Since 2005, it has been generally accepted that there are eight planets in the solar system. This is due to the discovery of M. Brownie, who proved that Pluto is a dwarf planet. Of course, the opinions of scientists were divided: some believe that this planet should not be classified as a dwarf, but that it should be returned to its former title, while others agree with Michael. There are even opinions that have proposed increasing the number of planets to twelve. Because of these discrepancies, scientists had to draw up criteria by which space objects are classified as planets:
- They must revolve around the sun.
- The mass of the planets in the solar system must be such as to allow the object to have gravity that keeps the spherical shape.
- The object must clear the orbital path from unnecessary bodies.
Pluto failed when evaluating it according to these criteria, for which it was excluded from the list of planets.
Mercury
Not far from the Sun is the first and closest planet to it - Mercury. The distance from it to the star is about 58 million kilometers. This object is considered the smallest planet in our system. Its diameter is only slightly more than 4,800 kilometers, and the duration of one year (by earthly standards) is eighty-seven days, with fifty-nine days being the duration of one day on Mercury. The mass of the planet of the solar system is only 0.055 of the earth's mass, i.e. 3.3011 x 10 23 kg.
Reminiscent of the moon. An interesting fact - this planet of our system has no satellites.
If a person weighs fifty kilograms on Earth, then on Mercury his weight will be about twenty. The temperature ranges from -170 to +400 ° С.
Venus
The next planet is Venus. It is removed one hundred and eight million kilometers from the star. The diameter and mass of the planet of the solar system is close to our Earth, but still it is smaller. is 0.81 of the earth, i.e. 4.886 x 10 24 kg. Here the year lasts two hundred twenty-five days. Venus has an atmosphere, but it is filled with sulfuric acid, nitrogen, and carbon dioxide.
This space object is clearly visible from Earth in the evening and morning: because of the bright glow, Venus is often mistaken for a UFO.
Earth
Our home is located from the luminary at a distance of one hundred and fifty million kilometers. The mass of the planet in the solar system is 5.97 x 10 24 kg. Our year lasts 365 days. The range of heating and cooling of the planet's surface is +60 to -90 degrees Celsius. constantly changing: the percentage of land and water fluctuates. We have a satellite - the Moon.
On Earth, the atmosphere is composed of nitrogen, oxygen, and other impurities. According to scientists, this is the only world where there is life.
Mars
From the Sun to Mars, almost three hundred million kilometers. This object has another name - the Red Planet. It is due to the reddish tint of the surface created by the iron oxide. On the axis of tilt and rotation, Mars strongly resembles the Earth: seasons are also formed on this planet.
On its surface there are many deserts, volcanoes, ice caps, mountains, valleys. The planet's atmosphere is very thin, the temperature drops to -65 degrees. The mass of the planet of the solar system is 6.4171 x 10 24 kg. Around the star, the planet makes a complete turn in 687 Earth days: if we were Martians, then our age would be half that.
According to the latest data, due to the mass and size, this planet of the solar system began to belong to terrestrial objects.
There is no oxygen in the atmosphere, but there is nitrogen, carbon and other impurities. The soil contains a large amount of iron.
Jupiter
It is a huge body located at a distance of almost eight hundred million kilometers from the Sun. The giant is 315 times larger than the Earth. There are very strong winds here, the speed of which reaches six hundred kilometers per hour. There are auroras that almost never stop.
The radius and mass of the planet of the solar system are impressive: it weighs 1.89 x 10 27 kg, and its diameter is almost half a million kilometers (for comparison, the diameter of the Earth is only twelve thousand seven hundred kilometers).
Jupiter resembles a separate system, where the planet acts as a luminary, and dozens of objects revolve around it. This impression is created by the numerous satellites (67) and moons. An interesting fact: if on Earth a person weighs about forty-five kilograms, then on Jupiter his weight will be more than a centner.
Saturn
Saturn is located at a distance of almost one and a half billion kilometers from the Sun. This is a beautiful planet with an unusual ring system. Saturn has layers of gas that are concentrated around the core.
The mass of the planet is 5.66 x 10 26 kg. One revolution around the star takes almost thirty Earth years. Despite such a long year, a day here lasts only eleven hours.
Saturn has 53 moons, although scientists managed to find nine more, but so far they have not been confirmed and do not belong to the moons of Saturn.
Uranus
The beautiful giant planet Uranus is located at a distance of almost three billion kilometers. It is classified as an ice gas giant due to the composition of the atmosphere: methane, water, ammonia and hydrocarbons. A large amount of methane lends a blueness.
A year on Uranus lasts eighty-four Earth years, but the length of the day is short, only eighteen hours.
Uranus is the fourth mass planet in the solar system: it weighs 86, 05 x 10 24 kg. It has twenty-seven satellites and a small ring system.
Neptune
At a distance of four and a half billion kilometers from the Sun is Neptune. This is another ice gas giant. The planet has satellites and a weak ring system.
The mass of the planet is 1.02 x 10 26 kg. Neptune flies around the sun in one hundred and sixty-five years. The day lasts only sixteen hours here.
The planet has water, methane, ammonia, helium.
Neptune has thirteen satellites and one more has not yet received the status of the moon. In the ring system, scientists distinguish six formations. Only one artificial satellite, Voyager 2, launched into space many years ago, was able to reach this planet.
Gas ice giants are very cold, here the temperature drops to -300 degrees and below.
Pluto
The former Pluto was able to maintain its status as a planet for a long century. However, in 2006 it was transferred to the status. Little is known about this object. Scientists cannot yet say exactly how long a year lasts here: it was discovered in 1930 and to this day it has passed only a third of the orbital path.
Pluto has satellites - there are five of them. The diameter of the planet is only 2300 kilometers, but there is a lot of water here: according to scientists, it is three times more than on Earth. The surface of Pluto is completely covered with ice, among which ridges and dark small areas can be seen.
Having considered the sizes and masses, one can draw conclusions about how different they are. There are large objects, and there are also small ones that look like ants near baseballs.
The solar system is a system of planets, which includes its center - the Sun, as well as other objects of the Cosmos. They revolve around the sun. Until recently, 9 objects of the Cosmos, which revolve around the Sun, were called "planet". Now scientists have found that outside the solar system there are planets that revolve around the stars.
In 2006, the Union of Astronomers proclaimed that the planets of the solar system are spherical space objects orbiting the sun. On the scale of the solar system, the Earth appears to be extremely small. In addition to the Earth, eight planets revolve around the Sun in their individual orbits. All of them are larger than the Earth. Rotate in the plane of the ecliptic.
Planets in the solar system: types
The location of the terrestrial group in relation to the Sun
The first planet is Mercury, behind it is Venus; followed by our Earth and, finally, Mars.
Terrestrial planets do not have many satellites or moons. Of these four planets, only Earth and Mars have satellites.
The planets that belong to the terrestrial group are distinguished by high density, consist of metal or stone. Basically, they are small and rotate on their own axis. Their rotation speed is also low.
Gas giants
These are four space objects that are at the greatest distance from the Sun: under number 5 is Jupiter, followed by Saturn, then Uranus and Neptune.
Jupiter and Saturn are both impressive planets, composed of compounds of hydrogen and helium. The density of gas planets is low. They rotate at high speed, have satellites and are surrounded by rings of asteroids.
The “ice giants”, which include Uranus and Neptune, are smaller; their atmospheres contain methane and carbon monoxide.
Gas giants have a strong gravitational field, so they can attract many space objects, in contrast to the terrestrial group.
Scientists hypothesize that asteroid rings are the remnants of moons altered by the gravitational field of planets. 
Dwarf planet
Dwarfs are space objects, the size of which does not reach the planet, but exceeds the dimensions of an asteroid. There are a great many such objects in the solar system. They are concentrated in the region of the Kuiper belt. The satellites of the gas giants are dwarf planets that have left their orbit. 
The planets of the solar system: the process of origin
According to the hypothesis of cosmic nebulae, stars are born in clouds of dust and gas, in nebulae.
Due to the force of attraction, substances unite. Under the influence of concentrated gravity, the center of the nebula contracts and stars are formed. Dust and gases are transformed into rings. The rings rotate under the influence of gravity, and planetazimals are formed in the whirlpools, which enlarge and attract cosmetic objects.
Under the influence of the force of gravity, the planetazimals contract and acquire spherical outlines. The spheres can merge and gradually turn into protoplanets. 
There are eight planets within the solar system. They revolve around the sun. Their location is as follows:
The closest "neighbor" of the Sun is Mercury, followed by Venus, followed by the Earth, followed by Mars and Jupiter, Saturn, Uranus and the last one, Neptune, are located even further from the Sun.
The solar system consists of eight planets and more than 63 of their satellites, which are being discovered more and more often, as well as several dozen comets and a large number of asteroids. All cosmic bodies move along their clear directed trajectories around the Sun, which is 1000 times heavier than all bodies in the solar system combined.
How many planets revolve around the sun
How the planets of the solar system originated: approximately 5-6 billion years ago, one of the disk-shaped gas and dust clouds of our large Galaxy (Milky Way) began to contract towards the center, gradually forming the present Sun. Further, according to one of the theories, under the influence of powerful forces of attraction, a large number of dust and gas particles revolving around the Sun began to stick together into balls - forming future planets. According to another theory, the gas-dust cloud immediately disintegrated into separate clusters of particles, which were compressed and compressed, forming the current planets. Now 8 planets revolve around the Sun constantly.
The center of the solar system is the Sun - the star around which the planets revolve in orbits. They do not emit heat and do not glow, but only reflect the light of the Sun. There are now 8 officially recognized planets in the solar system. Briefly, in order of distance from the sun, we list them all. And now a few definitions.
Satellites of the planets. The solar system also includes the Moon and natural satellites of other planets, which all of them have, except for Mercury and Venus. More than 60 satellites are known. Most of the satellites of the outer planets were discovered when they received photographs taken by robotic spacecraft. The smallest satellite of Jupiter - Leda - is only 10 km across.
The sun is a star, without which life on Earth could not exist. She gives us energy and warmth. According to the classification of stars, the Sun is a yellow dwarf. Age about 5 billion years. It has a diameter at the equator equal to 1,392,000 km, 109 times larger than the Earth's. The rotation period at the equator is 25.4 days and 34 days at the poles. The mass of the Sun is 2x10 to the power of 27 tons, about 332950 times the mass of the Earth. The temperature inside the core is about 15 million degrees Celsius. The surface temperature is about 5500 degrees Celsius.
In terms of chemical composition, the Sun consists of 75% hydrogen, and of the other 25% of the elements, most of all helium. Now, in order, let's figure out how many planets revolve around the sun, in the solar system and the characteristics of the planets.
The planets of the solar system in order from the sun in pictures
Mercury - 1st planet in the solar system
Mercury. The four inner planets (closest to the Sun) - Mercury, Venus, Earth and Mars - have a solid surface. They are smaller than four giant planets. Mercury moves faster than other planets, being burned by the sun's rays during the day and freezing at night.
Characteristics of the planet Mercury:
The period of revolution around the Sun: 87.97 days.
Diameter at the equator: 4878 km.
Rotation period (revolution around the axis): 58 days.
Surface temperature: 350 during the day and -170 at night.
Atmosphere: very thin, helium.
How many satellites: 0.
The main satellites of the planet: 0.
Venus - 2nd in the order of the planet in the solar system
Venus is more like Earth in size and brightness. Observing her is difficult because of the clouds that envelop her. The surface is a hot rocky desert.
Characteristics of the planet Venus:
The period of revolution around the Sun: 224.7 days.
Diameter at the equator: 12104 km.
Rotation period (revolution around the axis): 243 days.
Surface temperature: 480 degrees (average).
Atmosphere: dense, mostly carbon dioxide.
How many satellites: 0.
The main satellites of the planet: 0.
Earth - 3rd in order planet in the solar system
Apparently, the Earth was formed from a gas and dust cloud, like other planets of the solar system. Particles of gas and dust, colliding, gradually "grew" the planet. Surface temperatures reached 5,000 degrees Celsius. Then the Earth cooled down and was covered with hard stone crust. But the temperature in the bowels is still quite high - 4500 degrees. Rocks in the bowels are melted and, during volcanic eruptions, are poured onto the surface. Only on earth there is water. That is why life exists here. It is located relatively close to the Sun in order to receive the necessary heat and light, but far enough so as not to burn out.
Characteristics of the planet Earth:
The period of revolution around the Sun: 365.3 days.
Diameter at equator: 12756 km.
The period of the planet's rotation (revolution around the axis): 23 hours 56 minutes.
Surface temperature: 22 degrees (average).
Atmosphere: Mainly nitrogen and oxygen.
Number of satellites: 1.
The main satellites of the planet: the Moon.
Mars - 4th in order planet in the solar system
Due to its resemblance to Earth, it was believed that life exists here. But the spacecraft that landed on the surface of Mars showed no signs of life. This is the fourth planet in order.
Characteristics of the planet Mars:
The period of revolution around the Sun: 687 days.
Diameter of the planet at the equator: 6794 km.
Rotation period (revolution around the axis): 24 hours 37 minutes.
Surface temperature: -23 degrees (average).
Atmosphere of the planet: rarefied, mostly carbon dioxide.
How many satellites: 2.
The main satellites in order: Phobos, Deimos.
Jupiter - 5th in order planet in the solar system
Jupiter, Saturn, Uranus and Neptune are composed of hydrogen and other gases. Jupiter exceeds Earth by more than 10 times in diameter, 300 times in mass and 1,300 times in volume. It is more than twice as massive as all the planets in the solar system combined. How long does it take for the planet Jupiter to become a star? It is necessary to increase its mass by 75 times!
Characteristics of the planet Jupiter:
The period of revolution around the Sun: 11 years 314 days.
Diameter of the planet at the equator: 143884 km.
Rotation period (revolution around the axis): 9 hours 55 minutes.
Surface temperature of the planet: -150 degrees (average).
Number of satellites: 16 (+ rings).
The main satellites of the planets in order: Io, Europa, Ganymede, Callisto.
Saturn - 6th in order planet of the solar system
It is number 2, the largest of the planets in the solar system. Saturn is eye-catching thanks to its ring system made of ice, rocks and dust that orbits the planet. There are three main rings with an outer diameter of 270,000 km, but their thickness is about 30 meters.
Characteristics of the planet Saturn:
The period of revolution around the Sun: 29 years 168 days.
Diameter of the planet at the equator: 120,536 km.
Rotation period (revolution around the axis): 10 hours 14 minutes.
Surface temperature: -180 degrees (average).
Atmosphere: Mainly hydrogen and helium.
Number of satellites: 18 (+ rings).
Main satellites: Titan.
Uranus - 7th in order planet of the solar system
Unique planet in the solar system. Its peculiarity is that it revolves around the Sun not like everyone else, but “lying on its side”. Uranus also has rings, although they are more difficult to see. In 1986, Voyager-2 flew at a distance of 64,000 km, he had six hours of photography, which he successfully implemented.
Characteristics of the planet Uranus:
Circulation period: 84 years 4 days.
Diameter at the equator: 51,118 km.
The period of the planet's rotation (revolution around the axis): 17 hours 14 minutes.
Surface temperature: -214 degrees (average).
Atmosphere: Mainly hydrogen and helium.
How many satellites: 15 (+ rings).
Main satellites: Titania, Oberon.
Neptune - 8th in order planet in the solar system
At the moment, Neptune is considered the last planet in the solar system. Its discovery took place by means of mathematical calculations, and then they saw it through a telescope. In 1989, Voyager 2 flew by. He took striking photographs of the blue surface of Neptune and its largest moon, Triton.
Characteristics of the planet Neptune:
The period of revolution around the Sun: 164 years 292 days.
Diameter at the equator: 50538 km.
Period of rotation (revolution around the axis): 16 hours 7 minutes.
Surface temperature: -220 degrees (average).
Atmosphere: Mainly hydrogen and helium.
Number of satellites: 8.
Main satellites: Triton.
How many planets are there in the solar system: 8 or 9?
Earlier, for many years, astronomers recognized the presence of 9 planets, that is, Pluto was also considered a planet, like the others already known to everyone. But in the 21st century, scientists were able to prove that it is not a planet at all, which means that there are 8 planets in the solar system.
Now, if you are asked how many planets are in the solar system, boldly answer - there are 8 planets in our system. It has been officially recognized since 2006. When building the planets of the solar system in order from the sun, use the finished picture. What do you think, maybe Pluto shouldn't have been removed from the list of planets and this is scientific prejudice?
How many planets in the solar system: video, watch for free
SOLAR SYSTEM
The sun and the celestial bodies revolving around it - 9 planets, more than 63 satellites, four systems of rings near giant planets, tens of thousands of asteroids, a myriad of meteoroids ranging in size from boulders to dust grains, as well as millions of comets. Particles of the solar wind - electrons and protons - move in the space between them. The entire solar system has not yet been explored: for example, most of the planets and their satellites have only been skimmed from flyby trajectories, only one hemisphere of Mercury has been photographed, and there have been no expeditions to Pluto yet. But nevertheless, with the help of telescopes and space probes, a lot of important data has already been collected.
Almost all the mass of the solar system (99.87%) is concentrated in the sun. The size of the Sun is also significantly larger than any planet in its system: even Jupiter, which is 11 times the size of 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 shine with reflected sunlight (albedo), since they themselves are quite cold. They are arranged in the following order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Distances in the Solar System are usually measured in units of the Earth's average distance from the Sun, called an astronomical unit (1 AU = 149.6 million km). For example, Pluto's average distance from the Sun is 39 AU, but sometimes it moves away by 49 AU. Comets are known to fly away at 50,000 AU. The distance from Earth to the nearest star a Centaur is 272,000 AU, or 4.3 light years (ie, light traveling at a speed of 299,793 km / s travels this distance in 4.3 years). For comparison, light reaches 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, when 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. The orbital inclinations are measured from the ecliptic plane. Tilt angles less than 90 ° correspond to forward orbital motion (counterclockwise), and angles greater than 90 ° correspond to reverse motion. All the planets of the solar system move in a forward direction; Pluto has the greatest orbital inclination (17 °). Many comets move in the opposite direction, for example, the orbit of Halley's orbit is 162 °. The orbits of all bodies in the solar system are very close to ellipses. The size and shape of the elliptical orbit are characterized by the semi-major axis of the ellipse (the average distance of the planet from the Sun) and eccentricity, varying from e = 0 for circular orbits to e = 1 for extremely elongated ones. The point of the orbit closest to the Sun is called perihelion, and the most distant one is aphelion.
see also ORBIT; TAPERED SECTIONS. From the point of view of the terrestrial observer, the planets of the solar system are divided into two groups. Mercury and Venus, which are closer to the Sun than Earth, are called lower (internal) planets, and more distant (from Mars to Pluto) - upper (external). The lower planets have a limiting angle of distance from the Sun: 28 ° for Mercury and 47 ° for Venus. When such a planet is as far away as possible to the west (east) of the Sun, it is said to be in the greatest western (eastern) elongation. When the lower planet is visible directly in front of the Sun, it is said to be in lower conjunction; when directly behind the Sun - in the upper conjunction. Like the Moon, these planets go through all phases of solar illumination during the synodic period Ps - the time it takes for the planet to return to its original position relative to the Sun from the point of view of the terrestrial observer. The planet's true orbital period (P) is called sidereal. For the lower planets, these periods are related by the ratio:
1 / Ps = 1 / P - 1 / Po where Po is the Earth's orbital period. For the upper planets, a similar ratio has a different form: 1 / Ps = 1 / Po - 1 / P For the upper planets, a limited range of phases is characteristic. 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. The planet, observed at an angular distance of 90 ° from the Sun, is in square (east or west). The asteroid belt, passing between the orbits of Mars and Jupiter, divides the solar planetary system 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 densities are from 3.9 to 5.5 g / cm3. They rotate relatively slowly around their axes, are devoid of 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 lack solid surfaces. They all rotate rapidly, have many satellites and are surrounded by rings. Distant small Pluto and large satellites of giant planets are in many ways similar to the terrestrial planets. Ancient people knew planets visible to the naked eye, i.e. all internal and external up to Saturn. W. Herschel discovered Uranus in 1781. The first asteroid was discovered by J. Piazzi in 1801. Analyzing deviations in the motion of Uranus, W. Leverrier and J. Adams theoretically discovered Neptune; at the calculated place it was discovered by I. Galle in 1846. The most distant planet - Pluto - was discovered in 1930 by K. Thombo as a result of a long search for a Zaneptunian planet, organized by P. Lovell. Galileo discovered four large moons of Jupiter in 1610. Since then, with the help of telescopes and space probes, numerous satellites have been found on 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 by observing the covering of the star. The transparent stone rings of Jupiter were discovered in 1979 by the Voyager 1 interplanetary probe. Since 1983, at the moments of star coverage, there have been signs of inhomogeneous rings around Neptune; in 1989 the image of these rings was transmitted by Voyager 2.
see also
ASTRONOMY AND ASTROPHYSICS;
ZODIAC;
SPACE PROBE ;
HEAVENLY SPHERE.
SUN
In the center of the solar system is the Sun - 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 - photosphere - approx. 5800 K. The density of gas in the photosphere is thousands of times less than the density of air at the Earth's surface. Inside the Sun, temperature, density and pressure increase with depth, reaching in the center, respectively, 16 million K, 160 g / cm3 and 3.5 * 10 11 bar (air pressure in the room is about 1 bar). Under the influence of high temperatures 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 being compressed by 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 * 10 26 W. With such an intensity, the Sun has been emitting for 4.6 billion years, having processed 4% of its hydrogen into helium during this time; while 0.03% of the sun's mass was converted into energy. Stellar evolutionary models indicate that the Sun is now in the middle of its life (see also NUCLEAR SYNTHESIS). To determine the content of various chemical elements in the sun, astronomers study 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, absorbed by a certain chemical element. Emission lines, or emission lines, are brighter portions of the spectrum, indicating an excess of photons emitted by some chemical element. The frequency (wavelength) of a spectral line indicates which atom or molecule is responsible for its appearance; line contrast indicates the amount of light emitting or absorbing substance; the line width 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 understandable. However, it should be noted that only half of the lines in the solar spectrum have been identified so far. The sun is dominated by hydrogen. In second place is helium, the name of which ("helios" in Greek "Sun") reminds 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 also reluctantly manifests itself in the optical spectrum of the Sun - just one line, although many less abundant elements are represented in the solar spectrum by numerous lines. Here is the composition of the "solar" matter: 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, each 2 atoms of nickel, sodium and calcium, as well as a little bit of all other elements. Thus, the mass of the Sun is about 71% hydrogen and 28% helium; the rest of the elements account for just over 1%. From the point of view of planetary science, it is noteworthy that some objects in the solar system have almost the same composition as the sun (see below the 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 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 sun's surface where the vertical magnetic field is so strong (200-3000 Gs) 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 sunspot appears - a "shadow", surrounded by a hotter transition region - a "penumbra". The size of a typical sunspot is slightly larger than the diameter of the Earth; there is such a spot for several weeks. The number of sunspots increases and decreases with the duration of the cycle from 7 to 17 years, an average of 11.1 years. Typically, the more spots appear in a cycle, the shorter the cycle itself. The direction of the magnetic polarity of the sunspots changes to the opposite from cycle to cycle; therefore, the true cycle of sunspot-forming activity of the Sun is 22.2 years. At the beginning of each cycle, the first spots appear at high latitudes, approx. 40 °, and gradually the zone of their birth shifts to the equator to latitude approx. 5 °. see also STARS ; SUN . Fluctuations in the activity of the Sun are almost not reflected in the total power of its radiation (if it changed by only 1%, this would lead to serious changes in the climate on Earth). There have been many attempts to find a connection between sunspot cycles and the Earth's climate. The most remarkable event in this sense is the "Maunder minimum": from 1645 for 70 years there were almost no sunspots on the Sun, and at the same time the Earth experienced the Little Ice Age. It is still not clear if this amazing fact was a mere coincidence or if it points to a causal link.
see also
CLIMATE;
METEOROLOGY AND CLIMATOLOGY. 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 for 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-wave sound vibrations) excited in the planet's body by earthquakes, seismologists compiled a detailed map of the earth's interior: they found out the sizes and densities of the Earth's core and its mantle, and also obtained three-dimensional images of moving plates of her crust. Similar methods can be applied to the Sun, since there are waves on its surface with a period of approx. 5 min, caused by a multitude of seismic vibrations propagating in its depths. These processes are studied by helioseismology. Unlike earthquakes, which generate short bursts of waves, energetic 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 yielded no results, since the instruments are not yet able to detect the resulting oscillations. Above the Sun's photosphere 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 emission line Ha. The temperature almost doubles from the photosphere to the upper layers of the chromosphere, from which, for an unclear reason, the energy leaving the Sun is released in the form of heat. Above the chromosphere, the gas is heated to 1 million K. This region, called the corona, extends approximately 1 solar radius. The density of the gas 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 - eruptive prominences - appear in the atmosphere of the Sun. 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 lines of force of the magnetic field. Another interesting and extremely active phenomenon is solar flares, powerful emissions of energy and particles lasting up to two hours. The flux of photons generated by such a solar flare reaches the Earth at the speed of light in 8 minutes, and the flux of electrons and protons - in a few days. Solar flares occur in places of a sharp change in the direction of the magnetic field caused by the movement of matter in sunspots. The maximum solar flare activity usually occurs a year before the maximum of the sunspot cycle. This predictability is very important, because the flurry of charged particles generated by a powerful solar flare can damage even ground communications and power grids, not to mention astronauts and space technology.
SOLAR PROTUBERANTS, observed in the helium emission line (wavelength 304) from the Skylab space station.
From the plasma corona of the Sun, there is a constant outflow of charged particles, called the solar wind. It was suspected of its existence even before the start of space flights, since it was noticeable how something was "blowing off" cometary tails. Three components are distinguished in the solar wind: a high-speed stream (more than 600 km / s), a low-speed stream, and unsteady streams from solar flares. X-ray images of the Sun have shown that huge "holes" - regions of low density - are regularly formed in the corona. These coronal holes are the main source of high-speed solar wind. In the region of the Earth's orbit, the typical solar wind speed is about 500 km / s, and the density is about 10 particles (electrons and protons) in 1 cm3. The solar wind flux interacts with planetary magnetospheres and comet tails, significantly affecting their shape and processes occurring in them.
see also
GEOMAGNETISM;
;
COMET. Under the pressure of the solar wind, a giant cavity, the heliosphere, has formed in the interstellar medium around the Sun. On its border - 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. This is a very dramatic race against time: Pioneer 10 stopped working in 1998, and the rest are trying to reach the heliopause before the energy in their batteries runs out. According to calculations, Voyager 1 is flying exactly in 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 Earth through a telescope: it does not move away from the Sun at an angle of more than 28 °. It was studied using radar from the Earth, and the interplanetary probe "Mariner-10" photographed half of its surface. Around the Sun, Mercury revolves 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 AU. It rotates around the axis with a period of 58.6 days, exactly equal to 2/3 of the orbital period, so each point of its surface turns to the Sun only once every 2 Mercury years, i.e. sunny days there last 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 metal core that makes up 75% of the planet's radius (it occupies 50% of the Earth's radius). The surface of Mercury is similar to 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 2000 km Caloris Basin, which resembles lunar seas. However, unlike the Moon, Mercury has peculiar structures - several kilometers high ledges stretching 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 temperature of the planet's surface is about 700 K during the day, and about 100 K at night. 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. It is the second planet from the Sun and closest to the Earth - the brightest "star" in our sky; sometimes it is visible even during the day. Venus is similar to Earth in many ways: its size and density are only 5% less than that of 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. Venus rotates around the axis in the opposite direction (clockwise as viewed from the North Pole) with a period of 243 Earth days. Its orbital period is 225 days; therefore, the Venusian day (from sunrise to the next sunrise) lasts 116 Earth days.
see also RADAR ASTRONOMY.
VENUS. The ultraviolet image taken from the Pioneer-Venus interplanetary station shows the planet's atmosphere, densely filled with clouds that are lighter in the polar regions (top and bottom of the image).
The atmosphere of Venus is composed primarily of carbon dioxide (CO2), along with small amounts of nitrogen (N2) and water vapor (H2O). Hydrochloric acid (HCl) and hydrofluoric acid (HF) were found in the form of minor impurities. Surface pressure 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 in what is not quite accurately called the "greenhouse effect": the sun's rays pass relatively easily through the clouds of its atmosphere and heat the planet's surface, but thermal infrared radiation from the surface itself escapes through the atmosphere back into space with great difficulty. Venus's clouds are composed of microscopic droplets of concentrated sulfuric acid (H2SO4). The upper layer of clouds is 90 km away from the surface, the temperature there is approx. 200 K; lower layer - 30 km, temperature approx. 430 K. Even below 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 planet's surface, but much faster, making a revolution in 4 days; this phenomenon is called super-rotation, and no explanation has yet been found for it. Automatic stations descended on the day and night sides of Venus. During the day, the planet's surface is illuminated by diffused sunlight with approximately the same intensity as on a cloudy 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, in which rocky ground is visible. In general, the topography of Venus has been studied using radar images transmitted by the orbiters Pioneer-Venera (1979), Venera-15 and -16 (1983) and Magellan (1990). The finest details on the best of them are about 100 m in size. Unlike Earth, Venus does not have well-defined continental plates, but there are several global elevations, for example, Ishtar the size of Australia. On the surface of Venus, there are many meteorite craters and volcanic domes. Obviously, Venus's crust is thin, so molten lava comes close to the surface and is easily poured onto it after meteorites fall. Since there is no rain or strong winds near the surface of Venus, surface erosion occurs very slowly, and geological structures remain accessible for observation from space for hundreds of millions of years. Little is known about the internal structure of Venus. It probably has a metal core that occupies 50% of the radius. But the planet has no magnetic field due to its very slow rotation. Venus does not have satellites.
Earth. Our planet is the only one in which most of the surface (75%) is covered with liquid water. The Earth is an active planet and, perhaps, the only one in which surface renewal is due to plate tectonics processes, manifesting themselves as mid-oceanic ridges, island arcs and folded mountain belts. The distribution of heights on 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 (see also EARTH). Seismic data indicate the following structure of the earth's interior: crust (30 km), mantle (to a depth of 2900 km), metal core. Part of the core is melted; there the Earth's magnetic field is generated, which traps charged particles of the solar wind (protons and electrons) and forms two toroidal regions filled with them around the Earth - radiation belts (Van Allen belts), localized at heights of 4000 and 17000 km from the Earth's surface.
see also GEOLOGY; GEOMAGNETISM.
The Earth's atmosphere is 78% nitrogen and 21% oxygen; it is the result of a long evolution under the influence of geological, chemical and biological processes. Perhaps the primary atmosphere of the Earth was rich in hydrogen, which then evaporated. The degassing of the subsoil filled the atmosphere with carbon dioxide and water vapor. But the steam condensed in the oceans, and the carbon dioxide became trapped in the carbonate rocks. (Curiously, if all the CO2 filled the atmosphere in the form of gas, then the pressure would be 90 bar, like on Venus. And if all the water evaporated, then the pressure would be 257 bar!). Thus, nitrogen remained in the atmosphere, and oxygen appeared gradually as a result of the vital activity of the biosphere. Even 600 million years ago, the oxygen content in the air was 100 times lower than the current one (see also ATMOSPHERE; OCEAN). There are indications that the Earth's climate changes on a 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. Variations in the intensity of solar radiation are also not excluded. In our era, human activities also affect the climate: emissions of gases and dust into the atmosphere.
see also
ACID PRECIPITATION;
AIR POLLUTION ;
WATER POLLUTION ;
ENVIRONMENTAL DEGRADATION.
The Earth has a satellite - the Moon, the origin of which has not yet been figured out.
EARTH AND MOON from the Lunar Orbiter space probe.
Moon. One of the largest satellites, the Moon is in second place after Charon (the satellite of Pluto) in the ratio of the masses of the satellite to 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 / cm3, which indicates that it does not have a significant metal 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 in relation to the ecliptic - from 4 ° 59ў to 5 ° 19ў. The daily rotation and orbital rotation of the Moon are synchronized, so we always see only one hemisphere of it. 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 diurnal rotation of the Moon occurs at a constant speed, and the orbital revolution - with a variable (due to the eccentricity of the orbit). Sections of the lunar surface have long been conventionally divided into "sea" and "continental". The surface of the seas looks darker, lies lower and is much less often covered with meteorite craters than the mainland surface. The seas are filled with basaltic lavas, and the continents are composed of anorthosite 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 crushed, and turned the outer layer several meters into a powder called regolith. Astronauts and robotic probes delivered rock and regolith samples 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 falling meteorites and the formation of craters practically did not change and is still one crater with a diameter of 1 km in 105 years.
see also SPACE RESEARCH AND USE.
Lunar rocks are poor in volatile elements (H2O, 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 practically 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 have fallen with meteorites. The lack 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. Seismometers delivered to the moon made it possible to learn something about the lunar interior. Weak "moonquakes" often occur there, probably related to the tidal influence of the Earth. The moon is quite homogeneous, has a small dense core and a crust about 65 km thick from lighter materials, with the upper 10 km of the crust shattered by meteorites 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, therefore, it is on it that 70% of the sea surface is concentrated. The history of the lunar surface as a whole is known: after the end of the stage of intense meteorite bombardment 4 billion years ago, about 1 billion years ago, the bowels were hot enough and basaltic lava was 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 form together with the Earth as its satellite; finally, could have separated 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 matter ejected by the proto-Earth during a collision with a large celestial body is being seriously considered. Despite the unclear origin of the Earth-Moon system, their further evolution can be traced quite reliably. Tidal interaction significantly affects the movement of celestial bodies: the daily rotation of the Moon has practically stopped (its period equaled the orbital), and the Earth's rotation slows down, transferring its angular momentum to the orbital motion of the Moon, which as a result moves away from the Earth by about 3 cm per year. This will stop when the Earth's rotation aligns with the Moon's movement. Then the Earth and the Moon will be constantly turned to each other by one side (like Pluto and Charon), and their day and month will be equal to 47 present days; the Moon will move away from us 1.4 times. True, this situation will not last forever, because solar tides will not stop acting on the Earth's rotation. see also
MOON ;
MOONS ORIGIN AND HISTORY;
Ebb and flow.
Mars. Mars is similar to Earth, but almost half its size and has a slightly lower average density. The period of diurnal rotation (24 h 37 min) and the tilt of the axis (24 °) are almost the same as on Earth. To the terrestrial observer, Mars appears to be a reddish star, the brilliance of which changes noticeably; it is maximal during periods of confrontation, repeated after a little over two years (for example, in April 1999 and June 2001). Mars is especially close and bright during periods of great oppositions that occur if, at the moment of opposition, it passes near the perihelion; this happens every 15-17 years (the next 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 areas is likely to resemble terrestrial basalts, while the light ones are composed of finely dispersed material.
SURFACE OF MARS near the Viking-1 landing block. Large pieces of stone are about 30 cm in size.
Most of our knowledge about Mars comes from robotic stations. The most productive were two orbiters and two landing vehicles of the Viking expedition, which landed on Mars on July 20 and September 3, 1976 in the regions of Chryse (22 ° N, 48 ° W) and Utopia (48 ° N). ., 226 ° W), and "Viking-1" worked until November 1982. Both of them sat in the classic light areas and found themselves in a reddish sandy desert strewn with dark stones. On July 4, 1997, the Mars Passfinder probe (USA) into the Ares Valley (19 ° N, 34 ° W) was the first automatic self-propelled vehicle to detect mixed rocks and, possibly, pebbles cut with water and mixed with sand and clay , which indicates strong changes in the Martian climate and the presence of a large amount of water in the past. Mars' thin atmosphere is 95% carbon dioxide and 3% nitrogen. Water vapor, oxygen and argon are present in small amounts. The average pressure at the surface is 6 mbar (i.e. 0.6% of the earth's). 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 a climate of a cold, dehydrated high-mountainous desert. In the high latitudes of Mars in winter, temperatures drop below 150 K and atmospheric carbon dioxide (CO2) freezes and falls onto the surface as white snow, forming the polar cap. Periodic condensation and sublimation of 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 a small area remains of it (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, raising clouds of fine sand into the air. Particularly powerful dust storms occur in late spring in the southern hemisphere, when Mars passes through the perihelion of its orbit and the solar heat is especially high. For weeks or even months, the atmosphere becomes opaque with yellow dust. Viking orbiters transmitted images of massive sand dunes at the bottom of large craters. Dust deposits change the appearance of the Martian surface so much from season to season that it is noticeable even from Earth when observed through a telescope. In the past, these seasonal changes in surface color were considered by some astronomers to be a sign of vegetation on Mars. The geology of Mars is very diverse. Large areas of the southern hemisphere are covered with old craters left over from the era of the ancient meteorite bombardment (4 billion BC). years ago). Much of the northern hemisphere is covered with younger lava flows. Particularly interesting is the Farsis Upland (10 ° N, 110 ° W), on which there are several giant volcanic mountains. 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 lava flows does not exceed 100 million years, which is slightly compared to the age of the planet 4.6 billion years.
Although ancient volcanoes indicate the once-powerful activity of the Martian interior, there are no signs of plate tectonics: folded mountain belts and other indicators of crustal compression are absent. 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 images from spacecraft are branched meandering valleys hundreds of kilometers long, reminiscent of the dried-up beds 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 rivers were on Mars for a very long time, probably in the first 0.5 billion years of its evolution. Now the water lies on the surface in the form of ice of the polar caps and, possibly, under the surface in the form of 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 Viking-2 landing block did not record the planet's seismic activity for 2 years of operation (the seismometer did not work 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; later it will be destroyed by the gravity of the planet.
Jupiter. The largest planet in the solar system, Jupiter, is 11 times the size of Earth and 318 times more massive. Its low average density (1.3 g / cm3) indicates a composition close to that of the sun: it is mainly hydrogen and helium. Jupiter's rapid rotation around its axis causes its polar contraction by 6.4%. A telescope on Jupiter shows cloud bands parallel to the equator; light zones in them are interspersed with reddish belts. The bright zones are likely to be areas of updrafts where the tops of the ammonia clouds are visible; reddish belts are associated with downdrafts, the bright color of which is determined by ammonium hydrogen sulfate, as well as compounds of red phosphorus, sulfur and organic polymers. In addition to hydrogen and helium, CH4, NH3, H2O, C2H2, C2H6, HCN, CO, CO2, PH3, and GeH4 were spectroscopically detected in Jupiter's atmosphere. The temperature at the level of the tops of ammonia clouds is 125 K, but with depth it increases by 2.5 K / km. At a depth of 60 km, there should be a layer of water clouds. The velocities of the clouds in the zones and in neighboring zones differ significantly: for example, in the equatorial zone, clouds move to the east by 100 m / s faster than in neighboring zones. The difference in speed causes severe turbulence at the boundaries of zones and belts, which makes their shape very intricate. One of the manifestations of this is the 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 revolution around the axis in 6 days. The rest of the spots are smaller and for some reason all white.
Jupiter has no solid surface. The top layer of the planet, 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 transforms into a 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. Liquid metal bowels and the planet's rapid rotation have 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 thermonuclear reactions to take place. But Jupiter's internal reserves of heat, left over from the epoch of formation, are large. Even now, 4.6 billion years later, it emits about the same heat as it receives from the Sun; in the first million years of evolution, the radiation power of Jupiter was 104 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 nearest to it - Io and Europa - have a rather high density (3.5 and 3.0 g / cm3), and more distant - Ganymede and Callisto - contain a lot of water ice and therefore are less dense (1.9 and 1.8 g / cm3).
Satellites. Jupiter has at least 16 satellites and a faint ring: it is located 53 thousand km from the upper cloud layer, has a width of 6000 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 present names - Io, Europa, Ganymede and Callisto. The smallest of the moons, Europa, is slightly smaller than the Moon, and Ganymede is larger than Mercury. They are all visible through binoculars.
On the surface of Io, the Voyagers discovered several active volcanoes, ejecting matter hundreds of kilometers upward. Io's surface is covered with reddish sulfur deposits and light spots of sulfur dioxide - products of volcanic eruptions. As a gas, sulfur dioxide forms Io's extremely rarefied atmosphere. The energy of volcanic activity is drawn from the planet's tidal influence on the satellite. Io's orbit passes through the radiation belts of Jupiter, and it has long been established that the satellite strongly interacts with the magnetosphere, causing radio bursts in it. In 1973, a torus of luminous sodium atoms was discovered along Io's orbit; later, sulfur, potassium and oxygen ions were found there. These substances are knocked out by the energetic protons of the radiation belts either directly from the surface of Io or from the gas plumes of volcanoes. Although Jupiter's tidal influence on Europa is weaker than on Io, its interior can also be partially melted. Spectral studies show that Europa's surface is covered with water ice, and its reddish tint is likely related to sulfur pollution from Io. The almost complete absence of impact craters indicates the geological youth of the surface. The folds and faults of Europa's ice surface resemble the ice fields of the earth's polar seas; there is probably liquid water under the ice on Europa. Ganymede is the largest satellite in the solar system. Its density is low; it is probably half rock and half ice. Its surface looks strange and retains traces of crustal expansion, possibly accompanying the process of subsurface differentiation. Sections of the ancient cratered surface are separated by younger troughs, hundreds of kilometers long and 1–2 km wide, lying at a distance of 10–20 km from each other. This is probably a 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 fracture marks on its surface; it is all very old and heavily cratered. The surface of both satellites is covered with ice interspersed with rocks such as regolith. But if on Ganymede the ice is about 50%, then on Callisto - less than 20%. The composition of the rocks of Ganymede and Callisto is probably similar to the composition of carbonaceous meteorites. Jupiter's moons lack an atmosphere, except for the rarefied volcanic gas SO2 on Io. Of the dozen small satellites of Jupiter, four are closer to the Galilean planet; the largest of them Amalthea is a cratered object of irregular shape (dimensions 270 * 166 * 150 km). Its dark surface - very red - is possibly gray with 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 ones - in the opposite direction. They are all small and dark; probably they were captured by Jupiter from among the asteroids of the Trojan group (see ASTEROID).
Saturn. The second largest giant planet. It is a hydrogen-helium planet, but Saturn has less helium than Jupiter; lower and its average density. The rapid rotation of Saturn leads to its large flattening (11%).
SATURN and its satellites, photographed during the passage of the Voyager space probe.
In a telescope, Saturn's disk does not look as impressive 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 (NH3) fog. Saturn is farther 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 the Jupiterian: under the layer of clouds of ammonium hydrosulfate is a layer of water clouds. In addition to hydrogen and helium, CH4, NH3, C2H2, C2H6, C3H4, C3H8, and PH3 were spectroscopically detected in Saturn's atmosphere. In terms of internal structure, Saturn also resembles Jupiter, although due to its lower mass it has lower pressure and temperature in the center (75 million bar and 10,500 K). Saturn's magnetic field is comparable to that of the earth. Like Jupiter, Saturn emits internal heat, and twice as much as it receives from the Sun. True, this ratio is greater than that of Jupiter, because Saturn, located half the distance, 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 of the planet's radius. They are easily distinguishable when viewed through a telescope, and when viewed at close range show exceptional diversity: from the massive B ring to the narrow F ring, 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 in the dark rings of Uranus and Neptune; their study in different spectral ranges shows that these are "dirty snowballs" with dimensions on the order of a meter. The three classical rings of Saturn, in order from outer to inner, are designated by the letters A, B and C. Ring B is quite dense: radio signals from Voyager passed through it with difficulty. The 4000 km gap between rings A and B, called the Cassini division (or gap), is not really empty, but is comparable in density to the pale C ring, which was formerly called the crepe ring. There is a less visible Encke gap near the outer edge of ring A. In 1859, Maxwell concluded that Saturn's rings should be composed of individual particles orbiting the planet. At the end of the 19th century. this was confirmed by spectral observations showing that the inner parts of the rings revolve faster than the outer ones. Since the rings lie in the plane of the planet's equator, which means they are tilted to the orbital plane by 27 °, the Earth twice in 29.5 years falls into the plane of the rings, 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 from Pioneer 11 (1979) and Voyagers (1980 and 1981) showed a much more complex structure than expected. The rings are divided into hundreds of individual rings with a typical width of several hundred kilometers. Even in Cassini's crack, there were at least five rings. A detailed analysis showed that the rings are inhomogeneous both in size and, possibly, in the composition of particles. 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 F ring, discovered in 1979 by Pioneer at a distance of 4000 km from the outer edge of the A ring. Voyager 1 found that the F ring was twisted and braided like a braid, but had flown for 9 months. later, Voyager 2 found the structure of the F ring much simpler: the "strands" of the substance were no longer intertwined with each other. This structure and its rapid evolution are partly due to the influence of two small moons (Prometheus and Pandora) moving at the outer and inner edges of this ring; they are called "watchdogs". It is possible, however, that the presence of even smaller bodies or temporary accumulations of matter inside the F ring itself is possible.
Satellites. Saturn has at least 18 satellites. Most of them are probably icy. Some have very interesting orbits. For example, Janus and Epimetheus have almost the same orbital radii. Orbiting Dione 60 ° in front of her (this position is called the leading point of Lagrange) moves the smaller satellite Helena. Tefia is accompanied by two small moons - Telesto and Calypso - at the leading and lagging points of the Lagrange of her orbit. The radii and masses of the seven satellites of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Iapetus) have been measured with good accuracy. They are all mostly icy. Those that are smaller have a density of 1-1.4 g / cm3, which is close to the density of water ice with a greater or lesser admixture of rocks. Whether they contain methane and ammonia ice is not yet clear. The higher density of Titanium (1.9 g / cm3) is the result of its large mass, which causes the subsoil to contract. Titan is very similar in diameter and density to Ganymede; probably their internal structure is similar. Titan is the second largest satellite in the solar system, and it is unique in that it has a permanent, powerful atmosphere, consisting mainly of nitrogen and small amounts of methane. The pressure at its surface is 1.6 bar, the temperature is 90 K. Under these conditions, there can be liquid methane on the surface of Titan. The upper layers of the atmosphere up to heights of 240 km are filled with orange clouds, probably consisting of particles of organic polymers synthesized under the influence of ultraviolet rays from 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 impacts of meteorites lead to the outpouring of water and filling craters. Some astronomers believe that particles from the surface of Enceladus formed a wide ring of E, stretching 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 from the surface of Saturn's outer moon Phoebe by meteorite impacts falls on the front hemisphere of Iapetus. In principle, this is possible, since Phoebe orbits in the opposite direction. In addition, the surface of Phoebe is rather dark, but there is no exact data on it yet.
Uranus. Uranus is aquamarine and lackluster, as the upper atmosphere is filled with fog, through which the Voyager 2 probe that flew near it in 1986 barely managed to see several clouds. The axis of the planet is tilted to the orbital axis by 98.5 °, i.e. lies almost in the plane of the orbit. Therefore, each of the poles for some time is directed directly to the Sun, and then for six months (42 Earth years) it goes into the shadow. Uranus' atmosphere 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 terms of strength at the surface, and its axis is inclined to the axis of rotation of the planet by 55 °. Little is known about the inner structure of the planet. Probably, the cloud layer extends to a depth of 11,000 km, followed by a hot water ocean 8,000 km deep, and below it a molten rock core with a radius of 7,000 km.
Rings. In 1976, the unique rings of Uranus were discovered, consisting of individual thin rings, the widest of which is 100 km thick. 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 composed of large dark stones. It is believed that a small satellite or even two satellites are moving in each ring, as in the F ring of Saturn.
Satellites. 20 satellites of Uranus were discovered. The largest - Titania and Oberon - are 1500 km in diameter. There are 3 more large ones, more than 500 km in size, the rest are very small. The surface spectra of the five large satellites indicate a large amount of water ice. The surfaces of all satellites are covered with meteorite craters.
Neptune. Outwardly, Neptune is similar to Uranus; its spectrum is also dominated by the bands of methane and hydrogen. The heat flux from Neptune noticeably exceeds the power of the solar heat falling on it, which indicates the existence of an internal source of energy. It is possible that much of the internal heat is generated by the tides caused by the massive moon Triton, which revolves in the opposite direction at a distance of 14.5 of the planet's radius. 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 vortex flows were discovered in the atmosphere. Triton's pinkish surface reveals amazing geological features, 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 inclined orbit; at perihelion, it approaches the Sun at 29.6 AU. and is removed at aphelion by 49.3 AU. In 1989 Pluto passed perihelion; 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 temperature of Pluto's surface 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 when the planet passes perihelion, but its pressure is 100,000 times less than the pressure of the earth's atmosphere. Pluto cannot hold the atmosphere for a long time - after all, it is smaller than the Moon. Pluto's satellite Charon orbits in 6.4 days 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. Pluto's brightness changes regularly with a period of 6.4 days. Consequently, Pluto rotates synchronously with Charon and has large spots on the surface. In relation to the size of the planet, Charon is very large. The Pluto-Charon pair is often called a "double planet". At one time Pluto was considered an "escaped" satellite of Neptune, but after the discovery of Charon this looks unlikely.
PLANETS: A COMPARATIVE ANALYSIS
Internal structure. The objects of the solar system from the point of view of their internal structure can be divided into 4 categories: 1) comets, 2) small bodies, 3) terrestrial planets, 4) gas giants. Comets are simple ice 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 enough size to heat up due to the radioactive decay taking place in them. Terrestrial planets are very diverse: from "iron" Mercury to the mysterious ice system Pluto - Charon. In addition to the largest planets, the Sun is sometimes referred to the category of gas giants, according to formal criteria. 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 kind of planet is - "stone" (silicates, metals), "ice" (water, ammonia, methane) or "gaseous" (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 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 influence of gravity, but to a special chemical composition: Mercury contains 60-70% of metals and 30-40% of silicates by weight. Mercury has a much higher metal content per unit mass than any other planet. Venus rotates so slowly that its equatorial swelling is measured only by fractions of a meter (at the Earth - 21 km) and absolutely cannot 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 continents "float". Perhaps the continents of Venus are fixed by the rigidity of the mantle, but it is possible that the relief of Venus is dynamically supported by vigorous convection in its mantle. The surface of the Earth is significantly younger than the surfaces of other bodies in the solar system. This is mainly due to the intensive processing of crustal material as a result of plate tectonics. Erosion under the influence of liquid water is also noticeably affected. The surfaces of most planets and satellites are dominated by ring structures associated with impact craters or volcanoes; on Earth, plate tectonics has led to the fact that its largest highlands and lowlands are linear. An example is mountain ranges growing at the collision of two plates; oceanic trenches marking the places where one plate goes under the other (subduction zones); and also mid-ocean ridges in those places where two plates diverge under the influence of a young crust floating up 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. Ultrabasic inclusions are known (ultrabasites, 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 to a depth of approx. 200 km. The mineralogical composition of the deep mantle is not so well known, since there are no precise data on the distribution of temperature with depth and the main phases of deep minerals have not yet been reproduced in the laboratory. The core of the Earth is divided into external and internal. The outer core does not transmit transverse seismic waves; therefore, it is liquid. However, at a depth of 5200 km, the core material again begins to conduct transverse waves, but at a low speed; this means that the inner core is partially "frozen". The core density is lower than it would be for a pure nickel-iron liquid, probably due to the impurity of sulfur. A quarter of the Martian surface is occupied by the Tarsis Upland, which has risen by 7 km relative to the average radius of the planet. It is on it that most of the 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 relatively hot centers in the mantle, so volcanoes grow in one place for a long time. 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 subsoil.
Internal warmth. Many planets emit 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 the forming planet, meteorite bombardment is the main source of heat; then heat is released during the differentiation of the subsurface, when the densest components, such as iron and nickel, settle towards the center and form the core. Jupiter, Saturn and Neptune (but, for some reason, not Uranus) still radiate the heat they stored when they formed 4.6 billion years ago. In terrestrial planets, an important source of heating in the current era is the decay of radioactive elements - uranium, thorium and potassium, which were in small quantities in the original chondritic (solar) composition. The dissipation of energy of motion 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) slowed down the tides.
Convection in the mantle. If the liquid is heated sufficiently strongly, convection develops in it, since thermal conductivity and radiation cannot cope with the locally supplied heat flux. It may seem strange to say that the bowels of terrestrial planets are engulfed in convection, like a liquid. Do we not know that, according to seismological data, shear waves propagate in the earth's mantle and, therefore, the mantle does not consist of liquid, but of solid rocks? But let's take an ordinary glass putty: when pressed slowly, it behaves like a viscous liquid, when it is pressed hard, it behaves like an elastic body, and when it hits it, it behaves like a stone. This means that in order to understand how a substance behaves, we must take into account the time scale of the processes. Shear seismic waves travel through the earth's interior in minutes. On a geological time scale measured in millions of years, rocks are plastically deformed if significant stress is constantly applied to them. It is striking that the earth's crust is still straightening, returning to the previous shape it had before the last glaciation, which ended 10,000 years ago. Having studied the age of the ascended coasts of Scandinavia, N. Haskell calculated in 1935 that the viscosity of the earth's mantle is 1023 times higher 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 are likely to 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. Until now, 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 elongated along the axis of rotation of the planet and bent in accordance with its sphericity. Such convective cells are called "bananas" for their shape. The pressure of the gas giant planets varies from 1 bar at the cloud tops to about 50 Mbar at the center. Therefore, their main component - hydrogen - is at different levels in different phases. At pressures above 3 Mbar, ordinary molecular hydrogen becomes a liquid metal like 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 frame of reference in which the wind speed is measured in the cloudy atmosphere of the giant planet; it is this that indicates that there are powerful streams in the liquid metal core of the Earth, and active mixing occurs in the water mantles of Uranus and Neptune. On the contrary, the absence of a strong magnetic field for 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. Venus's lack of a strong magnetic field does not mean that its core has solidified: most likely, the planet's slow rotation prevents the dynamo effect. Uranus and Neptune have the same magnetic dipoles with a large inclination to the axes of the planets and displacement relative to their centers; this indicates that their magnetism originates in their mantles and not in their cores. Jupiter's moons Io, Europa and Ganymede have their own magnetic fields, but Callisto does not. Residual magnetism is found on 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 specific chemical composition and type of 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. In terrestrial planets, a thin (0.1 km) layer of the atmosphere near the surface constantly experiences heating or cooling from it, and when moving - friction and turbulence (due to the unevenness of the relief); this layer is called surface or boundary layer. Near the surface, molecular viscosity "sticks" 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 heats up from a warm surface, it becomes lighter and floats; rising in the low pressure area, it expands and radiates heat into space, which is why it cools, becomes denser and sinks. As a result of convection in the lower layers of the atmosphere, an adiabatic vertical temperature gradient is established: 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" - cessation), which limits the lower atmosphere, called the troposphere. This is where the changes that we call the weather take place. 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, which means the clouds that create the weather. On Venus, carbon dioxide and water vapor, along with sulfuric acid and sulfur dioxide, absorb almost all infrared radiation emitted by the surface. This causes a strong greenhouse effect, i.e. leads to the fact that the temperature of the surface 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 sulfur from leaving the atmosphere by binding in surface rocks. Sulfur dioxide is enriched in the lower atmosphere of Venus, so there is a dense layer of sulfuric acid clouds in it at altitudes from 50 to 80 km. A small amount of sulfur-containing substances is also found in the earth's atmosphere, especially after powerful volcanic eruptions. Sulfur is not registered in the atmosphere of Mars, therefore, its volcanoes are inactive at the current epoch. On Earth, a stable decrease in temperature with altitude in the troposphere changes above the tropopause to an increase in temperature with altitude. Therefore, there is an extremely stable layer called the stratosphere (Latin stratum - layer, flooring). The existence of permanent thin aerosol layers and the long-term presence of radioactive elements there from nuclear explosions serve as direct evidence of the absence of mixing in the stratosphere. In the earth's stratosphere, the temperature continues to rise with altitude 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 to heat. The chemistry of the stratosphere is complex. Ozone is mainly formed over the equatorial regions, but its greatest concentration is found above 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 above the poles, especially above the winter pole. In the dry atmosphere of Mars, there are relatively few hydroxyl radicals (OH), which deplete ozone. The temperature profiles of the atmospheres of the giant planets were determined from ground-based observations of the planetary coverings of stars and from the data of the probes, in particular, from the weakening of radio signals when the probe enters the planet. Each of the planets found a tropopause and 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 temperatures and relatively low gravity on Uranus allow the atmosphere to extend to the rings. This causes the deceleration and rapid fall of the dust particles. Since dust lanes are still observed 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 atmosphere. 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 of 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 it once had a hundred times more water than it does now. The explanation for this is usually sought within the framework of the theory of "greenhouse volatilization", 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 vapor, 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 the terrestrial planets were formed as a result of the degassing of the interior; in this case, mainly water vapor and carbon dioxide were released. On Earth, water is concentrated in the ocean, and carbon dioxide is trapped in sedimentary rocks. But Venus is closer to the Sun, it's hot and there is no life; therefore carbon dioxide remained in the atmosphere. Water vapor under the influence of sunlight dissociated into hydrogen and oxygen; hydrogen escaped into space (the earth's atmosphere is also rapidly losing hydrogen), and oxygen was 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 may slightly exceed the normal freezing point of water, but low atmospheric pressure prevents water on the surface of Mars from being liquid: ice immediately turns into steam. However, there are several canyons on Mars that resemble dried-up river beds. Some of them appear to have been dug by short-term but catastrophically powerful streams of water, while others show deep ravines and an extensive network of valleys, indicating the likely continued 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 strongly than the young ones, and this is possible only if the atmosphere of Mars was much denser than it is now. In the early 1960s, Mars' polar caps were thought to be composed of water ice. But in 1966 R. Leighton and B. Murray examined 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 contraction of the polar caps lead to pressure fluctuations in the Martian atmosphere by 20% (for example, in the cabins of old jet liners, the pressure drops during takeoff and landing were also about 20%). Space photographs of Mars' polar caps show the amazing spiral patterns and stepped terraces that the Mars Polar Lander (1999) probe was supposed to explore but failed to land. It is not known exactly why the pressure of the Martian atmosphere dropped so much, probably from a few bars in the first billion years to 7 mbar now. It is possible that weathering of surface rocks removed carbon dioxide from the atmosphere, binding 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; apparently, it has proved to be very difficult to maintain a warm and humid climate on Mars throughout the entire 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 significantly more carbon dioxide than it does now, and the temperature of the atmosphere was higher. The main difference in the evolution of the atmosphere of Earth and Mars is that on Earth plate tectonics supports the carbon cycle, whereas on Mars it is "locked" in rocks and polar caps.
Near-planetary rings. It is curious that each of the giant planets has a system of rings, but not a single planet of the terrestrial type. 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 other planets are almost invisible through a telescope. Jupiter's pale ring is experiencing a mysterious interaction with its magnetic field. Uranus and Neptune are each surrounded by several thin rings; the structure of these rings reflects their resonant interaction with nearby satellites. The three circular arcs of Neptune are especially intriguing for researchers, since they are clearly limited in both the radial and azimuthal directions. A big surprise was the discovery of the narrow rings of Uranus during the observation of its covering of the 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 (radiation braking) and plasma inhibition. From a practical point of view, narrow rings, the position of which can be measured with high accuracy, have proved to be a very convenient indicator of the orbital motion of particles. The precession of the rings of Uranus made it possible to find out 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 scatter light strongly in the direction of its fall. That is why it is difficult to detect dust in planetary rings, 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 received 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 long-known narrow rings. Rotating discs are the most important topic in modern astrophysics. Many of the dynamical theories developed to explain the structure of galaxies can be used to study planetary rings. Thus, the rings of Saturn became an object for testing the theory of self-gravitating disks. The self-gravity property of these rings is indicated by the presence of both spiral density waves and spiral bending waves in them, which are visible in detailed images. The wave packet found in Saturn's rings has been attributed to the planet's strong horizontal resonance with the moon Iapetus, which excites spiral density waves in the outer part of the Cassini fission. There have been many speculations about the origin of the rings. It is important that they lie within the Roche zone, i.e. at such a distance from the planet, where the mutual attraction of the 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. It is possible that the material of the rings has remained "unclaimed" since the formation of the planet itself. But maybe these are traces of a recent catastrophe - the 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 the rest of the planets, the substance is much less.
SMALL BODIES OF THE SOLAR SYSTEM
Asteroids. Many minor planets - asteroids - revolve around the Sun, mainly between the orbits of Mars and Jupiter. Astronomers have adopted the name "asteroid" because in a telescope they look like faint stars (aster is Greek for "star"). At first it was thought that these were fragments of a large planet that once existed, but then it became clear that asteroids never made up a single body; most likely, this substance could not unite into a planet due to the influence of Jupiter. It is estimated that the total mass of all asteroids in our epoch 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's 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 orbital elements of more than 8000 asteroids are known out of 33,000 discovered to date. There are at least two hundred asteroids with a radius of more than 50 km and about a thousand - more than 15 km. It is estimated that about a million asteroids have a radius of more 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 in order to discern surface details of even large asteroids using ground-based telescopes. The orbital radii of most asteroids are between 2.2 and 3.3 AU, this area 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 disturbances from the direction of Jupiter. All asteroids rotate in a forward direction, but the orbits of many of them are noticeably elongated and tilted. Some asteroids have very curious orbits. Thus, a group of Trojans is orbiting Jupiter; most of these asteroids are very dark and red. The asteroids of the Amur group have orbits approaching or crossing the orbit of Mars; including 433 Eros. Apollo group asteroids traverse Earth's orbit; among them 1533 Icarus, which is closest to the Sun. Obviously, sooner or later, these asteroids experience a dangerous approach to the planets, which ends with a collision or a major change in orbit. Finally, recently, asteroids of the Aton group have been allocated to a special class, whose orbits lie almost entirely within the Earth's orbit. They are all very small in size. The brightness of many asteroids changes periodically, which is natural for rotating irregular bodies. The periods of their rotation are in the range from 2.3 to 80 hours and on average are close to 9 hours. Asteroids owe their irregular shape to numerous mutual collisions. Examples of exotic form are given by 433 Eros and 643 Hector, in which the ratio of the lengths of the axes reaches 2.5. In the past, the entire inner solar system was probably similar to the main asteroid belt. Jupiter, located near this belt, strongly perturbs the movement of asteroids by its attraction, increasing their speed and leading to collisions, 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 hide 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, based on their reflectivity and color, are assigned to three groups, similar to the groups of meteorites: Type C asteroids have a dark surface, like carbonaceous chondrites (see Meteorites below), type S is brighter and redder, and type M is similar to iron-nickel meteorites ... For example, 1 Ceres is similar to carbonaceous chondrites, and 4 Vesta is similar to basaltic eucrites. 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 remains 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, there are now many characteristic infrared and ultraviolet spectral lines 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 contain other asteroids. From the large group of C-asteroids, classes B, F and G were further distinguished. The modern classification includes 14 types of asteroids, designated (in decreasing order of the number of members) by the letters S, C, M, D, F, P, G, E, B, T, A, V, Q, R. Since the albedo of C-asteroids is lower than that of S-asteroids, observational selection takes place: dark C-asteroids are more difficult to detect. Taking this into account, the most numerous type is precisely the C-asteroids. Comparison of the spectra of asteroids of various types with the spectra of samples of pure minerals formed three large groups: 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; magmatic ones are covered with complex minerals, probably formed from melt. The inner region of the main asteroid belt is richly populated by magmatic asteroids, metamorphic asteroids prevail in the middle of the belt, and primitive asteroids prevail on the periphery. This indicates that a sharp temperature gradient existed in the asteroid belt during the formation of the solar system. The classification of asteroids based on their spectra groups bodies according to surface composition. But if we consider the elements of their orbits (semi-major axis, eccentricity, inclination), then the dynamic families of asteroids, first described by K. Hirayama in 1918, stand out. The most populated of them are the families of Themis, Eos and Koronis. Probably each family is a swarm of debris from a relatively recent collision. A systematic study of the solar system leads us to understand that large collisions are the rule rather than the exception, and that the Earth is also not immune from them.
Meteorites. A meteoroid is a small body orbiting the sun. A meteor is a meteoroid that flew into the planet's atmosphere and heated to a blaze. 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 have observed its flight in the atmosphere; otherwise, it is called "found". There are much more "found" meteorites than "fallen" ones. They are often found by tourists or peasants working in the fields. Because meteorites are dark in color and easily distinguishable in the snow, the ice fields of Antarctica, where thousands of meteorites have already been found, are an excellent place to find them. For the first time a meteorite in Antarctica was discovered in 1969 by a group of Japanese geologists studying 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 glacial fields moving at a speed of several meters a year stop, running into mountain ranges. The wind destroys and dries up the upper layers of ice (dry sublimation occurs - ablation), and meteorites are concentrated 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 that are 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 the fragments of this meteorite found were combined under the name of Allende. The fall of the Allende meteorite coincided with the beginning of the Apollo lunar program and gave scientists the opportunity to work out methods for analyzing extraterrestrial samples. In recent years, it has been established that some meteorites containing white debris embedded in darker parent rock are 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, grain) - the oldest spherical particles that condensed in a protoplanetary nebula and then became part of later rocks. Such meteorites allow us to estimate the age of the solar system and its original composition. The calcium and aluminum-rich Allende meteorite inclusions, the first to condense due to their high boiling point, have an age measured by 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 on them of galactic cosmic rays, solar radiation and solar wind. By examining the damage caused by cosmic rays, you can tell how long the meteorite was in orbit before it came under the protection of the earth's atmosphere. The direct connection 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 that differ in content are volatiles, such as hydrogen and helium, abundantly evaporated from meteorites during their cooling, as well as lithium, partially "burned up" by the Sun in nuclear reactions. The terms "solar composition" and "chondritic composition" are used interchangeably when describing the above "recipe for solar matter". Stone meteorites, the composition of which differs from the solar one, are called achondrites.
Small fragments. 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 is deflected back and therefore slows down the particle's movement). 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 in the form of zodiacal light, stretching in a wide strip along the ecliptic in the west after sunset and in the east before sunrise. Near the Sun, the zodiacal light transforms 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 decreases rapidly, but at the anti-solar point of the ecliptic it increases again, forming an anti-radiance; this is because fine dust particles are intensely reflecting light back. From time to time, meteoroids fall into the Earth's atmosphere. Their speed is so high (on average 40 km / s) that almost all of them, except for the smallest and largest, burn up at an altitude of about 110 km, leaving long glowing 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, many meteors are observed annually around August 12 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.
see also METEOR. Particles less than 30 microns in size can be decelerated in the atmosphere and fall to the ground without being burnt; such micrometeorites are collected for laboratory analysis. If particles a few centimeters in size or more consist of a sufficiently dense substance, then they also do not burn out entirely and fall to the surface of the Earth 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 as asteroid fragments. Iron meteorites were once part of the cores of these bodies, destroyed by collisions. It is possible that some loose and volatile-rich meteorites originated from comets, but this is unlikely; most likely, large particles of comets burn up in the atmosphere, and only small ones remain. Given how difficult it is for comets and asteroids to reach Earth, it is clear how useful it is to study meteorites that independently "arrived" on our planet from the depths of the solar system.
see also METEORITE.
Comets. Usually comets arrive from the distant periphery of the solar system and for a short time become extremely spectacular luminaries; at this time they attract everyone's attention, but much in 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, the orbits of which are well known; but all these comets, which repeatedly approached the Sun, have already aged, almost completely lost their volatiles and became pale and inactive. Only one periodic comet still retained activity - this is Halley's comet. Her 30 appearances have been regularly recorded since 240 BC. and named the comet in honor of the astronomer E. Galley, who predicted its appearance in 1758. Halley's comet has an orbital period of 76 years, a perihelion distance of 0.59 AU. and aphelion 35 a.u. When in March 1986 she crossed the plane of the ecliptic, an armada of spacecraft with fifty scientific instruments rushed to meet her. Particularly important results were obtained by two Soviet probes "Vega" and the European "Giotto", which for the first time transmitted images of the cometary nucleus. They show a very uneven surface, covered with craters, and two gas jets gushing out on the sunny side of the core. The volume of Halley's comet nucleus was larger than expected; its surface, which reflects 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 was discovered earlier. They are often classified according to the duration of the orbital period: short-period (3 OTHER PLANETARY SYSTEMS
From modern views on the formation of stars, it follows that the birth of a solar-type star must be accompanied by the formation of a planetary system. Even if this applies only to stars completely similar to the Sun (ie, single stars of spectral class G), then in this case no less than 1% of the stars in the Galaxy (and this is about 1 billion stars) must have planetary systems. A more detailed analysis shows that all stars can have planets colder than spectral class F, and even those included in binary systems.
Indeed, in recent years, there have been reports of the discovery of planets in other stars. At the same time, the planets themselves are not visible: their presence is detected by the slight displacement of the star, caused by its attraction to the planet. The orbital motion of the planet causes the star to "wobble" and to periodically change its radial velocity, which can be measured by the position of the lines in the star's spectrum (Doppler effect). By the end of 1999, the discovery of Jupiter-type planets was reported in 30 stars, 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) only 15 St. years. Two radio pulsars (PSR 1257 + 12 and PSR B1628-26) also have planetary systems with masses of the order of the Earth's mass. So far it has not been possible to detect such light planets in normal stars with the help of optical technology. Around each star, you can specify an ecosphere in which the temperature of the planet's surface allows liquid water to exist. 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 movement of the planets with high accuracy obeys the three laws of I. Kepler (1571-1630), derived by him 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 for equal periods of time of the planet's orbital motion. 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 conservation of angular momentum and is the most general of the three. Newton established 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 is 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 Hooke's law of direct proportionality (describing 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 motion of each member of the planetary system can be calculated with high accuracy within the framework of the problem of the motion of two mutually gravitating bodies - the sun and the only planet next to it. Its mathematical solution is known: if the speed of the planet is not too high, then it moves in a closed periodic orbit, which can be accurately calculated. The problem of the motion of more than two bodies, generally called the "N-body problem", is much more difficult because of their chaotic motion in open orbits. This randomness of the orbits is fundamentally important and makes it possible to understand, for example, how meteorites fall from the asteroid belt to the Earth.
see also
KEPLER'S LAWS;
HEAVENLY MECHANICS;
ORBIT. In 1867, D. Kirkwood was the first to note that empty spaces ("hatches") in the asteroid belt are located at such distances from the Sun, where the average motion is commensurate (in integer ratio) 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 Jupiter's revolution. Kirkwood's two largest hatches are in 3: 1 and 2: 1 proportions. However, near the 3: 2 commensurability, there is an excess of asteroids, united on this basis into the Gilda group. There is also an excess of asteroids of the Trojan group at a 1: 1 ratio, orbiting Jupiter 60 ° in front and 60 ° behind it. The situation with the Trojans is clear - they are captured near the stable Lagrange points (L4 and L5) in Jupiter's orbit, but how to explain Kirkwood's hatches and Gilda's group? If there were only hatches on commensurate, then one could accept a simple explanation, proposed by Kirkwood himself, that asteroids were thrown out of resonance 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 fragments of asteroids that fall into this region change their orbits from circular to elongated elliptical, regularly bringing them to the central part of the solar system. In such orbits that cross the planetary paths, meteoroids do not live long (only a few million years) before crashing into Mars or Earth, and with a small miss, they are thrown out to the periphery of the solar system. So, the main source of meteorites falling to the Earth is Kirkwood's hatches, through which the chaotic orbits of asteroid fragments pass. There are, of course, many examples of highly ordered resonant motions in the solar system. This is how satellites close to the planets move, for example, the Moon, always facing the same hemisphere to the Earth, since its orbital period coincides with the axial one. An example of an even higher synchronization is provided by the Pluto-Charon system, in which "a day is equal to a month" not only on the satellite, but also on the planet. The motion of Mercury has an intermediate character, the axial rotation and orbital rotation of which are in a resonance ratio of 3: 2. However, not all bodies behave so simply: for example, in a nonspherical Hyperion, under the influence of the attraction of Saturn, the axis of rotation is chaotically inverted. 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, remote from the planet at different distances, are attracted to it in different ways; the same is true for the attraction from the satellite to the planet. This difference in forces causes the ebb and flow of the sea, and gives a slightly flattened shape to the synchronously rotating satellites. The satellite and the planet cause each other to tidal deformations, and this affects their orbital motion. The resonance of average motions 4: 2: 1 for the moons of Jupiter Io, Europa and Ganymede, first studied in detail by Laplace in his Celestial Mechanics (vol. 4, 1805), is called the Laplace resonance. Just days before Voyager 1 flew to Jupiter, on March 2, 1979, astronomers Peale, Kassen, and Reynolds published the work "Melting of Io by Tidal Dissipation", in which they predicted active volcanism on this satellite due to its leading role in maintaining a resonance of 4: 2: 1. Voyager 1 indeed discovered active volcanoes on Io, so powerful that not a single meteorite crater is visible on images of the satellite's surface: so quickly its surface is covered with eruption products.
FORMATION OF A SOLAR SYSTEM
The question of how the solar system was formed is perhaps the most difficult in planetary science. To answer it, we still have little data that would help to reconstruct the complex physical and chemical processes that took place in that distant era. The 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 one direction, in almost circular orbits, lying almost in the same plane. Most of them rotate on their axis in the same direction as the sun. All this indicates that the predecessor of the solar system was a rotating disk, which naturally forms when a self-gravitating system is compressed with conservation of angular momentum and the resulting increase in angular velocity. (Momentum, or angular momentum of a planet, is the product of its mass times its distance from the Sun and its orbital velocity. The momentum of the Sun is determined by its axial rotation and is approximately equal to the product of its mass times its radius and rotation speed; the axial moments of planets are negligible.) The sun contains in itself 99% of the mass of the solar system, but only approx. 1% of its angular momentum. The theory should explain why most of the system's mass is concentrated in the Sun, and the overwhelming part of the angular momentum is in the outer planets. The available theoretical models of the formation of the solar system indicate that in the beginning the sun rotated much faster than it is 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. An approximate rule for the regular distribution of planetary distances from the Sun (Titius-Bode rule) has been known for two centuries, but there is no explanation for it. In the systems of satellites of the outer planets, the same regularities are traced as in the planetary system as a whole; probably, the processes of their formation had a lot in common.
see also BODY LAW.
Chemical composition. In the solar system, there is a strong gradient (difference) in chemical composition: planets and satellites close to the Sun consist of refractory materials, while distant bodies contain many volatile elements. 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 original composition of the protoplanetary cloud was close to the composition of the interstellar medium and the Sun: up to 75% of hydrogen in mass, up to 25% of 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 examining meteorites, we begin to understand the chemistry of the primordial nebula. However, the process of agglomeration of large planets from small particles remains unclear.
Isotope data. The isotopic composition of meteorites indicates that the formation of the solar system took place 4.6 ± 0.1 billion years ago and lasted no more than 100 million years. Anomalies of the isotopes of neon, oxygen, magnesium, aluminum and other elements indicate that in the process of the collapse of the interstellar cloud that gave birth to the solar system, the products of the explosion of a nearby supernova got into it.
see also ISOTOPES; SUPERNOVA .
Star formation. Stars are born in the process of collapse (compression) of interstellar gas-dust clouds. This process has not yet been studied in detail. There are observational facts in favor of the fact that shock waves from supernova explosions can compress interstellar matter and stimulate the collapse of clouds into stars.
see also GRAVITY COLLAPSE. Before a young star reaches a stable state, it goes through 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. In the spectra of some T Tauri stars, there are forbidden lines that arise only in low-density gas; these are probably the remnants of a protostellar nebula surrounding the star. T Tauri stars are characterized by fast fluctuations of ultraviolet and X-ray radiation. Many of them exhibit powerful infrared radiation and silicon spectral lines, indicating that the stars are surrounded by dust clouds. Finally, T Tauri stars have a powerful stellar wind. It is believed that in the early period of its evolution, the Sun also passed through the T Tauri stage, and that it was during this period that volatile elements were expelled from the inner regions of the solar system. Some moderate-mass forming stars exhibit strong luminosity increases and envelope ejection in less than a year. Such phenomena are called FU-type Orion flares. At least once such an outburst has been experienced by a T Tauri star. It is believed that most young stars go through the FU Orion stage. Many people see the reason for the outburst in the fact that from time to time the rate of accretion onto a young star of matter from the surrounding gas-dust disk increases. If the Sun also experienced one or more FU Orion-type flares in its early evolutionary period, this should have strongly affected the 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 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.
Formation theories. Scientific theories of the formation of the solar system can be divided into three categories: tidal, accretionary, and nebular. The latter are attracting the greatest interest now. The tidal theory, apparently first proposed by Buffon (1707-1788), does not directly link the formation of stars and planets. It is assumed that another star flying past the Sun, by means of tidal interaction, pulled out from it (or from itself) a stream of matter from which the planets were formed. This idea faces many physical problems; for example, hot matter ejected by a star should spray, not condense. 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 the dense interstellar cloud. Indeed, young stars are usually found near large interstellar clouds. However, within the framework of accretion theory, it is difficult to explain the chemical composition gradient in the planetary system. The most developed and generally accepted now is the nebular hypothesis, proposed by Kant at the end of the 18th century. Its main idea is that the Sun and planets were formed simultaneously from a single rotating cloud. Compressing, 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 the cloud, and then, as it contracts, the centrifugal force tore off the gas rings from the equator, which later condensed into planets. Laplace's hypothesis is faced with 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 his 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. This behavior is typical for young stars, which are called T Tauri stars by the name of the prototype. The distribution of pressure and temperature of 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 in 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.
The growth of the planets. There are many scenarios for the growth of planets. The planets may have formed as a result of random collisions and sticking together of small bodies called planetesimals. But, perhaps, small bodies combined into larger ones at once in large groups as a result of gravitational instability. It is not clear whether the accumulation of planets took place in a gaseous or a gas-free environment. In a gaseous nebula, temperature drops are smoothed out, but when part of the gas condenses into dust grains and the remaining gas is swept away by the stellar wind, the nebula's transparency 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 grains, 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 outside the Earth, and perhaps it still exists. The advent of space technology made it possible to begin a direct test of this hypothesis. Mercury turned out to be too hot and devoid of atmosphere and water. Venus is also very hot - lead melts on its surface. The possibility of life in the upper cloud layer of Venus, where conditions are much milder, is still no more than a fantasy. The moon and asteroids look completely sterile. Great hopes were pinned on Mars. The systems of thin straight lines - "channels", seen through a telescope 100 years ago, gave then reason to talk about artificial irrigation structures 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. The devices 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 have once 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 river beds. 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 planet's temperature, 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 the samples of the Martian soil. But getting them to Earth is a daunting task. Fortunately, there is strong evidence that out 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 (Chassigny, France). The ALH 84001 meteorite found in Antarctica is significantly 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, containing glasses with inclusions of bubbles, in which the composition of noble gases differs from the terrestrial, but corresponds to the atmosphere of Mars. Although there are many organic molecules in the atmospheres of the giant planets, it is hard to believe that life can exist there in the absence of a solid surface. In this sense, the satellite of Saturn, Titan, is much more interesting, which has not only an atmosphere with organic components, but also a solid surface where fusion 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 satellite Europa, although it lacks an atmosphere, but apparently has 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, so far we have no evidence that life in the solar system exists anywhere outside the Earth. One may ask questions: what are the possibilities of scientific instruments in connection with the search for extraterrestrial life? Could a modern space probe detect the presence of life on a distant planet? For example, could the Galileo spacecraft detect life and intelligence on Earth when it flew past it twice, making gravity assist 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 proof of its presence. They are completely unlike 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 an unreasonable life manifest itself? The Galileo television camera captured images of the Earth in six narrow spectral ranges. In the 0.73 and 0.76 micron filters, some land areas appear green due to strong absorption of red light, which is not typical for deserts and rocks. The easiest way to explain this is that a certain carrier of non-mineral pigment that absorbs red light is present on the planet's surface. We know for sure that this unusual light absorption 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 the ice shell of Europe, then a vehicle flying by is unlikely to detect it.
Geography Dictionary
Loading...
