 Juno, 44 ​​Nisa, 1566 Icarus. The exact elements of the orbits of more than 8,000 asteroids out of 33,000 discovered to date are known.

There are at least two hundred asteroids with a radius of more than 50 km and about a thousand - more than 15 km. About a million asteroids are estimated to have a radius greater than 0.5 km. The largest of them is Ceres, a rather dark and difficult object to observe. Special methods of adaptive optics are required to distinguish surface details of even large asteroids using ground-based telescopes.

The radii of the orbits of most asteroids are between 2.2 and 3.3 AU, this region is called the "asteroid belt". But it is not entirely filled with asteroid orbits: at distances of 2.50, 2.82 and 2.96 AU. They are not here; these "windows" were formed under the influence of perturbations from Jupiter. All asteroids orbit in the forward direction, but the orbits of many of them are noticeably elongated and tilted. Some asteroids have very curious orbits. Yes, the group Troyantsev moves in the orbit of Jupiter; most of these asteroids are very dark and red. The asteroids of the Amur group have orbits that fit or cross the orbit of Mars; among them 433 Eros. Asteroids of the Apollo group cross the Earth's orbit; among them 1533 Icarus, closest to the Sun. Obviously, sooner or later, these asteroids experience a dangerous approach to the planets, which ends in a collision or a serious change in orbit. Finally, asteroids of the Aton group have recently been singled out as a special class, the orbits of which lie almost entirely within the orbit of the Earth. They are all very small.

The brightness of many asteroids changes periodically, which is natural for rotating irregular bodies. Their rotation periods range from 2.3 to 80 hours and are close to 9 hours on average. Asteroids owe their irregular shape to numerous mutual collisions. Examples of an exotic form are 433 Eros and 643 Hector, in which the ratio of the lengths of the axes reaches 2.5.

In the past, the entire interior of the solar system was likely similar to the main asteroid belt. Jupiter, located near this belt, strongly disturbs the movement of asteroids with its attraction, increasing their speed and leading to a collision, and this more often destroys than unites them. Like an unfinished planet, the asteroid belt gives us a unique opportunity to see parts of the structure before they disappear inside the finished body of the planet.

By studying the light reflected by asteroids, it is possible to learn a lot about the composition of their surface. Most asteroids, on the basis of their reflectance and color, are assigned to three groups similar to meteorite groups: asteroids of the type C have a dark surface like carbonaceous chondrites ( see below Meteorites), type S brighter and redder, and type M similar to iron-nickel meteorites. For example, 1 Ceres is similar to carbonaceous chondrites, and 4 Vesta is similar to basalt eukrites. This indicates that the origin of meteorites is associated with the asteroid belt. The surface of asteroids is covered with finely crushed rock - regolith. It is rather strange that it is kept on the surface after the impact of meteorites - after all, a 20-km asteroid has a gravity of 10 -3 g, and the speed of leaving the surface is only 10 m / s.

In addition to color, many characteristic infrared and ultraviolet spectral lines are now known to be used to classify asteroids. According to these data, 5 main classes are distinguished: A, C, D, S And T. Asteroids 4 Vesta, 349 Dembowska and 1862 Apollo did not fit into this classification: each of them occupied a special position and became the prototype of new classes, respectively. V, R And Q, which now contains other asteroids. From a large group WITH-asteroids further distinguished classes B, F And G. The modern classification has 14 types of asteroids, designated (in decreasing order of the number of members) by letters S, C, M, D, F, P, G, E, B, T, A, V, Q, R. Because the albedo WITH- asteroids lower than S-asteroids, observational selection occurs: dark WITH-Asteroids are harder to detect. With this in mind, the most numerous type is precisely WITH- asteroids.

From a comparison of the spectra of asteroids of various types with the spectra of samples of pure minerals, three large groups were formed: primitive ( C, D, P, Q), metamorphic ( F, G, B, T) and magmatic ( S, M, E, A,V, R). The surface of primitive asteroids is rich in carbon and water; metamorphic ones contain less water and volatiles than primitive ones; igneous are covered with complex minerals, probably formed from the melt. The inner region of the main asteroid belt is richly populated by igneous asteroids, metamorphic asteroids predominate in the middle part of the belt, and primitive asteroids predominate on the periphery. This indicates that during the formation of the solar system, there was a sharp temperature gradient in the asteroid belt.

The classification of asteroids based on their spectra groups the bodies according to their surface composition. But if we consider the elements of their orbits (the semi-major axis, eccentricity, inclination), then the dynamic families of asteroids are distinguished, first described by K. Hirayama in 1918. The most populated of them are the families of Themis, Eos and Coronids. Probably, each family is a swarm of fragments of a relatively recent collision. A systematic study of the solar system leads us to understand that major collisions are the rule rather than the exception, and that the Earth is also not immune to them.

Meteorites.

A meteoroid is a small body that revolves around the sun. A meteor is a meteoroid that flew into the atmosphere of the planet and became red-hot to a shine. And if its remnant fell to the surface of the planet, it is called a meteorite. A meteorite is considered "fallen" if there are eyewitnesses who observed its flight in the atmosphere; otherwise, it is called "found".

There are much more “found” meteorites than “fallen” meteorites. Often they are found by tourists or peasants working in the field. Since meteorites are dark in color and easily visible in the snow, the Antarctic ice fields, where thousands of meteorites have already been found, are an excellent place to look for them. For the first time, a meteorite in Antarctica was discovered in 1969 by a group of Japanese geologists who studied glaciers. They found 9 fragments lying side by side, but belonging to four different types of meteorites. It turned out that meteorites that fell on the ice in different places gather where the ice fields moving at a speed of several meters per year stop, resting on mountain ranges. The wind destroys and dries the upper layers of ice (dry sublimation occurs - ablation), and meteorites concentrate on the surface of the glacier. Such ice has a bluish color and is easily distinguishable from the air, which is what scientists use when studying places promising for collecting meteorites.

An important meteorite fall occurred in 1969 in Chihuahua (Mexico). The first of many large fragments was found near a house in the village of Pueblito de Allende, and, following tradition, all found fragments of this meteorite were united under the name Allende. The fall of the Allende meteorite coincided with the start of the Apollo lunar program and gave scientists the opportunity to work out methods for analyzing extraterrestrial samples. In recent years, some meteorites containing white fragments embedded in darker parent rock have been found to be lunar fragments.

The Allende meteorite belongs to chondrites, an important subgroup of stony meteorites. They are called so because they contain chondrules (from the Greek chondros, seed) - the oldest spherical particles that condensed in a protoplanetary nebula and then became part of later rocks. Such meteorites make it possible to estimate the age of the solar system and its initial composition. The inclusions of the Allende meteorite rich in calcium and aluminum, which were the first to condense due to their high boiling point, have an age measured from radioactive decay of 4.559 ± 0.004 billion years. This is the most accurate estimate of the age of the solar system. In addition, all meteorites carry "historical records" caused by the long-term influence of galactic cosmic rays, solar radiation and solar wind on them. By examining the damage caused by cosmic rays, we can tell how long the meteorite stayed in orbit before it fell under the protection of the earth's atmosphere.

A direct relationship between meteorites and the Sun follows from the fact that the elemental composition of the oldest meteorites - chondrites - exactly repeats the composition of the solar photosphere. The only elements whose content differs are volatiles, such as hydrogen and helium, which evaporated abundantly from meteorites during their cooling, as well as lithium, which was partially “burned out” on the Sun in nuclear reactions. The terms "solar composition" and "chondrite composition" are used interchangeably when describing the "recipe for solar matter" mentioned above. Stone meteorites, the composition of which differs from the sun, are called achondrites.

Small shards.

The near-solar space is filled with small particles, the sources of which are the collapsing nuclei of comets and collisions of bodies, mainly in the asteroid belt. The smallest particles gradually approach the Sun as a result of the Poynting-Robertson effect (it consists in the fact that the pressure of sunlight on a moving particle is not directed exactly along the Sun-particle line, but as a result of light aberration it is deflected back and therefore slows down the movement of the particle). The fall of small particles on the Sun is compensated by their constant reproduction, so that in the plane of the ecliptic there is always an accumulation of dust that scatters the sun's rays. On the darkest nights it is visible as zodiacal light, stretching in a wide band along the ecliptic in the west after sunset and in the east before sunrise. Near the Sun, zodiacal light passes into a false corona ( F-crown, from false - false), which is visible only during a total eclipse. With an increase in the angular distance from the Sun, the brightness of the zodiacal light rapidly decreases, but at the antisolar point of the ecliptic it increases again, forming a counterradiance; this is due to the fact that small dust particles intensively reflect light back.

From time to time, meteoroids enter the Earth's atmosphere. The speed of their movement is so high (on average 40 km/s) that almost all of them, except for the smallest and largest ones, burn out at an altitude of about 110 km, leaving long luminous tails - meteors, or shooting stars. Many meteoroids are associated with the orbits of individual comets, so meteors are observed more often when the Earth passes near such orbits at certain times of the year. For example, there are many meteors around August 12 each year as the Earth crosses the Perseid shower associated with particles lost by comet 1862 III. Another stream, the Orionids, around October 20 is associated with dust from Halley's comet.

Particles smaller than 30 microns can slow down in the atmosphere and fall to the ground without being burned; such micrometeorites are collected for laboratory analysis. If particles of a few centimeters or more in size consist of a sufficiently dense substance, then they also do not burn out completely and fall to the Earth's surface in the form of meteorites. More than 90% of them are stone; only a specialist can distinguish them from terrestrial rocks. The remaining 10% of meteorites are iron (in fact, they are composed of an alloy of iron and nickel).

Meteorites are considered fragments of asteroids. Iron meteorites were once in the composition of the nuclei of these bodies, destroyed by collisions. It is possible that some loose and volatile meteorites originated from comets, but this is unlikely; most likely, large particles of comets burn up in the atmosphere, and only small ones remain. Considering how difficult it is for comets and asteroids to reach the Earth, it is clear how useful it is to study meteorites that independently "arrived" on our planet from the depths of the solar system.

Comets.

Usually comets come from the far periphery of the solar system and for a short time become extremely spectacular luminaries; at this time they attract general attention, but much of their nature is still unclear. A new comet usually appears unexpectedly, and therefore it is almost impossible to prepare a space probe to meet it. Of course, you can slowly prepare and send a probe to meet with one of the hundreds of periodic comets whose orbits are well known; but all these comets, which have repeatedly approached the Sun, have already grown old, have almost completely lost their volatile substances and have become pale and inactive. Only one periodic comet is still active - Halley's Comet. Her 30 appearances have been regularly recorded since 240 BC. and named the comet in honor of the astronomer E. Halley, who predicted its appearance in 1758.

Comet Halley has an orbital period of 76 years, a perihelion distance of 0.59 AU. and aphelion 35 AU When in March 1986 it crossed the plane of the ecliptic, an armada of spacecraft with fifty scientific instruments rushed to meet it. Particularly important results were obtained by two Soviet probes "Vega" and the European "Giotto", which for the first time transmitted images of a cometary nucleus. They show a very uneven surface covered with craters, and two gas jets gushing on the sunny side of the core. The nucleus of Halley's comet was larger than expected; its surface, reflecting only 4% of the incident light, is one of the darkest in the solar system.

About ten comets are observed per year, of which only a third have been discovered earlier. They are often classified according to the length of their orbital period: short-period (3 P P P

In recent years, a rather rich population of the solar system has been discovered, stretching in the form of a disk just beyond the orbits of the giant planets; it is called the Kuiper Belt see below). It may also contain many comet nuclei.

It is customary to distinguish three parts of a comet: a small (1-10 km) solid core, a gas-dust cloud surrounding it - a head or coma, about 100 thousand km in size, and a tail stretching from it for about 100 million km, directed from the Sun . The nucleus of a comet is an icy body with an admixture of solid rocks. As it approaches the Sun, the core heats up, and the gas streams leaving its surface carry away dust and ice particles that form the comet's head. In the spectrum of the head, bands of molecules and radicals CN, CH, NH, OH, C 2 , C 3 are usually visible, representing "fragments" of more complex core molecules destroyed by solar radiation. Some molecules are ionized and begin to actively interact with the solar wind, forming a plasma or ion tail (type I); its spectrum shows emission lines of CO + , OH + and N 2 + ions. Dust particles form a curved dust tail (type II), the spectrum of which is scattered sunlight.

As the gases evaporate, the comet's nucleus also loses fine dust, but it is not clear whether it leaves behind larger debris. It is also interesting what is the fate of the core after the loss of all volatile substances: does it become like an ordinary asteroid? It is curious that the small asteroids of the Apollo group move in elongated orbits, very reminiscent of the orbits of short-period comets.

Search for planets in the solar system.

More than once, assumptions have been made about the possibility of the existence of a planet closer to the Sun than Mercury. Le Verrier (1811–1877), who predicted the discovery of Neptune, investigated anomalies in the movement of the perihelion of Mercury's orbit and, on the basis of this, predicted the existence of a new unknown planet inside its orbit. Soon there was a message about her observation and the planet was even given a name - Vulcan. But the discovery was not confirmed.

In 1977, the American astronomer Cowell discovered a very faint object, which was dubbed the "tenth planet". But the object turned out to be too small for the planet (about 200 km). It was named Chiron and attributed to the asteroids, among which it was then the most distant: the aphelion of its orbit was removed by 18.9 AU. and almost touches the orbit of Uranus, and the perihelion lies just beyond the orbit of Saturn at a distance of 8.5 AU. from the sun. With an orbital inclination of just 7°, it can indeed come close to Saturn and Uranus. Calculations show that such an orbit is unstable: Chiron will either collide with the planet or be ejected from the solar system.

From time to time, theoretical predictions about the existence of large planets beyond the orbit of Pluto are published, but so far they have not been confirmed. Analysis of cometary orbits shows that up to a distance of 75 AU. There are no planets larger than Earth beyond Pluto. However, the existence of a large number of small planets in this area is quite possible, which are not easy to detect. The existence of this cluster of non-Neptunian bodies has long been suspected and even received the name - the Kuiper belt, after the famous American planetary explorer. However, it was only recently that the first objects were found in it. In 1992–1994, 17 minor planets were discovered beyond the orbit of Neptune. Of these, 8 move at distances of 40–45 AU. from the Sun, i.e. even beyond the orbit of Pluto.

Due to their great distance, the brightness of these objects is extremely weak; only the largest telescopes in the world are suitable for their search. Therefore, only about 3 square degrees of the celestial sphere have been systematically examined so far, i.e. 0.01% of its area. Therefore, it is expected that beyond the orbit of Neptune there may be tens of thousands of objects similar to those discovered, and millions of smaller ones, with a diameter of 5–10 km. Judging by estimates, this cluster of small bodies is hundreds of times more massive than the asteroid belt located between Jupiter and Mars, but inferior in mass to the giant cometary Oort cloud.

Objects beyond Neptune are still difficult to attribute to any class of small bodies in the solar system - to asteroids or comet nuclei. The newly discovered bodies are 100–200 km in size and have a rather red surface, indicating its ancient composition and the possible presence of organic compounds. Bodies of the "Kuiper belt" have recently been discovered quite often (by the end of 1999, about 200 of them had been discovered). Some planetary scientists believe that it would be more correct to call Pluto not "the smallest planet", but "the largest body of the Kuiper belt."

OTHER PLANETARY SYSTEMS

From modern views on the formation of stars, it follows that the birth of a star of the solar type must be accompanied by the formation of a planetary system. Even if this applies only to stars that are completely similar to the Sun (i.e., single stars of the spectral class G), then in this case at least 1% of the stars in the Galaxy (and this is about 1 billion stars) should have planetary systems. A more detailed analysis shows that planets can be cooler than the spectral type for all stars. F, and even in binary systems.

Indeed, in recent years there have been reports of the discovery of planets around other stars. At the same time, the planets themselves are not visible: their presence is detected by the slight movement of the star, caused by its attraction to the planet. The planet's orbital motion causes the star to "wiggle" and its radial velocity to change periodically, which can be measured from the position of the lines in the star's spectrum (the Doppler effect). By the end of 1999, the discovery of Jupiter-type planets around 30 stars was reported, including 51 Peg, 70 Vir, 47 UMa, 55 Cnc, t Boo, u And, 16 Cyg, etc. All these are stars close to the Sun, and the distance to the nearest of them (Gliese 876) is only 15 sv. years. Two radio pulsars (PSR 1257+12 and PSR B1628–26) also have systems of planets with masses on the order of the Earth's. It is not yet possible to notice such light planets in normal stars with the help of optical technology.

Around each star, you can specify the ecosphere, in which the surface temperature of the planet allows the existence of liquid water. The solar ecosphere extends from 0.8 to 1.1 AU. It contains the Earth, but Venus (0.72 AU) and Mars (1.52 AU) do not fall. Probably, in any planetary system, no more than 1–2 planets fall into the ecosphere, on which conditions are favorable for life.

DYNAMICS OF ORBITAL MOTION

The motion of the planets with high accuracy obeys the three laws of I. Kepler (1571–1630), which he derived from observations:

1) The planets move in ellipses, in one of the focuses of which is the Sun.

2) The radius-vector connecting the Sun and the planet sweeps out equal areas in equal time intervals of the planet's orbit.

3) The square of the orbital period is proportional to the cube of the semi-major axis of the elliptical orbit.

Kepler's second law follows directly from the law of conservation of angular momentum and is the most general of the three. Newton found that Kepler's first law is valid if the force of attraction between two bodies is inversely proportional to the square of the distance between them, and the third law - if this force is also proportional to the masses of the bodies. In 1873, J. Bertrand proved that in general only in two cases the bodies will not move one around the other in a spiral: if they are attracted according to Newton's inverse square law or according to Hooke's direct proportionality law (which describes the elasticity of springs). A remarkable property of the solar system is that the mass of the central star is much greater than the mass of any of the planets, so the movement of each member of the planetary system can be calculated with high accuracy within the framework of the problem of the movement of two mutually gravitating bodies - the Sun and the only planet next to it. Its mathematical solution is known: if the planet's speed is not too high, then it moves in a closed periodic orbit, which can be accurately calculated.

In 1867, D. Kirkwood was the first to note that empty places ("hatches") in the asteroid belt are located at such distances from the Sun, where the average motion is in commensurability (in integer terms) with the motion of Jupiter. In other words, asteroids avoid orbits in which the period of their revolution around the Sun would be a multiple of the period of revolution of Jupiter. The two largest hatches of Kirkwood fall on the proportions of 3:1 and 2:1. However, near the 3:2 commensurability, there is an excess of asteroids grouped according to this feature into the Gilda group. There is also an excess of asteroids of the Trojan group at a 1:1 commensurability moving in the orbit of Jupiter 60° ahead and 60° behind it. The situation with the Trojans is understandable - they are captured near the stable Lagrange points (L 4 and L 5) in the orbit of Jupiter, but how to explain the Kirkwood hatches and the Gilda group?

If there were only hatches on the commensurations, then one could accept the simple explanation proposed by Kirkwood himself that the asteroids are ejected from the resonant regions by the periodic influence of Jupiter. But now this picture seems too simple. Numerical calculations have shown that chaotic orbits penetrate regions of space near the 3:1 resonance and that asteroid fragments that fall into this region change their orbits from circular to elongated elliptical ones, regularly bringing them to the central part of the solar system. In such interplanetary orbits, meteoroids do not live long (only a few million years) before crashing into Mars or the Earth, and with a small miss, they are ejected to the periphery of the solar system. So, the main source of meteorites falling to the Earth are the Kirkwood hatches, through which the chaotic orbits of asteroid fragments pass.

Of course, there are many examples of highly ordered resonant motions in the solar system. This is exactly how satellites close to the planets move, for example, the Moon, which always faces the Earth with the same hemisphere, since its orbital period coincides with the axial one. An example of even higher synchronization is given by the Pluto-Charon system, in which not only on the satellite, but also on the planet, “a day is equal to a month”. The motion of Mercury has an intermediate character, the axial rotation and orbital circulation of which are in a resonant ratio of 3:2. However, not all bodies behave so simply: for example, in a non-spherical Hyperion, under the influence of Saturn's attraction, the axis of rotation randomly flips over.

The evolution of satellite orbits is influenced by several factors. Since the planets and satellites are not point masses, but extended objects, and, in addition, the gravitational force depends on the distance, different parts of the satellite's body, distant from the planet at different distances, are attracted to it in different ways; the same is true for the attraction acting from the side of the satellite on the planet. This difference in forces causes the tides of the sea, and gives the synchronously rotating satellites a slightly flattened shape. The satellite and the planet cause tidal deformations in each other, and this affects their orbital motion. The 4:2:1 mean motion resonance of Jupiter's moons Io, Europa, and Ganymede, first studied in detail by Laplace in his Celestial mechanics(Vol. 4, 1805), is called the Laplace resonance. Just a few days before Voyager 1's approach to Jupiter, on March 2, 1979, astronomers Peale, Cassin, and Reynolds published "Tidal Dissipation of Io," in which they predicted active volcanism on this moon due to its leading role in maintaining a 4:2:1 resonance. Voyager 1 indeed discovered active volcanoes on Io, so powerful that not a single meteorite crater is visible on the satellite’s surface images: its surface is covered with eruptions so quickly.

FORMATION OF THE SOLAR SYSTEM

The question of how the solar system formed is perhaps the most difficult in planetary science. To answer it, we still have little data that would help to restore the complex physical and chemical processes that took place in that distant era. A theory of the formation of the solar system must explain many facts, including its mechanical state, chemical composition, and isotope chronology data. In this case, it is desirable to rely on real phenomena observed near forming and young stars.

mechanical condition.

The planets revolve around the Sun in the same direction, in almost circular orbits lying almost in the same plane. Most of them rotate around their axis in the same direction as the Sun. All this indicates that the predecessor of the solar system was a rotating disk, which is naturally formed by the compression of a self-gravitating system with the conservation of angular momentum and the consequent increase in angular velocity. (The angular momentum, or angular momentum, of a planet is the product of its mass times its distance from the Sun times its orbital speed. The momentum of the Sun is determined by its axial rotation and is approximately equal to the product of its mass times its radius times its speed of rotation; the axial moments of planets are negligible.)

The sun contains 99% of the mass of the solar system, but only approx. 1% of her angular momentum. The theory should explain why most of the mass of the system is concentrated in the Sun, and the vast majority of the angular momentum is in the outer planets. The available theoretical models for the formation of the solar system indicate that the Sun initially rotated much faster than it does now. Then the angular momentum from the young Sun was transferred to the outer parts of the solar system; astronomers believe that gravitational and magnetic forces slowed down the rotation of the Sun and accelerated the movement of the planets.

For two centuries now, an approximate rule for the regular distribution of planetary distances from the Sun (the Titius-Bode rule) has been known, but there is no explanation for it. In the systems of satellites of the outer planets, the same regularities can be traced as in the planetary system as a whole; probably, the processes of their formation had much in common.

Chemical composition.

In the solar system, there is a strong gradient (difference) of chemical composition: planets and satellites close to the Sun are made of refractory materials, and there are many volatile elements in the composition of distant bodies. This means that during the formation of the solar system there was a large temperature gradient. Modern astrophysical models of chemical condensation suggest that the initial composition of the protoplanetary cloud was close to the composition of the interstellar medium and the Sun: in terms of mass, up to 75% hydrogen, up to 25% helium, and less than 1% of all other elements. These models successfully explain the observed variations in chemical composition in the solar system.

The chemical composition of distant objects can be judged on the basis of their average density, as well as the spectra of their surface and atmosphere. This could be done much more accurately by analyzing samples of planetary matter, but so far we have only samples from the Moon and meteorites. By studying meteorites, we begin to understand the chemical processes in the primordial nebula. However, the process of agglomeration of large planets from small particles is still unclear.

isotopic data.

Star formation.

Stars are born in the process of collapse (compression) of interstellar gas and dust clouds. This process has not yet been studied in detail. There are observational evidence that shock waves from supernova explosions can compress interstellar matter and stimulate the collapse of clouds into stars.

Before a young star reaches a stable state, it undergoes a stage of gravitational contraction from the protostellar nebula. Basic information about this stage of stellar evolution is obtained by studying young T Tauri stars. Apparently, these stars are still in a state of compression and their age does not exceed 1 million years. Usually their masses are from 0.2 to 2 solar masses. They show signs of strong magnetic activity. The spectra of some T Tauri stars contain forbidden lines that appear only in low-density gas; these are probably remnants of a protostellar nebula surrounding the star. T Tauri stars are characterized by rapid fluctuations in ultraviolet and X-ray radiation. Many of them have powerful infrared radiation and silicon spectral lines - this indicates that the stars are surrounded by dust clouds. Finally, T Tauri stars have powerful stellar winds. It is believed that in the early period of its evolution, the Sun also passed through the stage of T Taurus, and that it was during this period that volatile elements were forced out of the inner regions of the solar system.

Some moderate-mass forming stars show a strong increase in luminosity and shell ejection in less than a year. Such phenomena are called FU Orion flares. At least once such an outburst was experienced by a T Tauri star. It is believed that most young stars go through a FU Orionic flare stage. Many see the cause of the outburst in the fact that from time to time the rate of accretion onto the young star of matter from the gas-dust disk surrounding it increases. If the Sun also experienced one or more Orionian FU-type flares early in its evolution, this must have had a strong effect on volatiles in the central solar system.

Observations and calculations show that there are always remnants of protostellar matter in the vicinity of a forming star. It can form a companion star or a planetary system. Indeed, many stars form binary and multiple systems. But if the mass of the companion does not exceed 1% of the mass of the Sun (10 masses of Jupiter), then the temperature in its core will never reach the value necessary for the occurrence of thermonuclear reactions. Such a celestial body is called a planet.

Theories of formation.

Scientific theories for the formation of the solar system can be divided into three categories: tidal, accretionary, and nebular. The latter are currently attracting the most interest.

The tidal theory, apparently first proposed by Buffon (1707–1788), does not directly link star and planet formation. It is assumed that another star flying past the Sun, through tidal interaction, pulled out of it (or from itself) a jet of matter from which the planets were formed. This idea runs into many physical problems; for example, hot matter ejected by a star should be sprayed out, not condensed. Now the tidal theory is unpopular because it cannot explain the mechanical features of the solar system and presents its birth as a random and extremely rare event.

The accretion theory suggests that the young Sun captured the material of the future planetary system, flying through a dense interstellar cloud. Indeed, young stars are usually found near large interstellar clouds. However, within the framework of the accretion theory, it is difficult to explain the gradient of the chemical composition in the planetary system.

The nebular hypothesis proposed by Kant at the end of the 18th century is the most developed and generally accepted now. Its main idea is that the Sun and the planets formed simultaneously from a single rotating cloud. Shrinking, it turned into a disk, in the center of which the Sun was formed, and on the periphery - the planets. Note that this idea differs from Laplace's hypothesis, according to which the Sun was first formed from a cloud, and then, as it contracted, the centrifugal force tore off gas rings from the equator, which later condensed into planets. The Laplace hypothesis faces physical difficulties that have not been overcome for 200 years.

The most successful modern version of the nebular theory was created by A. Cameron and colleagues. In their model, the protoplanetary nebula was about twice as massive as the current planetary system. During the first 100 million years, the forming Sun actively ejected matter from it. Such behavior is characteristic of young stars, which are called T Tauri stars after the name of the prototype. The distribution of pressure and temperature of the nebula matter in Cameron's model is in good agreement with the gradient of the chemical composition of the solar system.

Thus, it is most likely that the Sun and the planets formed from a single, collapsing cloud. In its central part, where the density and temperature were higher, only refractory substances were preserved, and volatile substances were also preserved on the periphery; this explains the gradient of the chemical composition. According to this model, the formation of a planetary system should accompany the early evolution of all stars like the Sun.

Planet growth.

There are many scenarios for the growth of planets. Perhaps the planets formed as a result of random collisions and sticking together of small bodies called planetesimals. But, perhaps, small bodies united into larger ones at once in large groups as a result of gravitational instability. It is not clear whether the planets accumulated in a gaseous or gasless environment. In a gaseous nebula, temperature drops are smoothed out, but when part of the gas condenses into dust particles, and the remaining gas is swept away by the stellar wind, the transparency of the nebula increases sharply, and a strong temperature gradient arises in the system. It is still not entirely clear what are the characteristic times of gas condensation into dust particles, accumulation of dust grains in planetesimals, and accretion of planetesimals into planets and their satellites.

LIFE IN THE SOLAR SYSTEM

It has been suggested that life in the solar system once existed beyond the Earth, and perhaps exists now. The advent of space technology made it possible to begin direct testing of this hypothesis. Mercury was too hot and devoid of atmosphere and water. Venus is also very hot - lead is melted on its surface. The possibility of life in the upper cloud layer of Venus, where conditions are much milder, is nothing more than a fantasy. The moon and asteroids look completely sterile.

Great hopes were pinned on Mars. Seen through a telescope 100 years ago, systems of thin straight lines - "channels" - then gave reason to talk about artificial irrigation facilities on the surface of Mars. But now we know that the conditions on Mars are unfavorable for life: cold, dry, very rarefied air and, as a result, strong ultraviolet radiation from the Sun, sterilizing the surface of the planet. Instruments of the Viking landing blocks did not detect organic matter in the soil of Mars.

True, there are signs that the climate of Mars has changed significantly and may once have been more favorable for life. It is known that in the distant past there was water on the surface of Mars, since detailed images of the planet show traces of water erosion, reminiscent of ravines and dry riverbeds. Long-term variations in the Martian climate may be associated with a change in the tilt of the polar axis. With a slight increase in the temperature of the planet, the atmosphere can become 100 times denser (due to the evaporation of ice). Thus, it is possible that life on Mars once existed. We will be able to answer this question only after a detailed study of Martian soil samples. But their delivery to Earth is a difficult task.

Fortunately, there is strong evidence that of the thousands of meteorites found on Earth, at least 12 came from Mars. They are called SNC meteorites, because the first of them were found near the settlements of Shergotty (Shergotti, India), Nakhla (Nakla, Egypt) and Chassigny (Chassignoy, France). The ALH 84001 meteorite found in Antarctica is much older than the others and contains polycyclic aromatic hydrocarbons, possibly of biological origin. It is believed that it came to Earth from Mars, since the ratio of oxygen isotopes in it is not the same as in terrestrial rocks or non-SNC meteorites, but the same as in the EETA 79001 meteorite, which contains glasses with inclusions of bubbles, in which the composition of noble gases different from the earth, but corresponds to the atmosphere of Mars.

Although there are many organic molecules in the atmospheres of giant planets, it is hard to believe that in the absence of a solid surface, life could exist there. In this sense, Saturn's satellite Titan is much more interesting, which has not only an atmosphere with organic components, but also a solid surface where synthesis products can accumulate. True, the temperature of this surface (90 K) is more suitable for oxygen liquefaction. Therefore, the attention of biologists is more attracted by Jupiter's moon Europa, although devoid of an atmosphere, but, apparently, having an ocean of liquid water under its icy surface.

Some comets almost certainly contain complex organic molecules dating back to the formation of the solar system. But it's hard to imagine life on a comet. So, until we have evidence that life in the solar system exists anywhere outside the Earth.

One can ask questions: what are the capabilities of scientific instruments in connection with the search for extraterrestrial life? Can a modern space probe detect the presence of life on a distant planet? For example, could the Galileo spacecraft have detected life and intelligence on Earth when it flew past it twice in gravitational maneuvers? On the images of the Earth transmitted by the probe, it was not possible to notice signs of intelligent life, but the signals of our radio and television stations caught by the Galileo receivers became obvious evidence of its presence. They are completely different from the radiation of natural radio stations - auroras, plasma oscillations in the earth's ionosphere, solar flares - and immediately betray the presence of a technical civilization on Earth. And how does unreasonable life manifest itself?

The Galileo TV camera took images of the Earth in six narrow bands of the spectrum. In the 0.73 and 0.76 µm filters, some areas of the land appear green due to the strong absorption of red light, which is not typical for deserts and rocks. The easiest way to explain this is that some carrier of a non-mineral pigment that absorbs red light is present on the surface of the planet. We know for sure that this unusual absorption of light is due to chlorophyll, which plants use for photosynthesis. No other body in the solar system has such a green color. In addition, the Galileo infrared spectrometer recorded the presence of molecular oxygen and methane in the earth's atmosphere. The presence of methane and oxygen in the Earth's atmosphere indicates biological activity on the planet.

So, we can conclude that our interplanetary probes are able to detect signs of active life on the surface of planets. But if life is hidden under Europa's ice shell, then a vehicle flying by is unlikely to detect it.



Hello, dear readers of the blog site. The solar system is a collection of planets revolving around the Sun in orbits, the Sun and a number of other celestial bodies of smaller sizes.

The composition includes only natural objects that make a revolution around a star or any planet. Of course, satellites launched from Earth are not among them.

But let's take a closer look at what the solar system is and what its structure is. Let's find out what small and large bodies form it. Which is the largest planet and which is the smallest. Let's list them all in order, look at it and the layouts.

Planets of the solar system

You can read about the sun itself (the central star of the system) at the link above or briefly read the information on it at the bottom of this article. Of the interesting facts, we can add that the mass of the sun is 99.86% of the mass of the entire solar system, which indicates its undeniable importance.

How many planets are in the solar system and their order

The next largest bodies after the Sun are the planets. How many planets are in the solar system? Until recently, it was believed that 9 planets revolve around our star:

For children, there are special models or drawings of the solar system to help them understand what it means to rotate around the Sun, such as the model pictured above.

The largest and smallest planet in the solar system

Is Pluto a planet or not?

Pluto recognized as the smallest planet in the solar system. However, recently there have been many questions about whether it is correct to consider Pluto a planet. Why? Here are some facts that reason to doubt in whether this object can be called a planet:

1. The mass of Pluto is less than the mass of the Moon - the satellite of the Earth. It is not enough for Pluto to clear the space in orbit from other bodies. The orbit of Pluto is inhabited by many objects that have the same composition.
2. Detection beyond the orbit of Pluto of a body with a large mass and . This object is called Eris.
3. The center of mass of the Pluto-Charon system (Charon is a satellite) lies outside these two bodies.

Much became clear after detailed studies of the Kuiper belt. It consists of many ice objects with a diameter of 100 km. Pluto itself has a diameter of 2400 km.

After a series of similar discoveries, astronomers faced the task of redefining the concept of a planet.

One of the requirements was that the planet must be able clear the space around its orbit. This is what caused Pluto to be excluded from the list of planets and given the name of a dwarf planet.

Terrestrial planets including the smallest

The planets of the solar system revolve in orbits. The first 4 in order of the planets of the solar system are summarized as a terrestrial group:

1. Mercury - this is the smallest and the planet closest to the star. The period of its rotation around the star takes 88 days.
2. Venus. It rotates around its axis in the opposite direction relative to its orbital motion. Another such planet is Uranus. Venus is the hottest planet. The temperature of the atmosphere reaches +470°C.
3. Earth is the third planet from the Sun in the solar system. It has the largest density and diameter in its group. There is free oxygen in the atmosphere here. The Earth has one natural satellite - the Moon.
4. Mars. The atmosphere of the fourth planet consists of carbon dioxide. Due to the presence of iron oxide in the soil, the planet has a reddish hue.

Giant planets including the largest

The four terrestrial planets are followed by the giant planets of the solar system:

1. Jupiter - largest planet. Its mass is 318 times the mass of our planet. It consists of H (hydrogen) and He (helium), has many satellites, one of which is larger than even Mercury.
2. Saturn. He is known to us thanks to his rings. The planet has many satellites.
3. Uranus. This planet has the smallest mass among the giants. It differs in that the angle of inclination of its axis to the plane is almost 100°. Therefore, we can say about this planet that it does not so much rotate as it rolls along its orbit.
4. Neptune. The rotation period is 248 years. It is the last planet, but far from the last body in the solar system.

The photo above shows the planets of the solar system and the actual ratio of their sizes.

Small bodies of the solar system

These are small bodies that make a revolution around our star. Most often they do not have a spherical shape, but look like stone blocks. They have. Asteroids may have satellites. They are not included in the solar system model.

After the orbit of the fourth planet is the asteroid belt. It ends before the orbit of the fifth planet - Jupiter. Asteroids are the most common small bodies in the solar system. Their sizes can vary from a few meters to hundreds of kilometers. Although they are much smaller than planets, such bodies can have satellites.

In addition to the asteroid belt, there are other asteroids. The paths of some of these bodies intersect with the orbit of our planet. However, we can not worry that the movement of the asteroid will disturb the alignment of the planets in the solar system.

dwarf planets

A number of asteroids that have a large mass and diameter began to be classified as dwarf planets. Among them:

1. Ceres.
2. Pluto (formerly considered a planet).
3. Eris (located beyond Pluto).

This is a celestial luminous object with a pronounced head and tail. The brightness of a comet is directly related to its distance from the Sun.

The comet consists of the following parts:

1. Core. It contains almost the entire weight of the comet.
2. Coma is a foggy shell around the nucleus.
3. Tail. It is located in the opposite direction from the Sun.

One of the famous comets is Halley's Comet. It moves closer to the sun, then moves away from it. The comet's head is made up of frozen water, metal particles, and various compounds. The diameter of the nucleus of this comet is 10 km. The period of passage of the orbit (ellipse) is about 75 years.

The point in the orbit at which the body is as close as possible to the Star is called perihelion, and the opposite (farthest) is called aphelion.

meteorites

These are relatively small bodies that fall on the surface of other celestial objects of greater magnitude. can be iron, stone or iron-stone. About 2,000 tons of meteorites fall on the surface of our planet every year. Some have a mass of several grams, while others have a mass of several tens of tons. For example, the Tunguska meteorite that fell to Earth in 1908 knocked down forests.

The exploration of our solar system will continue for many more years, so for sure in the future we will become aware of all the new facts and information about planets, comets, asteroids and other cosmic bodies.

The sun is the star of the solar system

, which is located in the center of our system and is the basis of the layout of the solar system. Its mass is 1.989 ∙ 10 30 kg, which occupies 99.86% of the mass of the system. The diameter of the star is 1.391 million km. It is a fireball of gas. Due to the processes occurring in the nucleus, a huge amount of energy is released.

The sun belongs to a group of stars called "yellow dwarfs". Yellow stars are those whose surface temperatures range from 5,000 to 7,500 K.

Structure of the Sun

Considering the structure of the solar system, it is worth starting from its center, namely from the center of the Sun. The luminary can be divided into several layers:

1. Core. Hydrogen atoms break apart in the depths, which is accompanied by the release of enormous energy. There also occurs the fusion of protons and neutrons into the nuclei of helium atoms. In the core, the temperature reaches 15 million K, which is 2.5 times higher than on the surface. The core extends for 173 thousand km from the center of the Sun, which is about 20% of the star.
2. radiation zone. In it, the photons emitted by the nucleus wander for about 200 thousand years and lose their energy due to collisions with plasma particles.
3. convective zone. It looks like a boiling mass, in which particles constantly rise to the surface, located on the border of the radiation and convective zones. Here, the path of particles to the surface of the star takes much less time than the duration of processes in the radiation zone. The convective zone extends from 70% and almost to the surface of the star.
4. Photosphere. It has an extremely small thickness - only 100 km (compared to the size of the Sun - this is really not much). This is the visible surface of the sun.
5. The chromosphere is a heterogeneous layer of the solar atmosphere, which is located directly above the photosphere. Here the temperature increases from 6,000 K to 20,000 K.
6. The corona is the outer layer of the atmosphere. Due to the fact that its brightness is much less than that of a star, the corona is not visible to the naked eye (without additional equipment, it is visible only during eclipses). The temperature here is the highest in the entire solar system - 1,000,000 K.

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Until recently, astronomers believed that such a concept as a planet refers exclusively to the solar system. Everything that is outside it is unexplored cosmic bodies, most often stars of very large scales. But, as it turned out later, the planets, like peas, are scattered throughout the universe. They are different in their geological and chemical composition, may or may not have an atmosphere, and all this depends on the interaction with the nearest star. The arrangement of the planets in our solar system is unique. It is this factor that is fundamental for the conditions that have formed on each individual space object.

Our space house and its features

In the center of the solar system is the star of the same name, which is included in the category of yellow dwarfs. Its magnetic field is enough to hold nine planets of various sizes around its axis. Among them there are dwarf stony cosmic bodies, vast gas giants that reach almost the parameters of the star itself, and objects of the "middle" class, which include the Earth. The positions of the planets in the solar system do not occur in ascending or descending order. We can say that with respect to the parameters of each individual astronomical body, their arrangement is chaotic, that is, the large alternates with the small.

SS structure

To consider the location of the planets in our system, it is necessary to take the Sun as a reference point. This star is located in the center of the SS, and it is its magnetic fields that correct the orbits and movements of all surrounding space bodies. Nine planets revolve around the Sun, as well as an asteroid ring that lies between Mars and Jupiter, and the Kuiper Belt, located outside of Pluto. In these intervals, individual dwarf planets are also distinguished, which are sometimes attributed to the main units of the system. Other astronomers believe that all these objects are nothing more than large asteroids, on which, under no circumstances, life can arise. They attribute Pluto itself to this category, leaving only 8 planetary units in our system.

The order of the planets

So, we will list all the planets, starting with the closest to the Sun. In the first place are Mercury, Venus, then Earth and Mars. After the Red Planet, a ring of asteroids passes, behind which a parade of giants consisting of gases begins. These are Jupiter, Saturn, Uranus and Neptune. The list is completed by the dwarf and icy Pluto, with its no less cold and black satellite Charon. As we said above, several more dwarf space units are distinguished in the system. The location of dwarf planets in this category coincides with the Kuiper belts and asteroids. Ceres is in an asteroid ring. Makemake, Haumea and Eris are in the Kuiper belt.

terrestrial planets

This category includes cosmic bodies, which in their composition and parameters have much in common with our home planet. Their bowels are also filled with metals and stone, either a full-fledged atmosphere is formed around the surface, or a haze that resembles it. The location of the terrestrial planets is easy to remember, because these are the first four objects that are directly next to the Sun - Mercury, Venus, Earth and Mars. Characteristic features are small size, as well as a long period of rotation around its axis. Also, of all the terrestrial planets, only the Earth itself and Mars have satellites.

Giants made of gases and hot metals

The location of the planets of the solar system, which are called gas giants, is the most distant from the main star. They are located behind the asteroid ring and stretch almost to the Kuiper belt. There are four giants in total - Jupiter, Saturn, Uranus and Neptune. Each of these planets consists of hydrogen and helium, and in the region of the core there are metals heated to a liquid state. All four giants are characterized by an incredibly strong gravitational field. Due to this, they attract numerous satellites to themselves, which form almost entire asteroid systems around them. SS gas balls rotate very quickly, therefore whirlwinds and hurricanes often occur on them. But, despite all these similarities, it is worth remembering that each of the giants is unique in its composition, size, and gravity.

dwarf planets

Since we have already considered in detail the location of the planets from the Sun, we know that Pluto is the farthest, and its orbit is the most gigantic in the SS. It is he who is the most important representative of dwarfs, and only he from this group is the most studied. Dwarfs are those cosmic bodies that are too small for planets, but also large for asteroids. Their structure can be comparable to Mars or Earth, or it can be just rocky, like any asteroid. Above, we have listed the brightest representatives of this group - these are Ceres, Eris, Makemake, Haumea. In fact, dwarfs are found not only in the two SS asteroid belts. Often they are called satellites of gas giants, which were attracted to them due to the huge

The planets of the solar system are in order in the following sequence:
1 - Mercury. The smallest of the real planets in the solar system
2 - Venus. The description of hell was taken from her: terrible heat, sulfur evaporation and eruptions of many volcanoes.
3 - Earth. The third planet in order from the Sun, our home.
4 - Mars. The most distant of the planets of the terrestrial group of the solar system.
Then the Main Asteroid Belt is located, where the dwarf planet Ceres and the minor planets Vesta, Pallas, etc. are located.
Next in order are the four giant planets:
5 - Jupiter. The largest planet in the solar system.
6 - Saturn with its famous rings.
7 - Uranus. The coldest planet.
8 - Neptune. It is the furthest "real" planet in order from the Sun.
And here's what's interesting:
9 - Pluto. A dwarf planet that is usually listed after Neptune. But, Pluto's orbit is such that it is sometimes closer to the Sun than Neptune. For example, this was the case from 1979 to 1999.
No, Neptune and Pluto cannot collide :) - their orbits are such that they do not intersect.
The location of the planets of the solar system in order in the photo:

How many planets are in the solar system

How many planets are in the solar system? This is not so easy to answer. For a long time it was believed that there were nine planets in the solar system:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

But, on August 24, 2006, Pluto ceased to be considered a planet. This was caused by the discovery of the planet Eris and other small planets of the solar system, in connection with which it was necessary to clarify which celestial bodies can be considered planets.
Several signs of "real" planets were identified and it turned out that Pluto does not fully satisfy them.
Therefore, Pluto was transferred to the category of dwarf planets, which include, for example, Ceres, the former asteroid No. 1 in the Main asteroid belt between Mars and Jupiter.

As a result, when trying to answer the question of how many planets are in the solar system, the situation is even more confused. Because in addition to the "real" now there are also dwarf planets.
But there are also small planets, which were called large asteroids. For example Vesta, asteroid number 2 in the mentioned Main asteroid belt.
Recently, the same Eris, Make-Make, Haumea and several other small planets of the solar system, data on which is insufficient and it is not clear what to consider them - dwarf or small planets. Not to mention that some small asteroids are mentioned in the literature as minor planets! For example, the asteroid Icarus, which is only about 1 kilometer in size, is often referred to as a minor planet...
Which of these bodies should be taken into account when answering the question "how many planets are there in the solar system"???
In general, "we wanted the best, but it turned out as always."

It is curious that many astronomers and even ordinary people "defend" Pluto, continuing to consider it a planet, sometimes arrange small demonstrations and diligently promote this idea on the Web (mainly abroad).

Therefore, when answering the question "how many planets in the solar system" it's easiest to say "eight" briefly and not even try to discuss something ... otherwise it will immediately turn out that there is simply no exact answer :)

The giant planets are the largest planets in the solar system.

There are four giant planets in the solar system: Jupiter, Saturn, Uranus and Neptune. Since these planets are located outside the main asteroid belt, they are called the "outer" planets of the solar system.
In size, two pairs clearly stand out among these giants.
The largest giant planet is Jupiter. Saturn is quite a bit inferior to him.
And Uranus and Neptune are sharply smaller than the first two planets and they are located farther from the Sun.
Look at the comparative sizes of the giant planets relative to the Sun:

The giant planets protect the inner planets of the solar system from asteroids.
Without these bodies in the solar system, our Earth would be hundreds of times more likely to be hit by asteroids and comets!
How do the giant planets protect us from the fall of intruders?

You can learn more about the largest planets in the solar system here:

terrestrial planets

The terrestrial planets are four planets in the solar system that are similar in size and composition: Mercury, Venus, Earth, and Mars.
Since one of them is the Earth, all these planets were assigned to the terrestrial group. Their sizes are very similar, and Venus and the Earth are generally almost the same. The temperatures are relatively high, which is explained by the proximity to the Sun. All four planets are formed by rocks, while the giant planets are gas and ice worlds.

Mercury is the closest planet to the Sun and the smallest planet in the solar system.
It is generally accepted that Mercury is very hot. Yes, it is, the temperature on the sunny side can reach +427°С. But, there is almost no atmosphere on Mercury, so on the night side it can be up to -170 ° С. And at the poles, because of the low Sun, a layer of underground permafrost is generally assumed ...

Venus. For a long time, it was considered the "sister" of the Earth, until Soviet research stations landed on its surface. It turned out to be a real hell! Temperature +475°C, pressure of almost a hundred atmospheres and an atmosphere of toxic compounds of sulfur and chlorine. To colonize it - you have to try very hard ...

Mars. The famous red planet. It is the most distant of the terrestrial planets in the solar system.
Like Earth, Mars has moons: Phobos and Deimos
Basically it is a cold, rocky and dry world. Only at the equator at noon can it get warmer up to + 20 ° С, at the rest of the time - fierce frost, up to -153 ° С at the poles.
The planet does not have a magnetosphere and cosmic radiation irradiates the surface mercilessly.
The atmosphere is very rarefied and not suitable for breathing, however, its density is enough to occasionally cause powerful dust storms on Mars.
Despite all the shortcomings. Mars is the most promising planet for colonization in the solar system.

Read more about the terrestrial planets in the article The largest planets in the solar system

The largest planet in the solar system

The largest planet in the solar system is Jupiter. This is the fifth planet from the Sun, its orbit is beyond the main asteroid belt. Look at the size comparison of Jupiter and Earth:
Jupiter is 11 times the diameter of Earth and 318 times its mass. Due to the large size of the planet, parts of its atmosphere rotate at different speeds, so Jupiter's belts are clearly visible in the image. Below, on the left, you can see Jupiter's famous Great Red Spot, a huge atmospheric vortex that has been observed for several centuries.

The smallest planet in the solar system

Which planet is the smallest planet in the solar system? This is not such a simple question...
Today it is generally accepted that the smallest planet in the solar system is Mercury, which we mentioned a little above. But, you already know that until August 24, 2006, Pluto was considered the smallest planet in the solar system.

More attentive readers may recall that Pluto is a dwarf planet. And there are five known. The smallest dwarf planet is Ceres, with a diameter of about 900 km.
But that's not all...

There are also so-called minor planets, the size of which starts at only 50 meters. Both the 1-kilometer Icarus and the 490-kilometer Pallas fall under this definition. It is clear that there are many of them, and it is difficult to choose the smallest one due to the complexity of observations and calculation of sizes. So, when answering the question "what is the name of the smallest planet in the solar system", it all depends on what exactly is meant by the word "planet".

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