Divided into 2 groups based on their planetary surfaces: gas giants and terrestrial planets. Terrestrial planets are characterized by a dense surface and, as a rule, consist of silicate compounds. There are only four such planets in the solar system: Mars, Earth, Venus and Mercury.
Terrestrial planets in the Solar System:
Mercury
Mercury is the smallest of the four Earth-like planets in the Solar System with an equatorial radius of 2439.7 ± 1.0 km. The planet is larger than moons such as Titan. However, Mercury has the second highest density (5427 grams per cubic centimeter) among the planets of the solar system, slightly inferior to Earth in this indicator. The high density provides clues to the planet's internal structure, which scientists believe is rich in iron. Mercury's core is believed to have the highest iron content of any planet in our system. Astronomers believe that the molten core makes up 55% of the planet's total volume. The outer layer of the iron-rich core is the mantle, which is mainly composed of silicates. The planet's rocky crust reaches 35 km in thickness. Mercury is located at a distance of 0.39 astronomical units from the Sun, which makes it the closest planet to our luminary. Due to its proximity to the Sun, the surface temperature of the planet rises to more than 400º C.
Venus
Venus is Earth's closest neighbor and one of the four terrestrial planets in the solar system. It is the second largest planet in this category with a diameter of 12,092 km; second only to Earth. However, Venus's thick atmosphere is considered the densest in the solar system, with an atmospheric pressure 92 times higher than the atmospheric pressure on our planet. The dense atmosphere consists of carbon dioxide, which has a greenhouse effect and causes the temperature on the surface of Venus to rise to 462º C, and is. The planet is dominated by volcanic plains, covering about 80% of its surface. Venus also has numerous impact craters, some of which reach a diameter of about 280 km.
Earth
Of the four terrestrial planets, Earth is the largest with an equatorial diameter of 12,756.1 km. It is also the only planet of this group known to have a hydrosphere. Earth is the third closest planet to the Sun, located at a distance of about 150 million km (1 astronomical unit) from it. The planet also has the highest density (5.514 grams per cubic centimeter) in the Solar System. Silicate and alumina are the two compounds found in the highest concentrations in the Earth's crust, accounting for 75.4% of the continental crust and 65.1% of the oceanic crust.
Mars
Mars is another terrestrial planet in the Solar System, located farthest from the Sun at a distance of 1.5 astronomical units. The planet has an equatorial radius of 3396.2±0.1 km, making it the second smallest planet in our system. The surface of Mars is mainly composed of basaltic rocks. The planet's crust is quite thick and ranges from 125 km to 40 km in depth.
Dwarf planets
There are other smaller dwarf planets that have some characteristics comparable to terrestrial planets, such as having a dense surface. However, the surface of dwarf planets is formed by a sheet of ice and therefore they do not belong to this group. Examples of dwarf planets in the solar system are Pluto and Ceres.
Terrestrial planets Terrestrial planets 4 planets of the solar system: Mercury, Venus, Earth and Mars. In structure and composition, some stony asteroids are close to them, for example, Vesta. Terrestrial planets have a high density and... ... Wikipedia
PLANETS AND SATELLITES.- PLANETS AND SATELLITES. The 9 large planets of the solar system are divided into terrestrial planets (Mercury... Physical encyclopedia
Planets- Planets suitable for the emergence of life Theoretical dependence of the zone of location of planets suitable for supporting life (highlighted in green) on the type of star. The orbital scale is not respected... Wikipedia
Giant planets- 4 planets of the solar system: Jupiter, Saturn, Uranus, Neptune; located outside the ring of minor planets. Compared to the solid-state planets of the terrestrial group (inner), they are all gas planets, have large sizes, masses ... Wikipedia
Planets- Planets. PLANETS, the most massive bodies of the Solar System, moving in elliptical orbits around the Sun (see Kepler's laws). 9 planets are known. The so-called terrestrial planets (Mercury, Venus, Earth, Mars) have solid... ... Illustrated Encyclopedic Dictionary
PLANETS- (from the Greek planetes wandering) the most massive bodies of the Solar System, moving in elliptical orbits around the Sun (see Kepler's laws), glow with reflected sunlight. The location of the planets in the direction from the Sun: Mercury, Venus, ... ... Big Encyclopedic Dictionary
Earth- Earth Photograph of the Earth from the Apollo 17 spacecraft Orbital characteristics Aphelion 152,097,701 km 1.0167103335 a. e... Wikipedia
Giant planets- For giant planets outside the solar system, see Gas planet ... Wikipedia
planets- (from the Greek planētēs wandering), massive celestial bodies moving around the Sun in elliptical orbits (see Kepler's laws) and glowing by reflected sunlight. The location of the planets in the direction from the Sun: Mercury, Venus, Earth, Mars... encyclopedic Dictionary
Giant planets- planets of the solar system: Jupiter, Saturn, Uranus, Neptune; located outside the ring of minor planets (See Minor planets). Compared to the terrestrial (inner) planets, they have larger sizes, masses, a lower average ... Great Soviet Encyclopedia
Books
- Buy for 2144 UAH (Ukraine only)
- Space. From the Solar System deep into the Universe, Mikhail Yakovlevich Marov. The book sets out in a fairly concise and popular form modern ideas about space and the bodies inhabiting it. This is, first of all, the Sun and the Solar System, the terrestrial planets and...
Chapter 8. Terrestrial planets: Mercury, Venus, Earth
Planet formation
One way or another, at the moment there are 8 planets known in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and several plutonoids, including Pluto, which until recently was listed among the planets. All planets move in orbits in the same direction and in the same plane and in almost circular orbits (with the exception of plutonoids). From the center to the outskirts of the solar system (to Pluto) 5.5 light hours. The distance from the Sun to the Earth is 149 million km, which is 107 of its diameters. The first planets from the Sun are strikingly different in size from the latter and, unlike them, are called terrestrial planets, and the distant ones are called giant planets.
Mercury
The planet closest to the Sun, Mercury, is named after the Roman god of trade, travelers and thieves. This small planet moves quickly in orbit and rotates very slowly around its axis. Mercury has been known since ancient times, but astronomers did not immediately realize that it was a planet, and that in the morning and evening they saw the same star.
Mercury is located at a distance of about 0.387 AU from the Sun. (1 AU is equal to the average radius of the Earth’s orbit), and the distance from Mercury to the Earth, as it and the Earth move in their orbits, changes from 82 to 217 million km. The inclination of the plane of Mercury's orbit to the plane of the ecliptic (plane of the solar system) is 7°. Mercury's axis is almost perpendicular to the plane of its orbit, and its orbit is elongated. Thus, there are no seasons on Mercury, and the changes of day and night occur very rarely, approximately once every two Mercury years. One side of it, facing the Sun for a long time, is very hot, and the other, turned away from the Sun for a long time, is in terrible cold. Mercury moves around the Sun at a speed of 47.9 km/s. The weight of Mercury is almost 20 times less than the weight of the Earth (0.055M), and its density is almost the same as that of the Earth (5.43 g/cm3). The radius of the planet Mercury is 0.38R (radius of the Earth, 2440 km).
Due to its proximity to the Sun, under the influence of gravity, powerful tidal forces arose in the body of Mercury, which slowed down its rotation around its axis. In the end, Mercury found itself in a resonant trap. The period of its revolution around the Sun, measured in 1965, was 87.95 Earth days, and the period of rotation around its axis was 58.65 Earth days. Mercury completes three full revolutions around its axis in 176 days. During the same period, the planet makes two revolutions around the Sun. In the future, tidal braking of Mercury should lead to equality of its revolution around its axis and revolution around the Sun. Then it will always face the Sun in one direction, just as the Moon faces the Earth.
Mercury has no satellites. Perhaps, once upon a time, Mercury itself was a satellite of Venus, but due to solar gravity it was “taken away” from Venus and became an independent planet. The planet is actually spherical in shape. The acceleration of free fall on its surface is almost 3 times less than that on Earth (g = 3.72 m/s 2 ).
Its proximity to the Sun makes observing Mercury difficult. In the sky, it does not move far from the Sun - a maximum of 29°; from the Earth it is visible either before sunrise (morning visibility) or after sunset (evening visibility).
In its physical characteristics, Mercury resembles the Moon; there are many craters on its surface. Mercury has a very thin atmosphere. The planet has a large iron core, which is a source of gravity and a magnetic field, the strength of which is 0.1 of the strength of the Earth's magnetic field. Mercury's core makes up 70% of the planet's volume. The surface temperature ranges from 90° to 700° K (–180° to +430° C). The sun's equatorial side heats up much more than the polar regions. Different degrees of surface heating create a difference in the temperature of the rarefied atmosphere, which should cause its movement - wind.
The inner region of the Solar System is inhabited by a variety of bodies: large 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 for many years was applied to asteroids, 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 a new group into the nomenclature of the Solar System 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, the structure of its upper layers (where airplanes and even balloons do not rise), and 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 all higher plants and animals live on the surface of our (or not only our?) planet, 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 components of the spacecraft itself. 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 thermal control systems for a sealed nacelle, so they limited themselves 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 about some 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 was spotted near Jupiter. 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 is geologically younger than the continents, but it seems that in Antarctica there is a crater 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 multiple 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 on the surface of Mercury there are no large seas filled with dark solidified lava and similar to the 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. The modern theory of magnetic field generation in celestial bodies, the so-called theory of turbulent dynamo, requires the presence in the interior 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 the Solar System - Heat Planum ( 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 an inhospitable (hot, swampy, with a poisonous atmosphere), but generally similar to the Earth 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. It is difficult to demonstrate on a general map, but on individual map fragments it is clearly visible. 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 highest points on Venus. This is the “Venusian Tibet” - the highest plateau. The 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? This is a 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 willing to risk their health and go to Mars one way 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
> Terrestrial planets
Terrestrial planets– the first four planets of the solar system with photos. Find out the characteristics and description of terrestrial planets, search for exoplanets, research.
Researchers have been studying the vastness of the solar system for many centuries, noting various planetary types. Since the opening of access to exoplanets, our information base has become even wider. In addition to gas giants, we also found terrestrial-type objects. What is this?
Definition of the Terrestrial planets
Terrestrial planet- a celestial body represented by silicate rocks or metal, and has a solid surface layer. This is the main difference from gas giants filled with gases. The term is taken from the Latin word "Terra", which translates as "Earth". Below is a list indicating which terrestrial planets there are.
Structure and features of the Terrestrial planets
All bodies have a similar structure: a metallic core filled with iron and surrounded by a silicate mantle. Their surface sphere is covered with craters, volcanoes, mountains, canyons and other formations.
There are secondary atmospheres created by volcanic activity or the arrival of comets. They have a small number of satellites or are completely devoid of such features. The Earth has the Moon, and Mars has Phobos and Deimos. Not equipped with ring systems. Let's see what the characteristics of the terrestrial planets look like, and also note what are their similarities and differences using the example of Mercury, Venus, Earth and Mars.
Basic Facts of the Terrestrial Planets
Mercury- the smallest planet in the system, reaching 1/3 of Earth's size. It is endowed with a thin atmospheric layer, which is why it constantly freezes and heats up. Characterized by high density with iron and nickel. The magnetic field reaches only 1% of the earth's. There are many deep crater scars and a faint layer of silicate particles visible on the surface. In 2012, traces of organic material were noticed. These are the building blocks for life, and have also been found in water ice.
Venus similar in size to Earth, but its atmosphere is too dense and full of carbon monoxide. Because of this, heat is retained on the planet, making it the hottest in the system. Most of the surface is covered by active volcanoes and deep canyons. Only a few devices managed to penetrate the surface and survive for a short period of time. There are few craters because meteors burn up.
Earth- the largest of the terrestrial type and has a huge amount of liquid water. It is needed for life, which develops in all forms. There is a rocky surface covered in canyons and hills, as well as a heavy metal core. There is water vapor in the atmosphere, which helps to moderate the daily temperature regime. There is a change of regular seasons. The greatest heating occurs in areas near the equatorial line. But now the rates are rising due to human activity.
Mars has the highest mountain in the solar system. Most of the surface is represented by ancient sediments and crater formations. But you can also find younger areas. There are polar caps that reduce their size in summer and spring. It is inferior in density to Earth, and the core is solid. Researchers have not yet obtained evidence of life, but there are all hints and conditions in the past. The planet has water ice, organic matter and methane.
Formation and general features of the Terrestrial planets
It is believed that terrestrial planets appeared first. Initially, dust grains merged to create large objects. They were located closer to the Sun, so volatile substances evaporated. Celestial objects grew to a kilometer size, becoming planetesimals. Then they accumulate more and more dust.
The analysis shows that at the early stage of the development of the Solar System there could have been about a hundred protoplanets, whose sizes varied between the Moon and Mars. They constantly collided, due to which they merged, throwing out garbage fragments. As a result, 4 large terrestrial planets survived: Mercury, Venus, Mars and Earth.
All of them are characterized by a high density, and the composition is represented by silicates and metallic iron. The largest representative of the terrestrial type is the Earth. These planets are also distinguished by their overall structural structure, which includes a core, mantle and crust. Only two planets (Earth and Mars) have satellites.
Current research on the Terrestrial planets
Researchers believe that Earth-like planets are the best candidates for detecting life. Of course, the conclusions are based on the fact that the only planet with life is Earth, so its characteristics and features serve as a kind of standard.
Everything suggests that life is capable of surviving in extreme conditions. Therefore, it is expected to be found even on Mercury and Venus, despite their high temperatures. Most attention is paid to Mars. Not only is it a prime candidate for finding life, but it is also a potential future colony.
If everything goes according to plan, then in the 2030s. The first batch of astronauts may be sent to the Red Planet. Nowadays, rovers and orbiters are constantly on the planet, looking for water and signs of life.
Terrestrial exoplanets
Many exoplanets found have turned out to be gas giants because they are much easier to find. But since 2005, we began to actively capture terrestrial objects thanks to the Kepler mission. Most of them were called the super-earth class.
Among these, it is worth remembering Gliese 876d, whose mass is 7-9 times greater than that of Earth. It orbits a red dwarf star 15 light years away from us. In the Gliese 581 system, 3 terrestrial exoplanets were found with a distance of 20 light years.
The smallest is Gliese 581e. It exceeds our mass by only 1.9 times, but is located extremely close to its star. The first confirmed terrestrial exoplanet was Kepler-10b, 3-4 times our mass. It is 460 light years away and was found in 2011. At the same time, the mission team issued a list of 1235 applicants, of which 6 were terrestrial type and located in the habitable zone.
Super-Earths
Among the exoplanets, it was possible to find many super-Earths (in size between the Earth and Neptune). This species is not found in our system, so it is not yet clear whether they look more like giants or an terrestrial type.
Now the scientific world is awaiting the launch of the James Webb Telescope, which promises to increase the search power and open us to the depths of space.
Categories of Terrestrial planets
There is a division of terrestrial planets. Silicates are typical objects of our system, represented by a rocky mantle and a metallic core. Iron - a theoretical variety consisting entirely of iron. This gives greater density, but reduces the radius. Such planets can only appear in areas with high temperatures.
Rocky is another theoretical type where there is silicate rock but no metallic core. They should form further away from the star. Carbonaceous - endowed with a metallic core, around which a carbon-containing mineral has accumulated.
Previously, we thought we had studied the process of planetary formation in detail. But considering exoplanets forces us to find many gaps and take on new research. This also expands the conditions for searching for life in alien worlds. Who knows what we'll see there if we can send a probe.