Density of terrestrial planets. Presentation of "terrestrial planets"

Planets terrestrial group- Mercury, Venus, Earth and Mars differ from the giant planets in their smaller sizes and lower mass. They move inside the belt of minor planets. Within the same group, planets are similar in physical characteristics such as density, size chemical composition, but one group differs sharply from the other. Each planet has its own unique characteristics

Mercury is the planet closest to the Sun in the Solar System. Located at a distance of 58 million km from the Sun. It completes a full revolution in the sky in 88 days. Due to its proximity to the Sun and small apparent size, Mercury has long remained a little-studied planet. Only in 1965, thanks to the use of radar, the rotation period of Mercury around its axis was measured, which turned out to be equal to 58.65 days, i.e. 2/3 of its revolution around the Sun. This rotation is dynamically stable. A solar day on Mercury lasts 176 days. Mercury's rotation axis is almost perpendicular to the plane of its orbit. As radio observations suggested, the temperature on the surface of Mercury at the point where the Sun is at its zenith reaches 620 K. The temperature of the night hemisphere is about 110 K. Using radio observations, it was possible to determine the thermal properties of the outer layer of the planet, which turned out to be close to the properties of finely crushed rocks of the lunar regolith. The reason for this state of the rocks, apparently, is the continuous impacts of meteorites, almost not weakened by the rarefied atmosphere of Mercury.

Photographing the surface of Mercury by the American spacecraft Mariner 10 in 1974-1975. showed that the planet resembles the Moon in appearance. The surface is dotted with craters of different sizes, and their distribution by diameter is similar to the distribution of craters on the Moon. This suggests that they were formed as a result of intense meteorite bombardment billions of years ago during the early stages of the planet's evolution. There are craters with light rays, with and without central hills, with dark and light bottoms, with sharp outlines of shafts (young) and dilapidated (ancient). Valleys have been discovered that resemble the famous Valley of the Alps on the Moon, smooth round plains called basins. The largest of them - Caloris - has a diameter of 1300 km. The presence of dark matter in pools and lava-filled craters indicates that in the initial period of its existence the planet experienced strong heating, followed by one or more eras of intense volcanism. The atmosphere of Mercury is very thin compared to the earth's atmosphere.

According to data obtained from Mariner 10, its density does not exceed the density of the earth's atmosphere at an altitude of 620 km. Small amounts of hydrogen, helium and oxygen were found in the atmosphere; some inert gases, such as argon and neon, are also present. Such gases could be released as a result of the decay of radioactive substances that make up the planet's soil. A weak magnetic field has been discovered, the intensity of which is less than that of the Earth and greater than that of Mars. The interplanetary magnetic field, interacting with the core of Mercury, can create electric currents in it. These currents, as well as the movement of charges in the ionosphere, which is weaker on Mercury compared to Earth’s, can maintain the planet’s magnetic field. Interacting with the solar wind (see Radiation from the Sun), it creates a magnetosphere. The average density of Mercury is significantly higher than that of the Moon and is almost equal to the average density of the Earth. It is hypothesized that Mercury has a thick silicate shell (500 - 600 km), and the remaining 50% of the volume is occupied by a ferrous core. Life on Mercury cannot exist due to the very high daytime temperatures and lack of liquid water. Mercury has no satellites

Venus is the second planet in the solar system in terms of distance from the Sun and the closest planet to Earth. The average distance from the Sun is 108 million km. The period of revolution around it is 225 days. During inferior conjunctions it can approach the Earth up to 40 million km, i.e. closer than any other large planet in the solar system. The synodic period (from one inferior connection to another) is 584 days. Venus is the brightest luminary in the sky after the Sun and Moon. Known to people since ancient times. The diameter of Venus is 12,100 km. (95% of the Earth's diameter), mass 81.5% of the Earth's mass or 1: 408,400 of the Sun's mass, average density 5.2 g/cm, surface gravity acceleration 8.6 m/s (90% of Earth's). The rotation period of Venus could not be established for a long time due to the dense atmosphere and cloud layer enveloping this planet. Only with the help of radar did they establish that it is equal to 243.2 days, and Venus rotates in the opposite direction compared to the Earth and other planets. The inclination of the axis of rotation of Venus to the plane of its orbit is almost 90 50 0. The existence of an atmosphere of Venus was discovered in 1761 by M. V. Lomonosov while observing its passage across the disk of the Sun.

In the 20th century, using spectral studies, carbon dioxide was found in the atmosphere of Venus, which turned out to be the main gas of its atmosphere. According to the Soviet interplanetary stations of the Venus series, carbon dioxide accounts for 97% of the total composition of the atmosphere of Venus. It also includes about 2% nitrogen and inert gases, no more than 0.1% oxygen and small amounts of carbon monoxide, hydrogen chromium and hydrogen fluoride. In addition, its atmosphere contains about 0.1% water vapor. Carbon dioxide and water vapor create a greenhouse effect in the atmosphere of Venus, leading to a strong heating of the planet. The reason for this is that both balls intensively absorb infrared (heat) rays emitted by the heated surface of Venus. Its temperature reaches about 500 C. The cloud layer of Venus, hiding its surface from us, as established by the Venus series stations, is located at an altitude of 49-68 km above the surface, and its density resembles a light fog.

But the large extent of the cloud layer makes it completely opaque to an earthly observer. It is assumed that the clouds consist of droplets of a hydrogen solution of sulfuric acid. Illumination on the surface during the daytime is similar to that on the ground on a cloudy day. From space, the clouds of Venus look like a system of stripes, usually located parallel to the equator of the planet, but sometimes they form details that were noticed from Earth, which made it possible to establish an approximately 4-day period of rotation of the cloud layer. This four-day rotation has been confirmed by spacecraft and is explained by the presence of constant winds at the cloud level, blowing in the direction of the planet's rotation at a speed of about 100 m/s. The atmospheric pressure at the surface of Venus is about 9 MPa, and the density is 35 times higher than the density of the earth's atmosphere. The amount of carbon dioxide in the atmosphere of Venus is 400 thousand times greater than in the earth's atmosphere. The reason for this is probably intense volcanic activity, and in addition, the absence on the planet of the two main carbon dioxide sinks of the ocean with its plankton and vegetation.

The uppermost layers of Venus's atmosphere are composed entirely of hydrogen. The hydrogen atmosphere extends to an altitude of 5500 km. Radar made it possible to study the relief of Venus, invisible because of the clouds. In the near-equatorial zone, more than 10 ring structures similar to the craters of the Moon and Mercury, with a diameter of 35 to 150 km, but highly smoothed and flat, were discovered. A fracture 1500 km long and 150 km wide has been discovered in the planet's crust. and a depth of about 2 km., mountain ranges, a volcano with a base diameter of 300-400 km. and about 1 km high, a huge basin with a length of 1500 km. from north to south and 1000 km. from west to east. The interplanetary stations "Venera-9" and "Venera-10" made it possible to study the relief of 55 regions of the planet from the orbits of artificial satellites of Venus; At the same time, mountainous areas with an elevation difference of 2-3 km were discovered, as well as relatively flat areas. The surface of Venus is relatively smoother than the surface of the Moon. An analysis of the nature and surface of Venus can be of great importance for constructing a theory of the evolution of all planets in the solar system, including our Earth. Venus has no satellites

Earth is one of the planets in the solar system. Like other planets, it moves around the Sun in an elliptical orbit. The distance from the Earth to the Sun at different points in the orbit is not the same. The average distance is about 149.6 million km. As our planet moves around the Sun, the plane of the Earth's equator (inclined to the plane of the orbit at an angle of 23 27") moves parallel to itself in such a way that in some parts of the orbit the globe is inclined towards the Sun with its northern hemisphere, and in others - with its southern hemisphere. For the most part, The World Ocean occupies up to 71% of the Earth's surface. Average depth The world ocean is 3900 m. The existence of sedimentary rocks whose age exceeds 3.5 billion years serves as evidence of the existence of vast bodies of water on Earth already in that distant time. On modern continents, plains, mainly low-lying ones, are more common, and mountains - especially high ones - occupy a small part of the planet's surface, as well as deep-sea depressions of the oceans. The shape of the Earth, as is known, is close to spherical, but with more detailed measurements it turns out to be very complex, even if we outline it with a flat ocean surface (not distorted by tides, winds, currents) and the conditional continuation of this surface under the continents.

The irregularities are maintained by the uneven distribution of mass in the Earth's interior. This surface is called a geoid. The geoid (with an accuracy of the order of hundreds of meters) coincides with the ellipsoid of rotation, the equatorial radius of which is 6378 km, and the polar radius is 21.38 km. less than equatorial. The difference in these radii arose due to the centrifugal force created by the daily rotation of the Earth. The daily rotation of the globe occurs at an almost constant angular velocity with a period of 23 hours 56 minutes. 4.1s. those. for one sidereal day, the number of which in a year is exactly one day more than solar days. The Earth's axis of rotation is directed at its northern end approximately to the star alpha Ursa Minor, which is therefore called the North Star. One of the features of the Earth is its magnetic field, thanks to which we can use a compass. The Earth's magnetic pole, to which the north end of the compass needle is attracted, does not coincide with the geographic North Pole. Under the influence of the solar wind (see Radiation from the Sun), the Earth's magnetic field is distorted and acquires a “trail” in the direction from the Sun, which extends for hundreds of thousands of kilometers. Our planet is surrounded by a vast atmosphere. The main gases that make up the lower layers of the atmosphere are nitrogen (about 78%), oxygen (about 21%) and argon (about 1%). There are very few other gases in the Earth's atmosphere, for example carbon dioxide is about 0.03%.

Atmospheric pressure at the ocean surface level is approximately 0.1 MPa under normal conditions. It is believed that the earth’s atmosphere has changed greatly in the process of evolution: it has become enriched with oxygen and acquired its modern composition as a result of long-term interaction with rocks and with the participation of the biosphere, i.e. plant and animal organisms. Evidence that such changes have actually occurred is provided, for example, by coal deposits and thick layers of carbonate deposits in sedimentary rocks. they contain enormous amounts of carbon, which was previously part of the earth's atmosphere in the form of carbon dioxide and carbon monoxide. Scientists believe that the ancient atmosphere came from gaseous products of volcanic eruptions; its composition is judged by chemical analysis of gas samples “embedded” in the cavities of ancient rocks. The studied samples, which are approximately 3.5 billion years old, contain approximately 60% carbon dioxide, and the remaining 40% are sulfur compounds, ammonia, hydrogen chloride and hydrogen fluoride.

A small amount of nitrogen and inert gases were found. All oxygen was chemically bound. One of the most important tasks of modern Earth science is the study of the evolution of the atmosphere, surface and outer layers of the Earth, as well as the internal structure of its interior. The internal structure of the Earth is primarily judged by the characteristics of the passage of mechanical vibrations through the various layers of the Earth that occur during earthquakes or explosions. Valuable information is also provided by measurements of the magnitude of the heat flow emerging from the depths, the results of determinations of the total mass, moment of inertia and polar compression of our planet. The mass of the Earth is found from experimental measurements of the physical constant of gravity and acceleration of gravity

The solid shell of the Earth is called the lithosphere. It can be compared to a shell covering the entire surface of the Earth. But this “shell” seems to have cracked into pieces and consists of several large lithospheric plates, slowly moving one relative to the other. The overwhelming number of earthquakes is concentrated along their boundaries. Upper layer lithosphere is the earth's crust, the minerals of which consist mainly of silicon and aluminum oxides, iron oxides and alkali metals. The earth's crust has an uneven thickness: 35-65 km. on continents and 6-8 km. under the ocean floor. Upper layer earth's crust consists of sedimentary rocks, the lower one is made of basalts. Between them there is a layer of granites, characteristic only of the continental crust. Under the crust is the so-called mantle, which has a different chemical composition and greater density. The boundary between the crust and mantle is called the Mohorovicic surface. In it, the speed of propagation of seismic waves increases abruptly. At a depth of 120-250 km under the continents and 60-400 km. Beneath the oceans lies a layer of mantle called the asthenosphere. Here the substance is in a state close to melting, its viscosity is greatly reduced. All lithospheric plates seem to float in a semi-liquid asthenosphere, like ice floes in water.

The terrestrial group of planets is closest to the Sun. It consists of metal or silicate rock, which is why such a planet is called rocky or telluric. The terrestrial planet is located inside the solar system. Such a planet is called terrestrial because it contains elements reminiscent of planet Earth. And it even got its name from Latin “ Terra "- translated means "earth".

While gas giant planets are composed of various types of water, helium and hydrogen, which can be transformed into a variety of physical states, the terrestrial group of planets have an exclusively solid surface. These planets are included in the same group because of the similarity of their structure: inside they have a metallic core, which is iron, and this core is surrounded by a special silicate mantle. And also these planets are combined into one group, since each of them has terrestrial components, which include volcanoes, mountains, canyons and others.

The terrestrial group of planets has an uncompressed density equal to zero pressure of the average density of matter on any planet. But since compression in planetary cores can increase its density, the real average density and uncompressed density may differ. Scientists determine the average density for each terrestrial planet separately, because the calculation of density depends on the size and what is included in its composition.

There is no way to know how much it actually was terrestrial planets when the solar system began to form. Perhaps they were expelled from the four planets, or merged (combined) with each other. The planetary nebula itself reorganized itself, and there were four such planets - Mars, Mercury, Venus, and of course, the Earth itself.

Features of terrestrial planets

Mars

This planet is half the Earth and the fourth from the sun. It has almost no atmosphere, only carbon dioxide, and is the coldest (from 00 degrees to minus 113C). A day on Mars is identical to that on Earth, but the year is longer - 687 days. There are no liquids on Mars; there are ice caps of gas and frozen water. Mars is famous for its volcanoes, craters and two satellites - Deimos and Phobos.

Mercury

It is closest to the Sun and the smallest in size of all four. It is slightly larger than the Moon. The surface of Mercury is littered with impact craters that have left traces on it. This happened due to the absence (or insignificant presence) of the atmosphere. The temperature on Mercury is off the charts, with a huge range from 4270 to minus 173C. This distinguishes it from other planets. The temperature range increases/decreases depending on the location towards the sun (high on the facing side, low on the non-facing side). You can turn around the sun in 88 days. This is possible due to its very close location (46 million kilometers). It is curious that the planet is very slow and one day there is equal to 59 Earth days.

Venus

This planet is almost an analogue of the Earth (density, size, structure). There is sulfuric acid present in the clouds and carbon dioxide. Although Venus is not close to the Sun, unlike Mercury, it is the hottest (4500C). Venus is famous for its retrograde rotation: west - the sun rises, east - it sets. A day on Venus is very long and consists of 243 Earth days. And a year lasts 225 days. Venus is beautiful and presents herself brightly, appearing as the Morning Star.

Earth

It is only the fifth largest in the planetary nebula of the solar system and the third from the Sun itself. Among all the planets, it is the only inhabited one. Possessing the liquid state of water, it gave birth to life. We breathe air that is only 28 percent oxygen, the rest is nitrogen and one percent argon and carbon dioxide. The habitable planet's seasons vary due to its 23-degree vertical tilt. A year is 365 days and a day is 24 hours.

Kreneva Evgenia

The work describes the planets belonging to the Terrestrial group. The conditions on these planets, their common features, as well as the characteristics of each planet are considered.

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TERRESTRIAL PLANETS Presentation on astronomy Prepared by 11th grade student Kreneva Evgenia GBOU Secondary School No. 8, Moscow

SOLAR SYSTEM

Terrestrial Planets These are the four planets of the solar system: Mercury, Venus, Earth and Mars. They are also called inner planets, in contrast to the outer planets - the giant planets.

Terrestrial planets have a high density and consist mainly of silicates and metal, as well as oxygen, silicon, iron, magnesium, aluminum and other heavy elements. Largest planet terrestrial group - Earth, but it is more than 14 times inferior in mass to the least massive gas planet - Uranus. All terrestrial planets have the following structure: - in the center, a core made of iron with an admixture of nickel, - a mantle, consisting of silicates, - a crust, formed as a result of partial melting of the mantle and also consisting of silicate rocks, but enriched in incompatible elements. Of the terrestrial planets, Mercury does not have a crust, which is explained by its destruction as a result of meteorite bombardment.

MERCURY Is closest to the sun. The existence of this planet was mentioned in ancient Sumerian writings, which date back to the third millennium BC. This planet got its name from the Roman pantheon, Mercury, the patron saint of merchants, who also had his Greek counterpart, Hermes. Mercury completely circles the sun in eighty-eight Earth days. It travels around its axis in less than sixty days, which by Mercury standards is two-thirds of a year. The temperature on the surface of Mercury can vary greatly - from + 430 degrees on the sun side to + 180 degrees on the shadow side. In our solar system, these differences are the strongest.

MERCURY This can be observed on Mercury unusual phenomenon, which is called the Joshua effect. When the sun on Mercury reaches a certain point, it stops and begins to go in the opposite direction, and not like on Earth - it must go around a full circle around the planet. Mercury is the smallest planet of the Earth group. It is smaller in size than even the largest satellites of the planets Jupiter and Saturn. The surface of Mercury is similar to the surface of the Moon - all strewn with craters. The only difference with the lunar surface is that Mercury has numerous oblique, jagged slopes that can extend for many hundreds of kilometers. These slopes were formed as a result of compression as the planet cooled.

MERCURY One of the most popular and visible parts of the planet is the so-called Heat Plain. This is a crater that gets its name due to its close location to the "hot longitudes". The crater has a diameter of one thousand three hundred kilometers. Most likely, the celestial body that made this crater in ancient times had a diameter of at least one hundred kilometers. Thanks to gravity, Mercury also captures particles of the solar wind, which in turn create a rather thin atmosphere around Mercury. Moreover, they are replaced every two hundred days. In addition, this planet is the fastest planet in our system. The average speed of its rotation around the sun is about forty-seven and a half kilometers per second, which is twice as fast as the Earth.

VENUS The atmosphere of Venus is quite aggressive, because relative to Earth it has a very high temperature and there are poisonous clouds in the sky. The atmosphere of Venus consists mainly of carbon dioxide. If you find yourself in the atmosphere of this planet, you will experience a pressure of about eighty-five kg per 1 square centimeter. In the Earth's atmosphere the pressure will be eighty-five times less. If you throw a coin in the atmosphere of Venus, it will fall as if in a layer of water. Thus, walking on the surface of this planet is just as difficult as walking on the bottom of the ocean. And if, God forbid, the wind rises on Venus, it will carry you like a sea wave carries a sliver.

VENUS The atmosphere of this planet is 96% carbon dioxide. This is what creates the greenhouse effect. The planet's surface is heated by the sun, and the resulting heat cannot be dissipated into space because it is reflected by a layer of carbon dioxide. That's why the temperature of this planet is about four hundred and eighty degrees, like an oven.

VENUS The surface of Venus is dotted with thousands of volcanoes. Science fiction writers described Venus as similar to Earth. It was believed that Venus was shrouded in clouds. This means that the surface of this planet should be dotted with swamps. This means that it probably has a very rainy climate, which leads to a lot of cloudiness and a lot of humidity. In reality, everything is completely different - in the early seventies, the union sent spaceships to the surface of Venus, which clarified the situation. It turned out that the surface of this planet is made up of continuous rocky deserts, where there is absolutely no water. Of course, at such a high temperature there could never be any water.

EARTH The Earth ranks fifth in size and mass among major planets, but of the terrestrial planets, it is the largest. Its most important difference from other planets in the solar system is the existence of life on it, which reached its highest, intelligent form with the advent of man. According to modern cosmogonic concepts, the Earth was formed ~4.5 billion years ago by gravitational condensation from gas and dust matter scattered in the circumsolar space, containing all the chemical elements known in nature.

EARTH The formation of the Earth was accompanied by differentiation of matter, which was facilitated by the gradual heating of the earth's interior, mainly due to the heat released during the decay of radioactive elements (uranium, thorium, potassium, etc.). The result of this differentiation was the division of the Earth into concentrically located layers - the geosphere, differing in chemical composition, state of aggregation and physical properties. The Earth's core formed in the center, surrounded by a mantle. From the lightest and most fusible components of the substance released from the mantle during melting processes, the earth's crust located above the mantle arose. The totality of these internal geospheres, bounded by solid earth's surface, is sometimes called the “solid” Earth.

EARTH “Solid” Earth contains almost the entire mass of the planet. Beyond its boundaries are the external geospheres - water (hydrosphere) and air (atmosphere), which were formed from vapors and gases released from the bowels of the Earth during degassing of the mantle. The differentiation of the substance of the Earth's mantle and the replenishment of the products of differentiation of the earth's crust, water and air shells occurred throughout geological history and continues to this day.

MARS This planet is named after the famous god of War in Rome, because the color of this planet is very reminiscent of the color of blood. This planet is also called the “red planet”. It is believed that this color of the planet is associated with iron oxide, which is present in the atmosphere of Mars. Mars is the seventh largest planet in the solar system. It is considered to be the home of the Valles Marineris - a canyon that is much longer and deeper than the famous Grand Canyon in the USA. By the way, there are quite a few mountains on Mars, and the height of these mountains is sometimes much higher than our Everest. Here, by the way, there is also Olympus - the highest and most famous mountain throughout the solar system.

MARS Mars has the largest volcanoes in the solar system. But the atmosphere of this planet is one hundred times less dense than Earth’s. But this is enough to maintain the weather system on the planet - that means wind and clouds. Mars boasts an average temperature of minus sixty degrees. A year on Mars = 687 Earth days. But a day on Mars is as close as possible to a day on Earth - it is 24 hours, 39 minutes. and 35 sec. Mars has a very thick crust - about fifty kilometers in cross section. Mars also has two moons - Deimos and Phobos.

Thank you for your attention!

What terrestrial planets do you know? List it in your head and check if you thought correctly :). Now we will tell you about them.

Planets Mercury, Venus, Earth and Mars The four sisters are so similar, but there is no complete similarity between them. Each of them developed in its own way.

The closest ones to the Sun formed in a very hot region. Under the influence of high temperatures, light gases moved to the periphery of the solar system, so the terrestrial planets consist of heavy elements such as carbon, iron, and silicon. That is, they are solid and rocky, unlike planets that formed far away and are made mostly of gas. The terrestrial planets have undergone dramatic changes since their formation. Their primary atmosphere disappeared; its replacements were light gases rising from the internal hot zones of the planets. Heavy elements moved inward and formed the core of such planets; volcanic eruptions changed their topography. The 4.5 billion years that have passed since then have changed the appearance of the planets, almost similar at the birth of such different ones today.

Mercury, a small planet close to the Sun with a very thin atmosphere, is a cratered desert scorched by the Sun. Unlike other terrestrial planets, Mercury is a planet on which nothing noteworthy happens, with the possible exception of a constant light meteor shower.

It's probably been a long time since we Venus there were oceans, well, since this planet is quite close to the Sun, the water evaporated and disappeared into space. Currently, the very dense atmosphere consists mainly of carbon dioxide. Several layers of sulfuric acid prevent the sun's rays from reaching the surface. Due to the greenhouse effect, the temperature rises to 500 degrees. The surface of the planet, hidden under the clouds, was studied using the Magellan interplanetary station in 1990. Vast plains, mountains, deep fissures, volcanoes and several meteorite craters were discovered.

Most of the surface Earth occupied by water, which remains in a liquid state due to the fact that the planet is not too close and not too far from the Sun. The atmosphere, a state of mostly nitrogen, oxygen, a small amount of carbon dioxide and water vapor, gives rise to the climate we know. Today's volcanic processes are much less significant than in the past.

U Mars Previously, there was a different, denser atmosphere that favored a mild climate, and there were laths and oceans. Well, since the planet is small, and the mass is not enough for the gravitational force to hold the gas, most of of them disappeared into space. The atmosphere now consists of carbon dioxide. The temperature has dropped, the water is now frozen under a layer of soil. From the inside, Mars also cooled faster than Venus and Earth, and huge volcanoes went extinct a billion years ago. Sometimes hurricane-force winds raise clouds of dust that take weeks to settle on the surface.

The inner region of the Solar System is inhabited by a variety of bodies: major planets, their satellites, as well as small bodies - asteroids and comets. Since 2006, a new subgroup has been introduced into the group of planets - dwarf planets, which have the internal qualities of planets (spheroidal shape, geological activity), but due to their low mass are not able to dominate in the vicinity of their orbit. Now the 8 most massive planets - from Mercury to Neptune - have been decided to be called simply planets, although in conversation astronomers, for the sake of clarity, often call them “major planets” to distinguish them from dwarf planets. The term " minor planet", which has been applied to asteroids for many years, is now recommended not to be used to avoid confusion with dwarf planets

In the region of large planets, we see a clear division into two groups of 4 planets each: the outer part of this region is occupied by giant planets, and the inner part is occupied by much less massive terrestrial planets. The group of giants is also usually divided in half: gas giants(Jupiter and Saturn) and ice giants (Uranus and Neptune). In the group of terrestrial planets, a division in half is also emerging: Venus and Earth are extremely similar to each other in many physical parameters, and Mercury and Mars are an order of magnitude inferior to them in mass and are almost devoid of an atmosphere (even Mars has an atmosphere hundreds of times smaller than Earth’s, and Mercury is practically absent).

It should be noted that among the two hundred satellites of the planets, at least 16 bodies can be distinguished that have the internal properties of full-fledged planets. They often exceed dwarf planets in size and mass, but at the same time they are controlled by the gravity of much more massive bodies. We are talking about the Moon, Titan, the Galilean satellites of Jupiter and the like. Therefore, it would be natural to introduce into the nomenclature of the Solar System new group for such “subordinate” objects of the planetary type, calling them “satellite planets”. But this idea is currently under discussion.

Let's return to terrestrial planets. Compared to giants, they are attractive because they have a solid surface on which space probes can land. Since the 1970s, automatic stations and self-propelled vehicles of the USSR and the USA have repeatedly landed and successfully operated on the surface of Venus and Mars. There have been no landings on Mercury yet, since flights to the vicinity of the Sun and landing on a massive atmosphereless body are associated with major technical problems.

While studying terrestrial planets, astronomers do not forget the Earth itself. Analysis of images from space has made it possible to understand a lot about the dynamics of the earth’s atmosphere and its structure. upper layers(where planes and even balloons do not rise), in the processes occurring in its magnetosphere. By comparing the structure of the atmospheres of Earth-like planets, much can be understood about their history and more accurately predict their future. And since everything higher plants and animals live on the surface of our (or not only our?) planet, then the characteristics of the lower layers of the atmosphere are especially important for us. This lecture is dedicated to terrestrial planets; mainly – their appearance and conditions on the surface.

The brightness of the planet. Albedo

Looking at the planet from afar, we can easily distinguish between bodies with and without an atmosphere. The presence of an atmosphere, or more precisely, the presence of clouds in it, makes the appearance of the planet changeable and significantly increases the brightness of its disk. This is clearly visible if we arrange the planets in a row from completely cloudless (without atmosphere) to completely covered by clouds: Mercury, Mars, Earth, Venus. Rocky, atmosphereless bodies are similar to each other to the point of almost complete indistinguishability: compare, for example, large-scale photographs of the Moon and Mercury. Even an experienced eye has difficulty distinguishing between the surfaces of these dark bodies, densely covered with meteorite craters. But the atmosphere gives any planet a unique appearance.

The presence or absence of an atmosphere on a planet is controlled by three factors: temperature and gravitational potential at the surface, as well as the global magnetic field. Only the Earth has such a field, and it significantly protects our atmosphere from solar plasma flows. The Moon lost its atmosphere (if it had one at all) due to the low critical speed at the surface, and Mercury - due to high temperatures and powerful solar wind. Mars, with almost the same gravity as Mercury, was able to retain the remnants of the atmosphere, since due to its distance from the Sun it is cold and not so intensely blown by the solar wind.

In terms of their physical parameters, Venus and Earth are almost twins. They have very similar size, mass, and therefore average density. Their internal structure should also be similar - crust, mantle, iron core - although there is no certainty about this yet, since seismic and other geological data on the bowels of Venus are missing. Of course, we did not penetrate deeply into the bowels of the Earth: in most places 3-4 km, in some places 7-9 km, and only in one place 12 km. This is less than 0.2% of the Earth's radius. But seismic, gravimetric and other measurements make it possible to judge the Earth’s interior in great detail, while for other planets there is almost no such data. Detailed gravitational field maps have been obtained only for the Moon; heat flows from the interior have been measured only on the Moon; Seismometers have so far only worked on the Moon and (not very sensitive) on Mars.

Geologists still judge the internal life of planets by the features of their solid surface. For example, the absence of signs of lithospheric plates on Venus significantly distinguishes it from the Earth, in the evolution of the surface of which tectonic processes (continental drift, spreading, subduction, etc.) play a decisive role. At the same time, some indirect evidence points to the possibility of plate tectonics on Mars in the past, as well as ice field tectonics on Europa, a moon of Jupiter. Thus, the external similarity of the planets (Venus - Earth) does not guarantee the similarity of their internal structure and the processes occurring in their depths. And planets that are not similar to each other can demonstrate similar geological phenomena.

Let's return to what is available to astronomers and other specialists for direct study, namely, the surface of planets or their cloud layer. In principle, the opacity of the atmosphere in the optical range is not an insurmountable obstacle to studying the solid surface of the planet. Radar from the Earth and from space probes made it possible to study the surfaces of Venus and Titan through their atmospheres opaque to light. However, these works are sporadic, and systematic studies of planets are still carried out with optical instruments. And more importantly, optical radiation from the Sun serves as the main source of energy for most planets. Therefore, the ability of the atmosphere to reflect, scatter and absorb this radiation directly affects the climate at the surface of the planet.

The brightest luminary in the night sky, not counting the Moon, is Venus. It is very bright not only because of its relative proximity to the Sun, but also because of the dense cloud layer of concentrated sulfuric acid droplets, which perfectly reflects light. Our Earth is also not too dark, since 30-40% of the Earth's atmosphere is filled with water clouds, and they also scatter and reflect light well. Here is a photograph (pic. above) where the Earth and the Moon were simultaneously included in the frame. This photo was taken by the Galileo space probe as it flew past Earth on its way to Jupiter. Look how much darker the Moon is than the Earth and generally darker than any planet with an atmosphere. This is a general pattern - atmosphereless bodies are very dark. The fact is that under the influence of cosmic radiation, any solid substance gradually darkens.

The statement that the surface of the Moon is dark usually causes confusion: at first glance, the lunar disk appears very bright; on a cloudless night it even blinds us. But this is only in contrast to the even darker night sky. To characterize the reflectivity of any body, a quantity called albedo is used. This is the degree of whiteness, that is, the coefficient of light reflection. Albedo equal to zero - absolute blackness, complete absorption of light. An albedo equal to one is total reflection. Physicists and astronomers have several different approaches to determining albedo. It is clear that the brightness of an illuminated surface depends not only on the type of material, but also on its structure and orientation relative to the light source and the observer. For example, fluffy snow that has just fallen has one reflectance value, but snow that you stepped on with your boot will have a completely different value. And the dependence on orientation can easily be demonstrated with a mirror, letting in sunbeams.

The entire range of possible albedo values ​​is covered by known space objects. Here is the Earth reflecting about 30% of the sun's rays, mostly due to clouds. And the continuous cloud cover of Venus reflects 77% of the light. Our Moon is one of the darkest bodies, reflecting on average about 11% of light; and its visible hemisphere, due to the presence of vast dark “seas,” reflects light even worse - less than 7%. But there are also even darker objects; for example, asteroid 253 Matilda with its albedo of 4%. On the other hand, there are surprisingly bright bodies: Saturn’s moon Enceladus reflects 81% of visible light, and its geometric albedo is simply fantastic - 138%, i.e. it is brighter than a perfectly white disk of the same cross-section. It's even difficult to understand how he manages to do this. Pure snow on Earth reflects light even worse; What kind of snow lies on the surface of this small and cute Enceladus?

Heat balance

The temperature of any body is determined by the balance between the influx of heat to it and its loss. There are three known mechanisms of heat exchange: radiation, conduction and convection. The last two of them require direct contact with the environment, therefore, in the vacuum of space, the first mechanism, radiation, becomes the most important and, in fact, the only one. This creates considerable problems for space technology designers. They have to take into account several heat sources: the Sun, the planet (especially in low orbits) and the internal units of the spacecraft. And there is only one way to release heat - radiation from the surface of the device. To maintain the balance of heat flows, space technology designers regulate the effective albedo of the device using screen-vacuum insulation and radiators. When such a system fails, conditions in a spacecraft can become quite uncomfortable, as the story of the Apollo 13 mission to the Moon reminds us.

But for the first time this problem was encountered in the first third of the 20th century by the creators of high-altitude balloons - the so-called stratospheric balloons. In those years they did not yet know how to create complex systems thermal regulation of the sealed nacelle, therefore, they were limited to simply selecting the albedo of its outer surface. How sensitive a body's temperature is to its albedo is revealed by the history of the first flights into the stratosphere.

Gondola of your stratospheric balloon FNRS-1 Swiss Auguste Picard painted it white on one side and black on the other. The idea was that the temperature in the gondola could be regulated by turning the sphere one way or the other towards the Sun. For rotation, a propeller was installed outside. But the device did not work, the sun was shining from the “black” side and the internal temperature on the first flight rose to 38 °C. On the next flight, the entire capsule was simply covered with silver to reflect the sun's rays. It became -16 °C inside.

American stratospheric balloon designers Explorer They took Picard's experience into account and adopted a compromise option: they painted the upper part of the capsule white and the lower part black. The idea was that the upper half of the sphere would reflect solar radiation, and the lower half would absorb heat from the Earth. This option turned out to be good, but also not ideal: during the flights in the capsule it was 5 °C.

Soviet stratonauts simply insulated the aluminum capsules with a layer of felt. As practice has shown, this decision was the most successful. Internal heat, mainly generated by the crew, was sufficient to maintain a stable temperature.

But if the planet does not have its own powerful heat sources, then the albedo value is very important for its climate. For example, our planet absorbs 70% of the sunlight falling on it, processing it into its own infrared radiation, supporting the water cycle in nature, storing it as a result of photosynthesis in biomass, oil, coal, and gas. The moon absorbs almost all of the sunlight, mediocrely turning it into high-entropy infrared radiation and thereby maintaining its rather high temperature. But Enceladus, with its perfectly white surface, proudly repels almost all sunlight, for which it pays with a monstrously low surface temperature: on average about –200 °C, and in some places up to –240 °C. However, this satellite - “all in white” - does not suffer much from the external cold, since it has an alternative source of energy - the tidal gravitational influence of its neighbor Saturn (), which maintains its subglacial ocean in a liquid state. But the terrestrial planets have very weak internal heat sources, so the temperature of their solid surface largely depends on the properties of the atmosphere - on its ability, on the one hand, to reflect part of the sun's rays back into space, and on the other, to retain the energy of radiation passing through atmosphere to the surface of the planet.

Greenhouse effect and planetary climate

Depending on how far the planet is from the Sun and what proportion of sunlight it absorbs, temperature conditions on the surface of the planet and its climate are formed. What does the spectrum of any self-luminous body, such as a star, look like? In most cases, the spectrum of a star is a “single-humped”, almost Planck, curve, in which the position of the maximum depends on the temperature of the star’s surface. Unlike a star, the planet’s spectrum has two “humps”: it reflects part of the starlight in the optical range, and the other part absorbs and re-radiates in the infrared range. The relative area under these two humps is precisely determined by the degree of light reflection, that is, albedo.

Let's look at the two planets closest to us - Mercury and Venus. At first glance, the situation is paradoxical. Venus reflects almost 80% of sunlight and absorbs only about 20%. But Mercury reflects almost nothing, but absorbs everything. In addition, Venus is further from the Sun than Mercury; 3.4 times less sunlight falls per unit of its cloud surface. Taking into account differences in albedo, each square meter of Mercury's solid surface receives almost 16 times more solar heat than the same surface on Venus. And yet, on the entire solid surface of Venus there are hellish conditions - enormous temperatures (tin and lead melt!), and Mercury is cooler! At the poles there is generally Antarctica, and at the equator the average temperature is 67 °C. Of course, during the day the surface of Mercury heats up to 430 °C, and at night it cools down to –170 °C. But already at a depth of 1.5-2 meters, daily fluctuations are smoothed out, and we can talk about an average surface temperature of 67 °C. It’s hot, of course, but you can live. And in the middle latitudes of Mercury there is generally room temperature.

What's the matter? Why is Mercury, which is close to the Sun and readily absorbs its rays, heated to room temperature, while Venus, which is farther from the Sun and actively reflects its rays, is heated like a furnace? How will physics explain this?

The Earth's atmosphere is almost transparent: it transmits 80% of incoming sunlight. The air cannot escape into space as a result of convection - the planet does not let it go. This means that it can only cool in the form of infrared radiation. And if IR radiation remains locked, then it heats those layers of the atmosphere that do not release it. These layers themselves become a source of heat and partially direct it back to the surface. Some of the radiation goes into space, but the bulk of it returns to the surface of the Earth and heats it until thermodynamic equilibrium is established. How is it installed?

The temperature rises, and the maximum in the spectrum shifts (Wien’s law) until it finds a “transparency window” in the atmosphere, through which IR rays will escape into space. The balance of heat flows is established, but at a higher temperature than it would be in the absence of an atmosphere. This is the greenhouse effect.

In our lives, we quite often encounter the greenhouse effect. And not only in the form of a garden greenhouse or a pan placed on the stove, which we cover with a lid to reduce heat transfer and speed up boiling. These examples do not demonstrate a pure greenhouse effect, since both radiative and convective heat removal are reduced in them. Much closer to the described effect is the example of a clear frosty night. When the air is dry and the sky is cloudless (for example, in a desert), after sunset the earth quickly cools, and moist air and clouds smooth out daily temperature fluctuations. Unfortunately, this effect is well known to astronomers: clear starry nights can be especially cold, which makes working at the telescope very uncomfortable. Returning to the figure above, we will see the reason: it is water vapor in the atmosphere that serves as the main obstacle to heat-carrying infrared radiation.

The Moon has no atmosphere, which means there is no greenhouse effect. On its surface, thermodynamic equilibrium is established explicitly; there is no exchange of radiation between the atmosphere and the solid surface. Mars has a thin atmosphere, but its greenhouse effect still adds 8 °C. And it adds almost 40 °C to the Earth. If our planet did not have such a dense atmosphere, the Earth's temperature would be 40 °C lower. Today it averages 15 °C around the globe, but it would be –25 °C. All the oceans would freeze, the surface of the Earth would turn white with snow, the albedo would increase, and the temperature would drop even lower. In general - a terrible thing! But it’s good that the greenhouse effect in our atmosphere works and warms us. And it works even more strongly on Venus - it raises the average Venusian temperature by more than 500 degrees.

Surface of planets

Until now, we have not begun a detailed study of other planets, mainly limiting ourselves to observing their surface. How important is information about the appearance of the planet for science? What valuable information can an image of its surface tell us? If it is a gas planet, like Saturn or Jupiter, or solid, but covered with a dense layer of clouds, like Venus, then we see only the upper cloud layer, therefore, we have almost no information about the planet itself. The cloudy atmosphere, as geologists say, is a super-young surface - today it is like this, but tomorrow it will be different, or not tomorrow, but in 1000 years, which is only a moment in the life of the planet.

The Great Red Spot on Jupiter or two planetary cyclones on Venus have been observed for 300 years, but tell us only some of the general properties of the modern dynamics of their atmospheres. Our descendants, looking at these planets, will see a completely different picture, and we will never know what picture our ancestors could have seen. Thus, looking from the outside at planets with dense atmospheres, we cannot judge their past, since we see only a changeable cloud layer. A completely different matter is the Moon or Mercury, the surfaces of which contain traces of meteorite bombardments and geological processes that have occurred over the past billions of years.

And such bombardments of giant planets leave virtually no traces. One of these events occurred at the end of the twentieth century right before the eyes of astronomers. We are talking about Comet Shoemaker-Levy 9. In 1993, a strange chain of two dozen small comets. The calculation showed that these are fragments of one comet that flew near Jupiter in 1992 and was torn apart by the tidal effect of its powerful gravitational field. Astronomers did not see the actual episode of the comet’s disintegration, but only caught the moment when the chain of cometary fragments moved away from Jupiter like a “locomotive.” If the disintegration had not occurred, then the comet, having approached Jupiter along a hyperbolic trajectory, would have gone into the distance along the second branch of the hyperbola and, most likely, would never have approached Jupiter again. But the comet’s body could not withstand the tidal stress and collapsed, and the energy expended on deformation and rupture of the comet’s body reduced the kinetic energy of its orbital motion, transferring the fragments from a hyperbolic orbit to an elliptical one, closed around Jupiter. The orbital distance at the pericenter turned out to be less than the radius of Jupiter, and the fragments crashed into the planet one after another in 1994.

The incident was huge. Each “shard” of the cometary nucleus is an ice block measuring 1×1.5 km. They took turns flying into the atmosphere of the giant planet at a speed of 60 km/s (the second escape velocity for Jupiter), having a specific kinetic energy of (60/11) 2 = 30 times greater than if it were a collision with the Earth. Astronomers watched with great interest the cosmic catastrophe on Jupiter from the safety of Earth. Unfortunately, fragments of the comet hit Jupiter from the side that was not visible from Earth at that moment. Fortunately, just at that time the Galileo space probe was on its way to Jupiter; it saw these episodes and showed them to us. Due to the rapid daily rotation of Jupiter, the collision regions within a few hours became accessible to both ground-based telescopes and, what is especially valuable, near-Earth telescopes, such as the Hubble Space Telescope. This was very useful, since each block, crashing into the atmosphere of Jupiter, caused a colossal explosion, destroying the upper cloud layer and creating a window of visibility deep into the Jovian atmosphere for some time. So, thanks to the comet bombardment, we were able to look there for a short time. But 2 months passed and no traces remained on the cloudy surface: the clouds covered all the windows, as if nothing had happened.

Another thing - Earth. On our planet, meteorite scars remain for a long time. Here is the most popular meteorite crater with a diameter of about 1 km and an age of about 50 thousand years. It is still clearly visible. But craters formed more than 200 million years ago can only be found using subtle geological techniques. They are not visible from above.

By the way, there is a fairly reliable relationship between the size of a large meteorite that fell to Earth and the diameter of the crater it formed - 1:20. A kilometer-diameter crater in Arizona was formed by the impact of a small asteroid with a diameter of about 50 m. And in ancient times, larger “projectiles” - both kilometer and even ten kilometers - hit the Earth. We know today about 200 large craters; they are called astroblemes (celestial wounds); and several new ones are discovered every year. The largest, with a diameter of 300 km, was found in southern Africa, its age is about 2 billion years. In Russia, the largest crater is Popigai in Yakutia with a diameter of 100 km. Surely there are larger ones, for example, on the bottom of the oceans, where they are more difficult to notice. True, the ocean floor in a geological sense younger than the continents, But it seems that there is a crater in Antarctica with a diameter of 500 km. It is underwater and its presence is indicated only by the profile of the bottom.

On a surface Moon, where there is no wind or rain, where there are no tectonic processes, meteorite craters persist for billions of years. Looking at the Moon through a telescope, we read the history of cosmic bombardment. On the reverse side is an even more useful picture for science. It seems that for some reason particularly large bodies never fell there, or, when falling, they could not break through the lunar crust, which on the back side is twice as thick as on the visible side. Therefore, the flowing lava did not fill large craters and did not hide historical details. On any patch of the lunar surface there is a meteorite crater, large or small, and there are so many of them that younger ones destroy those that formed earlier. Saturation has occurred: the Moon can no longer become more cratenated than it already is. There are craters everywhere. And this is a wonderful chronicle of the history of the solar system. Based on it, several episodes of active crater formation have been identified, including the era of heavy meteorite bombardment (4.1-3.8 billion years ago), which left traces on the surface of all terrestrial planets and many satellites. Why streams of meteorites fell on the planets in that era, we still have to understand. New data are needed on the structure of the lunar interior and the composition of matter at different depths, and not just on the surface from which samples have been collected so far.

Mercury outwardly similar to the Moon, because, like it, it is devoid of an atmosphere. Its rocky surface, not subject to gas and water erosion, retains traces of meteorite bombardment for a long time. Among the terrestrial planets, Mercury contains the oldest geological traces, dating back about 4 billion years. But there is no Mercury on the surface large seas, filled with dark solidified lava and similar to lunar seas, although there are no fewer large impact craters there than on the Moon.

Mercury is about one and a half times the size of the Moon, but its mass is 4.5 times greater than the Moon. The fact is that the Moon is almost entirely rocky, while Mercury has a huge metallic core, apparently consisting mainly of iron and nickel. The radius of its metallic core is about 75% of the planet's radius (and Earth's is only 55%). The volume of Mercury's metallic core is 45% of the planet's volume (and Earth's is only 17%). Therefore, the average density of Mercury (5.4 g/cm3) is almost equal to the average density of the Earth (5.5 g/cm3) and significantly exceeds the average density of the Moon (3.3 g/cm3). Having a large metallic core, Mercury could surpass the Earth in its average density if not for the low gravity on its surface. Having a mass of only 5.5% of the Earth's, it has almost three times less gravity, which is not able to compact its interior as much as the interior of the Earth, where even the silicate mantle has a density of about (5 g/cm3), has compacted.

Mercury is difficult to study because it moves close to the Sun. To launch an interplanetary apparatus from the Earth towards it, it must be strongly slowed down, that is, accelerated in the direction opposite to the orbital motion of the Earth; only then will it begin to “fall” towards the Sun. It is impossible to do this immediately using a rocket. Therefore, in the two flights to Mercury carried out so far, gravitational maneuvers in the field of the Earth, Venus and Mercury itself were used to decelerate the space probe and transfer it to Mercury's orbit.

Mariner 10 (NASA) first went to Mercury in 1973. It first approached Venus, slowed down in its gravitational field, and then passed close to Mercury three times in 1974-75. Since all three encounters took place in the same region of the planet's orbit, and its daily rotation is synchronized with the orbital one, all three times the probe photographed the same hemisphere of Mercury, illuminated by the Sun.

There were no flights to Mercury for the next few decades. And only in 2004 was it possible to launch the second device - MESSENGER ( Mercury Surface, Space Environment, Geochemistry, and Ranging; NASA). Having carried out several gravitational maneuvers near the Earth, Venus (twice) and Mercury (three times), the probe entered orbit around Mercury in 2011 and conducted research of the planet for 4 years.

Working near Mercury is complicated by the fact that the planet is on average 2.6 times closer to the Sun than the Earth, so the flow of solar rays there is almost 7 times greater. Without a special “solar umbrella,” the probe’s electronics would overheat. The third expedition to Mercury, called BepiColombo, Europeans and Japanese take part in it. The launch is scheduled for autumn 2018. Two probes will fly at once, which will enter orbit around Mercury at the end of 2025 after flying near Earth, two near Venus and six near Mercury. In addition to a detailed study of the surface of the planet and its gravitational field, a detailed study of the magnetosphere and magnetic field of Mercury, which poses a mystery to scientists, is planned. Although Mercury rotates very slowly, and its metallic core should have cooled and hardened long ago, the planet has a dipole magnetic field that is 100 times weaker than Earth's, but still maintains a magnetosphere around the planet. Modern theory of magnetic field generation celestial bodies, the so-called theory of turbulent dynamo, requires the presence in the bowels of the planet of a layer of liquid conductor of electricity (for the Earth this is the outer part of the iron core) and relatively rapid rotation. For what reason Mercury's core still remains liquid is not yet clear.

Mercury has an amazing feature that no other planet has. The movement of Mercury in its orbit around the Sun and its rotation around its axis are clearly synchronized with each other: during two orbital periods it makes three revolutions around its axis. Generally speaking, astronomers have been familiar with synchronous motion for a long time: our Moon synchronously rotates around its axis and revolves around the Earth, the periods of these two movements are the same, i.e. they are in a 1:1 ratio. And other planets have some satellites that exhibit the same feature. This is the result of the tidal effect.

To follow the movement of Mercury (fig. above), let's place an arrow on its surface. It can be seen that in one revolution around the Sun, i.e. in one Mercury year, the planet rotated around its axis exactly one and a half times. During this time, day in the area of ​​the arrow turned into night, and half of the sunny day passed. Another annual revolution - and daylight begins again in the area of ​​the arrow, one solar day has expired. Thus, on Mercury, a solar day lasts two Mercury years.

We will talk about tides in detail in Chap. 6. It was as a result of tidal influence from the Earth that the Moon synchronized its two movements - axial rotation and orbital rotation. The Earth greatly influences the Moon: it stretches its figure and stabilizes its rotation. The Moon's orbit is close to circular, so the Moon moves along it at an almost constant speed at an almost constant distance from the Earth (we discussed the extent of this "almost" in Chapter 1). Therefore, the tidal effect varies slightly and controls the rotation of the Moon along its entire orbit, leading to a 1:1 resonance.

Unlike the Moon, Mercury moves around the Sun in a substantially elliptical orbit, sometimes approaching the luminary, sometimes moving away from it. When it is far away, near the aphelion of the orbit, the tidal influence of the Sun weakens, since it depends on distance as 1/ R 3. When Mercury approaches the Sun, the tides are much stronger, so only in the perihelion region does Mercury effectively synchronize its two movements - diurnal and orbital. Kepler's second law tells us that the angular velocity of orbital motion is maximum at the perihelion point. It is there that “tidal capture” and synchronization of Mercury’s angular velocities – daily and orbital – occurs. At the perihelion point they are exactly equal to each other. Moving further, Mercury almost ceases to feel the tidal influence of the Sun and maintains its angular velocity of rotation, gradually reducing the angular velocity of orbital motion. Therefore, in one orbital period it manages to make one and a half daily revolutions and again falls into the clutches of the tidal effect. Very simple and beautiful physics.

The surface of Mercury is almost indistinguishable from the moon. Even professional astronomers, when the first detailed photographs of Mercury appeared, showed them to each other and asked: “Well, guess, is this the Moon or Mercury?” It's really hard to guess. Both there and there are surfaces battered by meteorites. But, of course, there are features. Although there are no large lava seas on Mercury, its surface is not homogeneous: there are older and younger areas (the basis for this is the count of meteorite craters). Mercury also differs from the Moon in the presence of characteristic ledges and folds on the surface, which arose as a result of the compression of the planet as its huge metal core cooled.

Temperature differences on the surface of Mercury are greater than on the Moon. During the daytime at the equator it is 430 °C, and at night –173 °C. But Mercury’s soil serves as a good heat insulator, so at a depth of about 1 m daily (or biannual?) temperature changes are no longer felt. So, if you fly to Mercury, the first thing you need to do is dig a dugout. It will be about 70 °C at the equator; It's a bit hot. But in the region of the geographic poles in the dugout it will be about –70 °C. So you can easily find the geographic latitude at which you will be comfortable in the dugout.

The lowest temperatures are observed at the bottom of polar craters, where the sun's rays never reach. It was there that deposits of water ice were discovered, which had previously been detected by radars from the Earth, and then confirmed by instruments of the MESSENGER space probe. The origin of this ice is still debated. Its sources can be both comets and water vapor emerging from the bowels of the planet.

Mercury has one of the largest impact craters in solar system– Plain of Heat ( Caloris Basin) with a diameter of 1550 km. This is the impact of an asteroid with a diameter of at least 100 km, which almost split the small planet. This happened about 3.8 billion years ago, during the period of the so-called “late heavy bombardment” ( Late Heavy Bombardment), when, for reasons that are not fully understood, the number of asteroids and comets in orbits intersecting the orbits of terrestrial planets increased.

When Mariner 10 photographed the Heat Plane in 1974, we did not yet know what happened on the opposite side of Mercury after this terrible impact. It is clear that if the ball is hit, sound and surface waves are excited, which propagate symmetrically, pass through the “equator” and gather at the antipodeal point, diametrically opposite to the point of impact. The disturbance there contracts to a point, and the amplitude of seismic vibrations rapidly increases. This is similar to the way cattle drivers crack their whip: the energy and momentum of the wave is essentially conserved, but the thickness of the whip tends to zero, so the vibration speed increases and becomes supersonic. It was expected that in the region of Mercury opposite the basin Caloris there will be a picture of incredible destruction. In general, it almost turned out that way: there was a vast hilly area with a corrugated surface, although I expected there to be an antipodean crater. It seemed to me that when the seismic wave collapses, a “mirror” phenomenon will occur to the fall of the asteroid. We observe this when a drop falls on a calm surface of water: first it creates a small depression, and then the water rushes back and throws a small new drop upward. This did not happen on Mercury, and we now understand why. Its depths turned out to be heterogeneous and precise focusing of the waves did not occur.

In general, the relief of Mercury is smoother than that of the Moon. For example, the walls of Mercury's craters are not so high. The likely reason for this is the greater force of gravity and the warmer and softer interior of Mercury.

Venus- the second planet from the Sun and the most mysterious of the terrestrial planets. It is not clear what the origin of its very dense atmosphere, consisting almost entirely of carbon dioxide (96.5%) and nitrogen (3.5%) and causing a powerful greenhouse effect, is. It is not clear why Venus rotates so slowly around its axis - 244 times slower than the Earth, and also in the opposite direction. At the same time, the massive atmosphere of Venus, or rather its cloud layer, flies around the planet in four Earth days. This phenomenon is called atmospheric superrotation. At the same time, the atmosphere rubs against the surface of the planet and should have slowed down long ago. After all, it cannot move for a long time around a planet whose solid body practically stands still. But the atmosphere rotates, and even in the direction opposite to the rotation of the planet itself. It is clear that friction with the surface dissipates the energy of the atmosphere, and its angular momentum is transferred to the body of the planet. This means that there is an influx of energy (obviously solar), due to which the heat engine operates. Question: how is this machine implemented? How is the energy of the Sun transformed into the movement of the Venusian atmosphere?

Due to the slow rotation of Venus, the Coriolis forces on it are weaker than on Earth, so atmospheric cyclones there are less compact. In fact, there are only two of them: one in the northern hemisphere, the other in the southern hemisphere. Each of them “winds” from the equator to its own pole.

The upper layers of the Venusian atmosphere were studied in detail by flybys (carrying out a gravity maneuver) and orbital probes - American, Soviet, European and Japanese. Soviet engineers launched Venera series devices there for several decades, and this was our most successful breakthrough in the field of planetary exploration. The main task was to land the descent module on the surface to see what was there under the clouds.

The designers of the first probes, like the authors of science fiction works of those years, were guided by the results of optical and radio astronomical observations, from which it followed that Venus is a warmer analogue of our planet. That is why in the middle of the 20th century all science fiction writers - from Belyaev, Kazantsev and Strugatsky to Lem, Bradbury and Heinlein - presented Venus as inhospitable (hot, swampy, with a poisonous atmosphere), but in general earth-like world. For the same reason, the first landing vehicles of the Venus probes were not very durable, unable to withstand high pressure. And they died, descending into the atmosphere, one after another. Then their bodies began to be made stronger, designed for a pressure of 20 atmospheres. But this turned out to be not enough. Then the designers, “biting the bit,” made a titanium probe that can withstand a pressure of 180 atm. And he landed safely on the surface (“Venera-7”, 1970). Note that not every submarine can withstand such pressure, which prevails at a depth of about 2 km in the ocean. It turned out that the pressure on the surface of Venus does not drop below 92 atm (9.3 MPa, 93 bar), and the temperature is 464 °C.

The dream of a hospitable Venus, similar to the Earth of the Carboniferous period, was finally ended precisely in 1970. For the first time, a device designed for such hellish conditions (“Venera-8”) successfully descended and worked on the surface in 1972. From this moment of landing to the surface of Venus have become a routine operation, but it is not possible to work there for a long time: after 1-2 hours the inside of the device heats up and the electronics fail.

The first artificial satellites appeared near Venus in 1975 (“Venera-9 and -10”). In general, the work on the surface of Venus by the Venera-9...-14 descent vehicles (1975-1981) turned out to be extremely successful, studying both the atmosphere and the surface of the planet at the landing site, even managing to take soil samples and determine its chemical composition and mechanical properties. But the greatest effect among fans of astronomy and cosmonautics was caused by the photo panoramas they transmitted of the landing sites, first in black and white, and later in color. By the way, the Venusian sky, when viewed from the surface, is orange. Beautiful! Until now (2017), these images remain the only ones and are of great interest to planetary scientists. They continue to be processed and new parts are found on them from time to time.

American astronautics also made a significant contribution to the study of Venus in those years. The Mariner 5 and 10 flybys studied the upper atmosphere. Pioneer Venera 1 (1978) became the first American Venus satellite and carried out radar measurements. And “Pioneer-Venera-2” (1978) sent 4 descent vehicles into the planet’s atmosphere: one large (315 kg) with a parachute to the equatorial region of the daytime hemisphere and three small (90 kg each) without parachutes - to mid-latitudes and at the north of the day hemisphere, as well as the night hemisphere. None of them were designed to work on the surface, but one of the small devices landed safely (without a parachute!) and worked on the surface for more than an hour. This case allows you to feel how high the density of the atmosphere is near the surface of Venus. The atmosphere of Venus is almost 100 times more massive than the Earth's atmosphere, and its density at the surface is 67 kg/m 3, which is 55 times denser than Earth's air and only 15 times less dense than liquid water.

It was not easy to create strong scientific probes that can withstand the pressure of the Venusian atmosphere, the same as at a kilometer depth in our oceans. But it was even more difficult to get them to withstand the ambient temperature of 464 ° C in the presence of such dense air. The heat flow through the body is colossal. Therefore, even the most reliable devices worked for no more than two hours. In order to quickly descend to the surface and prolong its work there, the Venus dropped its parachute during landing and continued its descent, slowed down only by a small shield on its hull. The impact on the surface was softened by a special damping device - a landing support. The design turned out to be so successful that Venera 9 landed on a slope with an inclination of 35° without any problems and worked normally.

Given Venus's high albedo and colossal density of its atmosphere, scientists doubted there would be enough sunlight near the surface to photograph. In addition, a dense fog could well be hanging at the bottom of the gas ocean of Venus, scattering sunlight and preventing a contrast image from being obtained. Therefore, the first landing vehicles were equipped with halogen mercury lamps to illuminate the soil and create light contrast. But it turned out that there is quite enough natural light there: it is as light on Venus as on a cloudy day on Earth. And the contrast in natural light is also quite acceptable.

In October 1975, the Venera 9 and 10 landing vehicles, through their orbital blocks, transmitted to Earth the first ever photographs of the surface of another planet (if we do not take into account the Moon). At first glance, the perspective in these panoramas looks strangely distorted: the reason is the rotation of the shooting direction. These images were taken by a telephotometer (optical-mechanical scanner), the “look” of which slowly moved from the horizon under the feet of the landing vehicle and then to the other horizon: a 180° scan was obtained. Two telephotometers on opposite sides of the device were supposed to provide a complete panorama. But the lens caps did not always open. For example, on “Venera-11 and -12” none of the four opened.

One of the most beautiful experiments in the study of Venus was carried out using the VeGa-1 and -2 probes (1985). Their name stands for “Venus-Halley”, because after the separation of the descent modules aimed at the surface of Venus, the flight parts of the probes went to explore the nucleus of Comet Halley and for the first time did so successfully. The landing devices were also not entirely ordinary: the main part of the device landed on the surface, and during descent, a balloon made by French engineers was separated from it, and for about two days it flew in the atmosphere of Venus at an altitude of 53-55 km, transmitting data on temperature and pressure to Earth , illumination and visibility in clouds. Thanks to the powerful wind blowing at this altitude at a speed of 250 km/h, the balloons managed to fly around a significant part of the planet. Beautiful!

Photographs from the landing sites show only small areas of the Venusian surface. Is it possible to see all of Venus through the clouds? Can! The radar sees through the clouds. Two Soviet satellites with side-looking radars and one American flew to Venus. Based on their observations, radio maps of Venus were compiled with very high resolution. On general map it is difficult to demonstrate, but it is clearly visible on certain fragments of the map. The colors on the radio maps show the levels: light blue and dark blue are lowlands; If Venus had water, it would be oceans. But liquid water cannot exist on Venus. And there is practically no gaseous water there either. Greenish and yellowish are the continents, let's call them that. Red and white are the most high points on Venus. This is the “Venusian Tibet” - the highest plateau. highest peak on it – Mount Maxwell – rises 11 km.

There are no reliable facts about the depths of Venus, about its internal structure, since seismic research has not yet been carried out there. In addition, the slow rotation of the planet does not allow measuring its moment of inertia, which could tell us about the distribution of density with depth. So far, theoretical ideas are based on the similarity of Venus with the Earth, and the apparent absence of plate tectonics on Venus is explained by the absence of water on it, which on Earth serves as a “lubricant”, allowing the plates to slide and dive under each other. Coupled with the high surface temperature, this leads to a slowdown or even complete absence of convection in the body of Venus, reduces the cooling rate of its interior and may explain its lack of a magnetic field. All this looks logical, but requires experimental verification.

By the way, about Earth. I will not discuss the third planet from the Sun in detail, since I am not a geologist. In addition, each of us has a general idea of ​​the Earth, even based on school knowledge. But in connection with the study of other planets, I note that we also do not fully understand the interior of our own planet. Almost every year there are major discoveries in geology, sometimes even new layers are discovered in the bowels of the Earth. We don't even know exactly the temperature at the core of our planet. Look at the latest reviews: some authors believe that the temperature at the boundary of the inner core is about 5000 K, while others believe that it is more than 6300 K. These are the results of theoretical calculations, which include not entirely reliable parameters that describe the properties of matter at a temperature of thousands of kelvins and a pressure of millions bar. Until these properties are reliably studied in the laboratory, we will not receive accurate knowledge about the interior of the Earth.

The uniqueness of the Earth among similar planets lies in the presence of a magnetic field and liquid water on the surface, and the second, apparently, is a consequence of the first: the Earth’s magnetosphere protects our atmosphere and, indirectly, the hydrosphere from solar wind flows. To generate a magnetic field, as it now appears, in the interior of the planet there must be a liquid electrically conductive layer, covered by convective motion, and rapid daily rotation, providing the Coriolis force. Only under these conditions does the dynamo mechanism turn on, enhancing the magnetic field. Venus barely rotates, so it has no magnetic field. The iron core of little Mars has long cooled and hardened, so it also lacks a magnetic field. Mercury, it would seem, rotates very slowly and should have cooled down before Mars, but it has a quite noticeable dipole magnetic field with a strength 100 times weaker than the Earth’s. Paradox! The tidal influence of the Sun is now believed to be responsible for maintaining Mercury's iron core in a molten state. Billions of years will pass, the iron core of the Earth will cool and harden, depriving our planet of magnetic protection from the solar wind. And the only rocky planet with a magnetic field will remain, oddly enough, Mercury.

Now let's turn to Mars. Its appearance immediately attracts us for two reasons: even in photographs taken from afar, the white polar caps and translucent atmosphere are visible. This is similar between Mars and the Earth: the polar caps give rise to the idea of ​​the presence of water, and the atmosphere – the possibility of breathing. And although on Mars not everything is as good with water and air as it seems at first glance, this planet has long attracted researchers.

Previously, astronomers studied Mars through a telescope and therefore eagerly awaited moments called “Mars oppositions.” What is opposing what at these moments?

From the point of view of an earthly observer, at the moment of opposition, Mars is on one side of the Earth, and the Sun is on the other. It is clear that it is at these moments that the Earth and Mars approach the minimum distance, Mars is visible in the sky all night and is well illuminated by the Sun. Earth orbits the Sun every year, and Mars every 1.88 years, so the average time between oppositions is just over two years. The last opposition of Mars was in 2016, although it was not particularly close. Mars's orbit is noticeably elliptical, so Earth's closest approaches to Mars occur when Mars is near the perihelion of its orbit. On Earth (in our era) this is the end of August. Therefore, the August and September confrontations are called “great”; At these moments, which occur once every 15-17 years, our planets come closer to each other by less than 60 million km. This will happen in 2018. And a super-close confrontation took place in 2003: then Mars was only 55.8 million km away. In this regard, a new term was born - “the greatest oppositions of Mars”: these are now considered approaches of less than 56 million km. They occur 1-2 times a century, but in the current century there will be even three of them - wait for 2050 and 2082.

But even during moments of great opposition, little is visible on Mars through a telescope from Earth. Here is a drawing of an astronomer looking at Mars through a telescope. An unprepared person will look and be disappointed - he will not see anything at all, just a small pink “drop”. But with the same telescope, the experienced eye of an astronomer sees more. Astronomers noticed the polar cap a long time ago, centuries ago. And also dark and light areas. The dark ones were traditionally called seas, and the light ones – continents.

Increased interest in Mars arose during the era of the great opposition of 1877: - by that time, good telescopes had already been built, and astronomers had made several important discoveries. American astronomer Asaph Hall discovered the moons of Mars - Phobos and Deimos. And the Italian astronomer Giovanni Schiaparelli sketched mysterious lines on the surface of the planet - Martian canals. Of course, Schiaparelli was not the first to see the channels: some of them were noticed before him (for example, Angelo Secchi). But after Schiaparelli, this topic became dominant in the study of Mars for many years.

Observations of features on the surface of Mars, such as “channels” and “seas,” marked the beginning of a new stage in the study of this planet. Schiaparelli believed that the “seas” of Mars could indeed be bodies of water. Since the lines connecting them needed to be given a name, Schiaparelli called them “canals” (canali), meaning sea straits, and not man-made structures. He believed that water actually flows through these channels in the polar regions during the melting of the polar caps. After the discovery of “channels” on Mars, some scientists suggested their artificial nature, which served as the basis for hypotheses about the existence of intelligent beings on Mars. But Schiaparelli himself did not consider this hypothesis scientifically substantiated, although he did not exclude the presence of life on Mars, perhaps even intelligent.

However, the idea of ​​an artificial irrigation canal system on Mars began to gain ground in other countries. This was partly due to the fact that the Italian canali was represented in English as canal (man-made waterway), rather than channel (natural sea strait). And in Russian the word “canal” means an artificial structure. The idea of ​​Martians captivated many people at that time, and not only writers (remember H.G. Wells with his “War of the Worlds,” 1897), but also researchers. The most famous of them was Percival Lovell. This American received an excellent education at Harvard, equally mastering mathematics, astronomy and humanities. But as the scion of a noble family, he would rather become a diplomat, writer or traveler than an astronomer. However, after reading Schiaparelli's works on canals, he became fascinated by Mars and believed in the existence of life and civilization on it. In general, he abandoned all other matters and began studying the Red Planet.

With money from his wealthy family, Lovell built an observatory and began drawing canals. Note that photography was then in its infancy, and the eye of an experienced observer is able to notice the smallest details in conditions of atmospheric turbulence, distorting images of distant objects. The maps of Martian canals created at the Lovell Observatory were the most detailed. In addition, being a good writer, Lovell wrote several interesting books - Mars and its channels (1906), Mars as the abode of life(1908), etc. Only one of them was translated into Russian even before the revolution: “Mars and life on it” (Odessa: Matezis, 1912). These books captivated an entire generation with the hope of meeting Martians.

It should be admitted that the story of the Martian canals has never received a comprehensive explanation. There are old drawings with channels and modern photographs without them. Where are the channels? What was it? Astronomers' conspiracy? Mass insanity? Self-hypnosis? It is difficult to blame scientists who have given their lives to science for this. Perhaps the answer to this story lies ahead.

And today we study Mars, as a rule, not through a telescope, but with the help of interplanetary probes. (Although telescopes are still used for this and sometimes bring important results.) The flight of probes to Mars is carried out along the most energetically favorable semi-elliptical trajectory. Using Kepler's Third Law, it is easy to calculate the duration of such a flight. Due to the high eccentricity of the Martian orbit, the flight time depends on the launch season. On average, a flight from Earth to Mars lasts 8-9 months.

Is it possible to send a manned expedition to Mars? It's big and interesting topic. It would seem that all that is needed for this is a powerful launch vehicle and a convenient spaceship. No one yet has sufficiently powerful carriers, but American, Russian and Chinese engineers are working on them. There is no doubt that such a rocket will be created in the coming years by state-owned enterprises (for example, our new Angara rocket in its most powerful version) or private companies (Elon Musk - why not).

Is there a ship in which astronauts will spend many months on their way to Mars? There is no such thing yet. All existing ones (Soyuz, Shenzhou) and even those undergoing testing (Dragon V2, CST-100, Orion) are very cramped and are only suitable for flying to the Moon, where it is only 3 days away. True, there is an idea to inflate additional rooms after takeoff. In the fall of 2016, the inflatable module was tested on the ISS and performed well. Thus, the technical possibility of flying to Mars will soon appear. So what's the problem? In a person!

We are constantly exposed to natural radioactivity of the earth's rocks, streams of cosmic particles or artificially created radioactivity. At the Earth's surface, the background is weak: we are protected by the magnetosphere and atmosphere of the planet, as well as its body, covering the lower hemisphere. In low Earth orbit, where ISS cosmonauts work, the atmosphere no longer helps, so the background radiation increases hundreds of times. In outer space it is even several times higher. This significantly limits the duration of a person’s safe stay in space. Let us note that nuclear industry workers are prohibited from receiving more than 5 rem per year - this is almost safe for health. Cosmonauts are allowed to receive up to 10 rem per year (an acceptable level of danger), which limits the duration of their work on the ISS to one year. And a flight to Mars with a return to Earth, in the best case (if there are no powerful flares on the Sun), will lead to a dose of 80 rem, which will create a high probability of cancer. This is precisely the main obstacle to human flight to Mars. Is it possible to protect astronauts from radiation? Theoretically, it’s possible.

We are protected on Earth by an atmosphere whose thickness per square centimeter is equivalent to a 10-meter layer of water. Light atoms better dissipate the energy of cosmic particles, so the protective layer of a spacecraft can be 5 meters thick. But even in a cramped ship, the mass of this protection will be measured in hundreds of tons. Sending such a ship to Mars is beyond the power of a modern or even promising rocket.

OK then. Let's say there were volunteers who were ready to risk their health and go to Mars in one direction without radiation protection. Will they be able to work there after landing? Can they be counted on to complete the task? Remember how astronauts, after spending six months on the ISS, feel immediately after landing on the ground? They are carried out in their arms, placed on a stretcher, and for two to three weeks they are rehabilitated, restoring bone strength and muscle strength. And on Mars no one will carry them in their arms. There you will need to go out on your own and work in heavy void suits, like on the Moon. After all, the atmospheric pressure on Mars is practically zero. The suit is very heavy. On the Moon it was relatively easy to move in it, since the gravity there is 1/6 of the Earth's, and during the three days of flight to the Moon the muscles do not have time to weaken. Astronauts will arrive on Mars after spending many months in conditions of weightlessness and radiation, and the gravity on Mars is two and a half times greater than the lunar one. In addition, on the surface of Mars itself, the radiation is almost the same as in outer space: Mars has no magnetic field, and its atmosphere is too rarefied to serve as protection. So the movie “The Martian” is fantasy, very beautiful, but unreal.

How did we imagine a Martian base before? We arrived, set up laboratory modules on the surface, live and work in them. And now here’s how: we flew in, dug in, built shelters at a depth of at least 2-3 meters (this is quite reliable protection from radiation) and try to go to the surface less often and not for long. Resurrections are sporadic. We basically sit under the ground and control the work of the Mars rovers. So they can be controlled from Earth, even more efficiently, cheaper and without risk to health. This is what has been done for several decades.

About what robots learned about Mars - .

Illustrations prepared by V. G. Surdin and N. L. Vasilyeva using NASA photographs and images from public sites