How many months does an astronomical day last on Mercury? And the day lasts longer than a year

Mercury in natural color (Mariner 10 image)

Mercury- the planet closest to the Sun in the Solar System, orbits the Sun in 88 Earth days. Mercury is classified as an inner planet because its orbit is closer to the Sun than the main asteroid belt. After Pluto was deprived of its planetary status in 2006, Mercury acquired the title of the smallest planet in the solar system. Mercury's apparent magnitude ranges from −2.0 to 5.5, but it is not easily visible due to its very small angular distance from the Sun (maximum 28.3°). At high latitudes, the planet can never be seen in the dark night sky: Mercury is always hidden in the morning or evening dawn. The optimal time for observing the planet is morning or evening twilight during periods of its elongations (periods of Mercury's maximum distance from the Sun in the sky, occurring several times a year).

It is convenient to observe Mercury at low latitudes and near the equator: this is due to the fact that the duration of twilight there is shortest. In mid-latitudes it is much more difficult to find Mercury and only during the period of best elongations, and in high latitudes it is impossible at all.

Relatively little is known about the planet yet. The Mariner 10 apparatus, which studied Mercury in -1975, managed to map only 40-45% of the surface. In January 2008, the interplanetary station MESSENGER flew past Mercury, which will enter orbit around the planet in 2011.

In its physical characteristics, Mercury resembles the Moon and is heavily cratered. The planet has no natural satellites, but has a very thin atmosphere. The planet has a large iron core, which is the source of a magnetic field in its totality that is 0.1 of the Earth’s. Mercury's core makes up 70 percent of the planet's total volume. The temperature on the surface of Mercury ranges from 90 to 700 (−180 to +430 °C). The solar side heats up much more than the polar regions and the far side of the planet.

Despite its smaller radius, Mercury still exceeds in mass such satellites of the giant planets as Ganymede and Titan.

The astronomical symbol of Mercury is a stylized image of the winged helmet of the god Mercury with his caduceus.

History and name

The oldest evidence of observations of Mercury can be found in Sumerian cuneiform texts dating back to the third millennium BC. e. The planet is named after the god of the Roman pantheon Mercury, analogue of Greek Hermes and Babylonian Naboo. The ancient Greeks of Hesiod's time called Mercury "Στίλβων" (Stilbo, the Shining One). Until the 5th century BC. e. The Greeks believed that Mercury, visible in the evening and morning skies, were two different objects. In ancient India, Mercury was called Buddha(बुध) and Roginea. In Chinese, Japanese, Vietnamese and Korean, Mercury is called water star(水星) (in accordance with the ideas of the “Five Elements”. In Hebrew, the name of Mercury sounds like “Kohav Hama” (כוכב חמה) (“Solar Planet”).

Planet movement

Mercury moves around the Sun in a fairly elongated elliptical orbit (eccentricity 0.205) at an average distance of 57.91 million km (0.387 AU). At perihelion, Mercury is 45.9 million km from the Sun (0.3 AU), at aphelion - 69.7 million km (0.46 AU). At perihelion, Mercury is more than one and a half times closer to the Sun than at aphelion. The inclination of the orbit to the ecliptic plane is 7°. Mercury spends 87.97 days on one orbital revolution. The average speed of the planet's orbit is 48 km/s.

For a long time, it was believed that Mercury constantly faces the Sun with the same side, and one revolution around its axis takes the same 87.97 days. Observations of details on the surface of Mercury, carried out at the limit of resolution, did not seem to contradict this. This misconception was due to the fact that the most favorable conditions for observing Mercury repeat after a triple synodic period, that is, 348 Earth days, which is approximately equal to six times the rotation period of Mercury (352 days), therefore approximately the same surface area was observed at different times planets. On the other hand, some astronomers believed that Mercury's day was approximately equal to Earth's. The truth was revealed only in the mid-1960s, when radar was carried out on Mercury.

It turned out that a Mercury sidereal day is equal to 58.65 Earth days, that is, 2/3 of a Mercury year. This commensurability of the periods of rotation and revolution of Mercury is a unique phenomenon for the Solar System. It is presumably explained by the fact that the tidal action of the Sun took away angular momentum and retarded the rotation, which was initially faster, until the two periods were related by an integer ratio. As a result, in one Mercury year, Mercury manages to rotate around its axis by one and a half revolutions. That is, if at the moment Mercury passes perihelion a certain point on its surface is facing exactly the Sun, then at the next passage of perihelion the exact opposite point on the surface will be facing the Sun, and after another Mercury year the Sun will again return to the zenith above the first point. As a result, a solar day on Mercury lasts two Mercury years or three Mercury sidereal days.

As a result of this movement of the planet, “hot longitudes” can be distinguished on it - two opposite meridians, which alternately face the Sun during Mercury’s passage of perihelion, and which, because of this, are especially hot even by Mercury standards.

The combination of planetary movements gives rise to another unique phenomenon. The speed of rotation of the planet around its axis is practically constant, while the speed of orbital motion is constantly changing. In the orbital region near perihelion, for approximately 8 days, the speed of orbital motion exceeds the speed of rotational motion. As a result, the Sun stops in the sky of Mercury and begins to move in the opposite direction - from west to east. This effect is sometimes called the Joshua effect, named after the main character of the Book of Joshua from the Bible, who stopped the movement of the Sun (Joshua, X, 12-13). For an observer at longitudes 90° away from the “hot longitudes,” the Sun rises (or sets) twice.

It is also interesting that although Mars and Venus are the closest in orbit to Earth, it is Mercury that is most of the time the closest planet to Earth than any other (since the others move away more, not being so “tied” to the Sun).

Physical characteristics

Comparative sizes of Mercury, Venus, Earth and Mars

Mercury is the smallest terrestrial planet. Its radius is only 2439.7 ± 1.0 km, which is smaller than the radius of Jupiter's moon Ganymede and Saturn's moon Titan. The mass of the planet is 3.3 × 10 23 kg. The average density of Mercury is quite high - 5.43 g/cm³, which is only slightly less than the density of Earth. Considering that the Earth is larger in size, the density value of Mercury indicates an increased content of metals in its depths. The acceleration of gravity on Mercury is 3.70 m/s². The second escape velocity is 4.3 km/s.

Kuiper Crater (just below center). Photo from MESSENGER spacecraft

One of the most noticeable features of the surface of Mercury is the Plain of Heat (lat. Caloris Planitia). This crater got its name because it is located near one of the “hot longitudes”. Its diameter is about 1300 km. Probably, the body whose impact formed the crater had a diameter of at least 100 km. The impact was so strong that the seismic waves, having passed through the entire planet and focused at the opposite point on the surface, led to the formation of a kind of rugged “chaotic” landscape here.

Atmosphere and physical fields

When the Mariner 10 spacecraft flew past Mercury, it was established that the planet had an extremely rarefied atmosphere, the pressure of which was 5 × 10 11 times less than the pressure of the Earth’s atmosphere. Under such conditions, atoms collide more often with the surface of the planet than with each other. It consists of atoms captured from the solar wind or knocked out from the surface by the solar wind - helium, sodium, oxygen, potassium, argon, hydrogen. The average lifetime of a certain atom in the atmosphere is about 200 days.

Mercury has a magnetic field whose strength is 300 times less than the Earth's magnetic field. Mercury's magnetic field has a dipole structure and is highly symmetrical, and its axis deviates only 2 degrees from the planet's axis of rotation, which imposes a significant limitation on the range of theories explaining its origin.

Research

An image of a section of Mercury's surface taken by MESSENGER

Mercury is the least studied terrestrial planet. Only two devices were sent to study it. The first was Mariner 10, which flew past Mercury three times in -1975; the closest approach was 320 km. As a result, several thousand images were obtained, covering approximately 45% of the planet's surface. Further research from Earth showed the possibility of the existence of water ice in polar craters.

Mercury in art

• In Boris Lyapunov's science fiction story "Nearest to the Sun" (1956), Soviet cosmonauts land on Mercury and Venus for the first time to study them.
• Isaac Asimov's story "Mercury's Big Sun" (Lucky Starr series) takes place on Mercury.
• Isaac Asimov's stories "Runaround" and "The Dying Night", written in 1941 and 1956 respectively, describe Mercury with one side facing the Sun. Moreover, in the second story, the solution to the detective plot is based on this fact.
• In the science fiction novel The Flight of the Earth by Francis Karsak, along with the main plot, a scientific station for studying the Sun, located at the North Pole of Mercury, is described. Scientists live on a base located in the eternal shadow of deep craters, and observations are carried out from giant towers constantly illuminated by the luminary.
• In Alan Nurse's science fiction story "Across the Sunny Side", the main characters cross the side of Mercury facing the Sun. The story was written in accordance with the scientific views of its time, when it was assumed that Mercury was constantly facing the Sun with one side.
• In the anime animated series Sailor Moon, the planet is personified by the warrior girl Sailor Mercury, aka Ami Mitsuno. Her attack is based on the power of water and ice.
• In Clifford Simak's science fiction story "Once Upon a Time on Mercury", the main field of action is Mercury, and the energy form of life on it - balls - surpasses humanity by millions of years of development, having long passed the stage of civilization.

Notes

See also

Literature

• Bronshten V. Mercury is closest to the Sun // Aksenova M.D. Encyclopedia for children. T. 8. Astronomy - M.: Avanta+, 1997. - P. 512-515. - ISBN 5-89501-008-3
• Ksanfomality L.V. Unknown Mercury // In the world of science. - 2008. - № 2.

Links

• Website about the MESSENGER mission (English)
  • Photos of Mercury taken by Messenger (English)
• BepiColombo mission section on the JAXA website
• A. Levin. Iron Planet Popular Mechanics No. 7, 2008
• “The closest” Lenta.ru, October 5, 2009, photographs of Mercury taken by Messenger
• “New photographs of Mercury have been published” Lenta.ru, November 4, 2009, about the rapprochement of Messenger and Mercury on the night of September 29-30, 2009

Mercury is the planet closest to the Sun in the Solar System, revolving around the Sun in 88 Earth days. The duration of one sidereal day on Mercury is 58.65 Earth days, and the duration of a solar day is 176 Earth days. The planet is named after the ancient Roman god of trade Mercury, an analogue of the Greek Hermes and Babylonian Nabu.

Mercury is an inner planet because its orbit lies within the orbit of the Earth. After Pluto was deprived of its planetary status in 2006, Mercury acquired the title of the smallest planet in the solar system. Mercury's apparent magnitude ranges from 1.9 to 5.5, but it is not easily visible due to its small angular distance from the Sun (maximum 28.3°). Relatively little is known about the planet yet. It was only in 2009 that scientists compiled the first complete map of Mercury, using images from Mariner 10 and Messenger. The presence of any natural satellites on the planet has not been detected.

Mercury is the smallest terrestrial planet. Its radius is only 2439.7 ± 1.0 km, which is less than the radius of Jupiter's moon Ganymede and Saturn's moon Titan. The mass of the planet is 3.3·1023 kg. The average density of Mercury is quite high - 5.43 g/cm3, which is only slightly less than the density of Earth. Considering that the Earth is larger in size, the density value of Mercury indicates an increased content of metals in its depths. The acceleration of gravity on Mercury is 3.70 m/s. The second escape velocity is 4.25 km/s. Despite its smaller radius, Mercury still exceeds in mass the satellites of the giant planets such as Ganymede and Titan.

The astronomical symbol of Mercury is a stylized image of the winged helmet of the god Mercury with his caduceus.

Planet movement

Mercury moves around the Sun in a fairly elongated elliptical orbit (eccentricity 0.205) at an average distance of 57.91 million km (0.387 AU). At perihelion, Mercury is 45.9 million km from the Sun (0.3 AU), at aphelion - 69.7 million km (0.46 AU). At perihelion, Mercury is more than one and a half times closer to Sun than at aphelion. The inclination of the orbit to the ecliptic plane is 7°. Mercury spends 87.97 Earth days on one orbital revolution. The average speed of the planet's orbit is 48 km/s. The distance from Mercury to Earth varies from 82 to 217 million km.

For a long time, it was believed that Mercury constantly faces the Sun with the same side, and one rotation around its axis takes the same 87.97 Earth days. Observations of details on the surface of Mercury did not contradict this. This misconception was due to the fact that the most favorable conditions for observing Mercury repeat after a period approximately equal to six times the rotation period of Mercury (352 days), therefore approximately the same section of the planet’s surface was observed at different times. The truth was revealed only in the mid-1960s, when radar surveys of Mercury were carried out.

It turned out that a Mercury sidereal day is equal to 58.65 Earth days, that is, 2/3 of a Mercury year. Such commensurability of the periods of rotation around the axis and revolution of Mercury around the Sun is a unique phenomenon for the Solar System. It is presumably explained by the fact that the tidal action of the Sun took away angular momentum and retarded the rotation, which was initially faster, until the two periods were related by an integer ratio. As a result, in one Mercury year, Mercury manages to rotate around its axis by one and a half revolutions. That is, if at the moment Mercury passes perihelion, a certain point on its surface is facing exactly the Sun, then at the next passage of perihelion, exactly the opposite point on the surface will be facing the Sun, and after another Mercury year, the Sun will again return to the zenith above the first point. As a result, a solar day on Mercury lasts two Mercury years or three Mercury sidereal days.

As a result of this movement of the planet, “hot longitudes” can be distinguished on it - two opposite meridians, which alternately face the Sun during Mercury’s passage of perihelion, and which, because of this, are especially hot even by Mercury standards.

There are no seasons on Mercury like on Earth. This occurs because the planet's rotation axis is at right angles to the orbital plane. As a result, there are areas near the poles that the sun's rays never reach. A survey carried out by the Arecibo radio telescope suggests that there are glaciers in this icy and dark zone. The glacial layer can reach 2 m and is covered with a layer of dust.

The combination of planetary movements gives rise to another unique phenomenon. The speed of rotation of the planet around its axis is practically constant, while the speed of orbital motion is constantly changing. In the orbital region near perihelion, for approximately 8 days the angular velocity of orbital motion exceeds the angular velocity of rotational motion. As a result, the Sun stops in the sky of Mercury and begins to move in the opposite direction - from west to east. This effect is sometimes called the Joshua effect, named after the main character in the Book of Joshua from the Bible, who stopped the movement of the Sun (Joshua 10:12-13). For an observer at longitudes 90° away from the “hot longitudes,” the Sun rises (or sets) twice.

It is also interesting that, although the closest orbits to Earth are Mars and Venus, Mercury is often the closest planet to Earth (since the others move away more, not being so “tied” to the Sun).

Anomalous orbital precession

Mercury is close to the Sun, so the effects of general relativity are manifested in its motion to the greatest extent among all the planets in the Solar System. Already in 1859, the French mathematician and astronomer Urbain Le Verrier reported that there was a slow precession in the orbit of Mercury that could not be fully explained by calculating the influence of the known planets according to Newtonian mechanics. The precession of Mercury's perihelion is 5600 arcseconds per century. Calculation of the influence of all other celestial bodies on Mercury according to Newtonian mechanics gives a precession of 5557 arcseconds per century. Trying to explain the observed effect, he suggested that there was another planet (or perhaps a belt of small asteroids) whose orbit was closer to the Sun than Mercury, and which was introducing a perturbing influence (other explanations considered the unaccounted for polar compression of the Sun). Thanks to previously achieved successes in the search for Neptune, taking into account its influence on the orbit of Uranus, this hypothesis became popular, and the desired hypothetical planet even received the name Vulcan. However, this planet was never discovered.

Since none of these explanations stood up to the test of observations, some physicists began to put forward more radical hypotheses that it was necessary to change the law of gravity itself, for example, change the exponent in it or add terms to the potential that depend on the speed of bodies. However, most of these attempts have proven controversial. At the beginning of the 20th century, general relativity provided an explanation for the observed precession. The effect is very small: the relativistic "addition" is only 42.98 arcseconds per century, which is 1/130 (0.77%) of the total rate of precession, so it would take at least 12 million revolutions of Mercury around the Sun for perihelion to return to the position predicted by classical theory. A similar, but smaller displacement exists for other planets - 8.62 arc seconds per century for Venus, 3.84 for Earth, 1.35 for Mars, as well as asteroids - 10.05 for Icarus.

Hypotheses for the formation of Mercury

Since the 19th century, there has been a scientific hypothesis that Mercury in the past was a satellite of the planet Venus, which was subsequently “lost” by it. In 1976, Tom van Flandern (English) Russian. and K.R. Harrington, on the basis of mathematical calculations, it was shown that this hypothesis well explains the large deviations (eccentricity) of the orbit of Mercury, its resonant nature of revolution around the Sun and the loss of angular momentum of both Mercury and Venus (the latter also - acquisition of rotation opposite to the main one in the Solar system).

Currently, this hypothesis is not confirmed by observational data and information from automatic stations on the planet. The presence of a massive iron core with a large amount of sulfur, the percentage of which is greater than in the composition of any other planet in the Solar System, the features of the geological and physical-chemical structure of the surface of Mercury indicate that the planet was formed in the solar nebula independently of other planets, that is Mercury has always been an independent planet.

Now there are several versions to explain the origin of the huge core, the most common of which says that Mercury initially had a ratio of the mass of metals to the mass of silicates similar to those in the most common meteorites - chondrites, the composition of which is generally typical for solid bodies of the Solar system and internal planets, and the mass of the planet in ancient times was approximately 2.25 times its present mass. In the history of the early Solar System, Mercury may have experienced an impact with a planetesimal of approximately 1/6 of its own mass at a speed of ~20 km/s. Most of the crust and upper layer of the mantle were blown into outer space, which, crushed into hot dust, were scattered in interplanetary space. But the core of the planet, consisting of heavier elements, has been preserved.

According to another hypothesis, Mercury formed in the inner part of the protoplanetary disk, which was already extremely depleted in light elements, which were swept out by the Sun into the outer regions of the Solar System.

Surface

In its physical characteristics, Mercury resembles the Moon. The planet has no natural satellites, but has a very thin atmosphere. The planet has a large iron core, which is a source of a magnetic field in its totality that is 0.01 of the Earth’s. Mercury's core makes up 83% of the planet's total volume. The temperature on the surface of Mercury ranges from 90 to 700 K (from +80 to +430 °C). The solar side heats up much more than the polar regions and the far side of the planet.

The surface of Mercury is also in many ways reminiscent of the Moon - it is heavily cratered. The density of craters varies in different areas. It is assumed that the more densely dotted areas with craters are more ancient, and the less densely dotted ones are younger, formed when the old surface was flooded with lava. At the same time, large craters are less common on Mercury than on the Moon. The largest crater on Mercury is named after the great Dutch painter Rembrandt; its diameter is 716 km. However, the similarity is incomplete - formations are visible on Mercury that are not found on the Moon. An important difference between the mountainous landscapes of Mercury and the Moon is the presence on Mercury of numerous jagged slopes, extending for hundreds of kilometers, called scarps. A study of their structure showed that they were formed during compression that accompanied the cooling of the planet, as a result of which the surface area of ​​Mercury decreased by 1%. The presence of well-preserved large craters on the surface of Mercury suggests that over the past 3-4 billion years there was no large-scale movement of sections of the crust, and there was no erosion of the surface; the latter almost completely excludes the possibility of the existence of any significant atmosphere.

During research conducted by the Messenger probe, over 80% of the surface of Mercury was photographed and found to be homogeneous. In this way, Mercury is not similar to the Moon or Mars, in which one hemisphere is sharply different from the other.

The first data from a study of the elemental composition of the surface using the X-ray fluorescence spectrometer of the Messenger spacecraft showed that it is poor in aluminum and calcium compared to the plagioclase feldspar characteristic of the continental regions of the Moon. At the same time, the surface of Mercury is relatively poor in titanium and iron and rich in magnesium, occupying an intermediate position between typical basalts and ultramafic rocks such as terrestrial komatiites. Sulfur was also found to be relatively abundant, suggesting reducing conditions for planet formation.

Craters

Craters on Mercury range in size from small bowl-shaped depressions to multi-ringed impact craters hundreds of kilometers across. They are in various stages of destruction. There are relatively well-preserved craters with long rays around them, which were formed as a result of the ejection of material at the moment of impact. There are also heavily destroyed remains of craters. Mercury craters differ from lunar craters in that the area of ​​their cover from the ejection of matter upon impact is smaller due to the greater gravity on Mercury.

One of the most noticeable features of the surface of Mercury is the Plain of Heat (lat. Caloris Planitia). This relief feature received this name because it is located near one of the “hot longitudes.” Its diameter is about 1550 km.

Probably, the body whose impact formed the crater had a diameter of at least 100 km. The impact was so strong that the seismic waves, having passed through the entire planet and focused at the opposite point on the surface, led to the formation of a kind of rugged “chaotic” landscape here. The force of the impact is also evidenced by the fact that it caused the ejection of lava, which formed high concentric circles at a distance of 2 km around the crater.

The point with the highest albedo on the surface of Mercury is the 60 km diameter Kuiper crater. This is probably one of the youngest large craters on Mercury.

Until recently, it was assumed that in the depths of Mercury there is a metallic core with a radius of 1800-1900 km, containing 60% of the planet’s mass, since the Mariner 10 spacecraft discovered a weak magnetic field, and it was believed that a planet with such a small size cannot have liquid kernels. But in 2007, Jean-Luc Margot's group summed up five years of radar observations of Mercury, during which they noticed variations in the planet's rotation that were too large for a model with a solid core. Therefore, today we can say with a high degree of confidence that the planet’s core is liquid.

The percentage of iron in Mercury's core is higher than that of any other planet in the solar system. Several theories have been proposed to explain this fact. According to the most widely supported theory in the scientific community, Mercury originally had the same ratio of metal to silicates as a normal meteorite, having a mass 2.25 times greater than now. However, at the beginning of the history of the Solar System, a planet-like body with 6 times less mass and several hundred kilometers in diameter hit Mercury. As a result of the impact, much of the original crust and mantle was separated from the planet, causing the relative proportion of the core in the planet's composition to increase. A similar process, known as the giant impact theory, has been proposed to explain the formation of the Moon. However, the first data from a study of the elemental composition of the surface of Mercury using the AMS Messenger gamma spectrometer does not confirm this theory: the abundance of the radioactive isotope potassium-40 of the moderately volatile chemical element potassium compared to the radioactive isotopes thorium-232 and uranium-238 of the more refractory elements uranium and thorium does not cope with the high temperatures inevitable during a collision. It is therefore assumed that the elemental composition of Mercury corresponds to the primary elemental composition of the material from which it formed, similar to enstatite chondrites and anhydrous cometary particles, although the iron content of enstatite chondrites examined to date is not sufficient to explain the high average density of Mercury.

The core is surrounded by a silicate mantle 500-600 km thick. According to data from Mariner 10 and observations from Earth, the thickness of the planet's crust ranges from 100 to 300 km.

Geological history

Like the Earth, Moon and Mars, Mercury's geological history is divided into eras. They have the following names (from earlier to later): pre-Tolstoyan, Tolstoyan, Kalorian, late Kalorian, Mansurian and Kuiper. This division periodizes the relative geological age of the planet. The absolute age, measured in years, is not precisely established.

After the formation of Mercury 4.6 billion years ago, the planet was intensively bombarded by asteroids and comets. The last major bombardment of the planet occurred 3.8 billion years ago. Some regions, for example, the Plain of Heat, were also formed due to their filling with lava. This led to the formation of smooth planes inside the craters, similar to those on the Moon.

Then, as the planet cooled and contracted, ridges and faults began to form. They can be observed on the surface of larger relief features of the planet, such as craters and plains, which indicates a later time of their formation. The period of volcanism on Mercury ended when the mantle had shrunk enough to prevent lava from reaching the planet's surface. This probably happened in the first 700-800 million years of its history. All subsequent changes in relief are caused by impacts of external bodies on the surface of the planet.

Magnetic field

Mercury has a magnetic field whose strength is 100 times less than that of Earth. Mercury's magnetic field has a dipole structure and is highly symmetrical, and its axis deviates only 10 degrees from the planet's axis of rotation, which imposes a significant limitation on the range of theories explaining its origin. Mercury's magnetic field may be generated by a dynamo effect, much like on Earth. This effect is the result of the circulation of the planet's liquid core. Due to the pronounced eccentricity of the planet, an extremely strong tidal effect occurs. It maintains the core in a liquid state, which is necessary for the dynamo effect to occur.

Mercury's magnetic field is strong enough to change the direction of the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, although small enough to fit inside the Earth, is powerful enough to trap plasma from the solar wind. Observations obtained by Mariner 10 detected low-energy plasma in the magnetosphere on the night side of the planet. Explosions of active particles were discovered in the magnetotail, indicating the dynamic qualities of the planet's magnetosphere.

During its second flyby of the planet on October 6, 2008, Messenger discovered that Mercury's magnetic field may have a significant number of windows. The spacecraft encountered the phenomenon of magnetic vortices - intertwined knots of the magnetic field connecting the ship with the planet’s magnetic field. The vortex reached 800 km in diameter, which is a third of the radius of the planet. This vortex form of magnetic field is created by the solar wind. As the solar wind flows around the planet's magnetic field, it binds and sweeps along with it, curling into vortex-like structures. These magnetic flux vortices form windows in the planetary magnetic shield through which the solar wind penetrates and reaches the surface of Mercury. The process of coupling between planetary and interplanetary magnetic fields, called magnetic reconnection, is a common phenomenon in space. It also occurs near the Earth when it generates magnetic vortices. However, according to Messenger observations, the frequency of reconnection of Mercury's magnetic field is 10 times higher.

Conditions on Mercury

Its proximity to the Sun and the planet's rather slow rotation, as well as its extremely weak atmosphere, mean that Mercury experiences the most dramatic temperature changes in the Solar System. This is also facilitated by the loose surface of Mercury, which conducts heat poorly (and with a completely absent or extremely weak atmosphere, heat can be transferred inward only due to thermal conductivity). The surface of the planet quickly heats up and cools down, but already at a depth of 1 m, daily fluctuations cease to be felt, and the temperature becomes stable, equal to approximately +75 ° C.

The average daytime surface temperature is 623 K (349.9 °C), the nighttime temperature is only 103 K (170.2 °C). The minimum temperature on Mercury is 90 K (183.2 °C), and the maximum, reached at noon at “hot longitudes” when the planet is near perihelion, is 700 K (426.9 °C).

Despite these conditions, there have recently been suggestions that ice may exist on the surface of Mercury. Radar studies of the planet's circumpolar regions have shown the presence of depolarization areas there from 50 to 150 km; the most likely candidate for a substance reflecting radio waves may be ordinary water ice. Entering the surface of Mercury when comets hit it, water evaporates and travels around the planet until it freezes in the polar regions at the bottom of deep craters, where the Sun never looks, and where ice can persist almost indefinitely.

When the Mariner 10 spacecraft flew past Mercury, it was established that the planet had an extremely rarefied atmosphere, the pressure of which was 5·1011 times less than the pressure of the Earth’s atmosphere. Under such conditions, atoms collide more often with the surface of the planet than with each other. The atmosphere is made up of atoms captured from the solar wind or knocked out from the surface by the solar wind - helium, sodium, oxygen, potassium, argon, hydrogen. The average lifetime of an individual atom in the atmosphere is about 200 days.

Hydrogen and helium likely enter the planet via the solar wind, diffuse into its magnetosphere, and then escape back into space. Radioactive decay of elements in Mercury's crust is another source of helium, sodium and potassium. Water vapor is present, released as a result of a number of processes, such as comet impacts on the surface of the planet, the formation of water from hydrogen in the solar wind and oxygen from rocks, and sublimation from ice that is found in permanently shadowed polar craters. The discovery of a significant number of water-related ions, such as O+, OH+ H2O+, was a surprise.

Since a significant number of these ions were found in the space surrounding Mercury, scientists hypothesized that they were formed from water molecules destroyed on the surface or in the exosphere of the planet by the solar wind.

On February 5, 2008, a group of astronomers from Boston University led by Jeffrey Baumgardner announced the discovery of a comet-like tail on the planet Mercury more than 2.5 million km long. It was discovered during observations from ground-based observatories in the sodium line. Before this, it was known about a tail no more than 40,000 km long. The team's first image was taken in June 2006 by the Air Force's 3.7-meter telescope on Mount Haleakala, Hawaii, and then used three smaller instruments, one at Haleakala and two at McDonald Observatory, Texas. A telescope with a 4-inch aperture (100 mm) was used to create images with a large field of view. The image of Mercury's long tail was taken in May 2007 by Jody Wilson (senior scientist) and Carl Schmidt (graduate student). The apparent length of the tail for an observer from Earth is about 3°.

New data about Mercury's tail appeared after the second and third flybys of the Messenger spacecraft in early November 2009. Based on these data, NASA employees were able to propose a model of this phenomenon.

Features of observation from Earth

Mercury's apparent magnitude ranges from -1.9 to 5.5, but it is not easily visible due to its small angular distance from the Sun (maximum 28.3°). At high latitudes, the planet can never be seen in the dark night sky: Mercury is visible for a very short period of time after dusk. The optimal time for observing the planet is morning or evening twilight during periods of its elongations (periods of Mercury's maximum distance from the Sun in the sky, occurring several times a year).

The most favorable conditions for observing Mercury are at low latitudes and near the equator: this is due to the fact that the duration of twilight there is shortest. In mid-latitudes, finding Mercury is much more difficult and is possible only during the period of best elongations, and in high latitudes it is impossible at all. The most favorable conditions for observing Mercury in the middle latitudes of both hemispheres occur around the equinoxes (the duration of twilight is minimal).

The earliest known observation of Mercury was recorded in the tables of Mul apin (a collection of Babylonian astrological tables). This observation was most likely made by Assyrian astronomers around the 14th century BC. e. The Sumerian name used for Mercury in the Mul Apin tables can be transcribed as UDU.IDIM.GUU4.UD ("jumping planet"). The planet was originally associated with the god Ninurta, and in later records it is called "Nabu" in honor of the god of wisdom and scribal arts.

In Ancient Greece, during the time of Hesiod, the planet was known under the names (“Stilbon”) and (“Hermaon”). The name "Hermaon" is a form of the name of the god Hermes. Later the Greeks began to call the planet "Apollo".

There is a hypothesis that the name “Apollo” corresponded to visibility in the morning sky, and “Hermes” (“Hermaon”) in the evening sky. The Romans named the planet after the fleet-footed god of commerce, Mercury, who is equivalent to the Greek god Hermes for moving through the sky faster than the other planets. The Roman astronomer Claudius Ptolemy, who lived in Egypt, wrote about the possibility of a planet moving across the disk of the Sun in his work “Hypotheses about the Planets.” He suggested that such a transit had never been observed because a planet like Mercury was too small to observe or because the moment of transit occurred infrequently.

In ancient China, Mercury was called Chen-hsing, "Morning Star". It was associated with the direction north, the color black and the element of water in Wu-hsing. According to the Hanshu, the synodic period of Mercury was recognized by Chinese scientists as equal to 115.91 days, and according to the Hou Hanshu - 115.88 days. In modern Chinese, Korean, Japanese and Vietnamese cultures, the planet began to be called “Water Star”.

Indian mythology used the name Budha for Mercury. This god, the son of Soma, was dominant on Wednesdays. In Germanic paganism, the god Odin was also associated with the planet Mercury and the environment. The Mayans represented Mercury as an owl (or perhaps as four owls, with two corresponding to the morning appearance of Mercury and two to the evening appearance), which was a messenger of the afterlife. In Hebrew, Mercury was called "Kokha in Hama."
Mercury in the starry sky (above, above the Moon and Venus)

In the Indian astronomical treatise "Surya-siddhanta", dating from the 5th century, the radius of Mercury was estimated at 2420 km. The error compared to the true radius (2439.7 km) is less than 1%. However, this estimate was based on an imprecise assumption of the planet's angular diameter, which was taken to be 3 arcminutes.

In medieval Arab astronomy, the Andalusian astronomer Az-Zarqali described the deferent of Mercury's geocentric orbit as an oval like an egg or a pine nut. However, this conjecture had no impact on his astronomical theory and his astronomical calculations. In the 12th century, Ibn Bajjah observed two planets as spots on the surface of the Sun. Later, the astronomer of the Maragha observatory Al-Shirazi suggested that his predecessor had observed the passage of Mercury and (or) Venus. In India, the astronomer of the Kerala school Nilakansa Somayaji (English) Russian. in the 15th century, developed a partially heliocentric planetary model in which Mercury revolved around the Sun, which in turn revolved around the Earth. This system was similar to that of Tycho Brahe, developed in the 16th century.

Medieval observations of Mercury in the northern parts of Europe were hampered by the fact that the planet is always observed at dawn - morning or evening - against the background of a twilight sky and quite low above the horizon (especially in northern latitudes). The period of its best visibility (elongation) occurs several times a year (lasting about 10 days). Even during these periods, it is not easy to see Mercury with the naked eye (a relatively dim star against a fairly light background of the sky). There is a story that Nicolaus Copernicus, who observed astronomical objects in the northern latitudes and foggy climate of the Baltic states, regretted that he had never seen Mercury in his entire life. This legend arose based on the fact that Copernicus’s work “On the Rotations of the Celestial Spheres” does not provide a single example of observations of Mercury, but he described the planet using the results of observations of other astronomers. As he himself said, Mercury can still be “caught” from northern latitudes by showing patience and cunning. Consequently, Copernicus could well have observed Mercury and observed it, but he described the planet based on other people’s research results.

Observations using telescopes

The first telescopic observation of Mercury was made by Galileo Galilei at the beginning of the 17th century. Although he observed the phases of Venus, his telescope was not powerful enough to observe the phases of Mercury. In 1631, Pierre Gassendi made the first telescopic observation of the passage of a planet across the disk of the Sun. The moment of passage was previously calculated by Johannes Kepler. In 1639, Giovanni Zupi discovered with a telescope that the orbital phases of Mercury were similar to those of the Moon and Venus. Observations have definitively demonstrated that Mercury orbits the Sun.

A very rare astronomical event is the overlap of one planet with the disk of another, observed from Earth. Venus occludes Mercury once every few centuries, and this event has only been observed once in history - on May 28, 1737 by John Bevis at the Royal Greenwich Observatory. Venus' next occultation of Mercury will be on December 3, 2133.

The difficulties accompanying the observation of Mercury have led to the fact that for a long time it was studied less than other planets. In 1800, Johann Schröter, who observed features on the surface of Mercury, announced that he had observed mountains 20 km high on it. Friedrich Bessel, using Schröter's sketches, erroneously determined the period of rotation around its axis to be 24 hours and the inclination of the axis to be 70°. In the 1880s, Giovanni Schiaparelli mapped the planet more precisely and proposed a rotation period of 88 days, coinciding with the sidereal period of revolution around the Sun due to tidal forces. The work of mapping Mercury was continued by Eugene Antoniadi, who in 1934 published a book containing old maps and his own observations. Many features of Mercury's surface are named after Antoniadi's maps.

Italian astronomer Giuseppe Colombo (English)Russian. noticed that the rotation period was 2/3 of Mercury's sidereal period, and suggested that these periods fall into a 3:2 resonance. Data from Mariner 10 subsequently confirmed this point of view. This does not mean that Schiaparelli and Antoniadi's maps are incorrect. It’s just that astronomers saw the same details of the planet every second revolution around the Sun, entered them into maps and ignored observations at a time when Mercury was facing the Sun on the other side, since due to the geometry of the orbit at that time the conditions for observation were bad.

The proximity of the Sun also creates some problems for the telescopic study of Mercury. For example, the Hubble telescope has never been used and will not be used to observe this planet. Its device does not allow observations of objects close to the Sun - if you try to do this, the equipment will suffer irreversible damage.

Research of Mercury using modern methods

Mercury is the least studied terrestrial planet. In the 20th century, radio astronomy, radar and research using spacecraft were added to the telescopic methods of studying it. Radio astronomy measurements of Mercury were first made in 1961 by Howard, Barrett and Haddock using a reflector with two radiometers mounted on it. By 1966, based on the accumulated data, good estimates of the surface temperature of Mercury were obtained: 600 K at the subsolar point and 150 K on the unlit side. The first radar observations were carried out in June 1962 by V. A. Kotelnikov’s group at the IRE; they revealed the similarity of the reflective properties of Mercury and the Moon. In 1965, similar observations at the Arecibo radio telescope led to an estimate of Mercury's rotation period: 59 days.

Only two spacecraft were sent to explore Mercury. The first was Mariner 10, which flew past Mercury three times in 1974-1975; the closest approach was 320 km. The result was several thousand images covering approximately 45% of the planet's surface. Further research from Earth showed the possibility of the existence of water ice in polar craters.

Of all the planets visible to the naked eye, only Mercury has never had its own artificial satellite. NASA is currently conducting a second mission to Mercury called Messenger. The device was launched on August 3, 2004, and in January 2008 it made its first flyby of Mercury. To enter orbit around the planet in 2011, the device performed two more gravity assist maneuvers near Mercury: in October 2008 and in September 2009. Messenger also performed one gravity assist maneuver near Earth in 2005 and two near Venus in October 2006 and June 2007, during which it tested its equipment.

Mariner 10 is the first spacecraft to reach Mercury.

The European Space Agency (ESA), together with the Japanese Aerospace Exploration Agency (JAXA), is developing the Bepi Colombo mission, consisting of two spacecraft: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO). The European MPO will explore Mercury's surface and depths, while the Japanese MMO will observe the planet's magnetic field and magnetosphere. BepiColombo is scheduled to launch in 2013, and in 2019 it will enter orbit around Mercury, where it will split into two components.

The development of electronics and computer science has made it possible to ground-based observations of Mercury using CCD radiation detectors and subsequent computer processing of images. One of the first series of observations of Mercury with CCD receivers was carried out in 1995-2002 by Johan Varell at the observatory on the island of La Palma on a half-meter solar telescope. Varell selected the best shots without using computer mixing. The reduction began to be applied at the Abastumani Astrophysical Observatory to series of photographs of Mercury obtained on November 3, 2001, as well as at the Skinakas Observatory of the University of Heraklion to series from May 1-2, 2002; To process the observation results, the correlation combination method was used. The resulting resolved image of the planet was similar to the Marinera 10 photomosaic; the outlines of small formations measuring 150-200 km in size were repeated. This is how a map of Mercury was compiled for longitudes 210-350°.

On March 17, 2011, the interplanetary probe Messenger entered Mercury orbit. It is assumed that with the help of equipment installed on it, the probe will be able to explore the landscape of the planet, the composition of its atmosphere and surface; Messenger's equipment also allows for research into energetic particles and plasma. The service life of the probe is determined to be one year.

On June 17, 2011, it became known that, according to the first studies carried out by the Messenger spacecraft, the planet’s magnetic field is not symmetrical relative to the poles; Thus, different numbers of solar wind particles reach Mercury's north and south poles. An analysis of the prevalence of chemical elements on the planet was also carried out.

Features of the nomenclature

The rules for naming geological objects located on the surface of Mercury were approved at the XV General Assembly of the International Astronomical Union in 1973:
The small crater Hun Kal (indicated by an arrow), serving as a reference point for Mercury's system of longitudes. Photo by AMS Mariner 10

The largest object on the surface of Mercury, with a diameter of about 1300 km, was given the name Heat Plain, since it is located in the region of maximum temperatures. This is a multi-ring structure of impact origin, filled with solidified lava. Another plain, located in the region of minimum temperatures, near the north pole, is called the Northern Plain. Other similar formations were called the planet Mercury or an analogue of the Roman god Mercury in the languages ​​of different peoples of the world. For example: Suisei Plain (planet Mercury in Japanese) and Budha Plain (planet Mercury in Hindi), Sobkou Plain (ancient Egyptian planet Mercury), Plain Odin (Norse god) and Tire Plain (ancient Armenian deity).
Mercury craters (with two exceptions) are named after famous people in the humanitarian field (architects, musicians, writers, poets, philosophers, photographers, artists). For example: Barma, Belinsky, Glinka, Gogol, Derzhavin, Lermontov, Mussorgsky, Pushkin, Repin, Rublev, Stravinsky, Surikov, Turgenev, Feofan the Greek, Fet, Tchaikovsky, Chekhov. The exceptions are two craters: Kuiper, named after one of the main developers of the Mariner 10 project, and Hun Kal, which means the number “20” in the language of the Mayan people, who used the base-20 number system. The last crater is located near the equator at meridian 200 west longitude and was chosen as a convenient reference point for reference in the coordinate system of the surface of Mercury. Initially, larger craters were given the names of celebrities, who, according to the IAU, had correspondingly greater significance in world culture. The larger the crater, the stronger the influence of the individual on the modern world. The top five included Beethoven (643 km in diameter), Dostoevsky (411 km), Tolstoy (390 km), Goethe (383 km) and Shakespeare (370 km).
Escarps (ledges), mountain ranges and canyons are named after ships of explorers who made history because the god Mercury/Hermes was considered the patron saint of travelers. For example: Beagle, Zarya, Santa Maria, Fram, Vostok, Mirny). An exception to the rule are two ridges named after astronomers, the Antoniadi Ridge and the Schiaparelli Ridge.
Valleys and other features on Mercury's surface are named after large radio observatories, in recognition of the importance of radar in planetary exploration. For example: Highstack Valley (radio telescope in the USA).
Subsequently, in connection with the discovery of grooves on Mercury by the automatic interplanetary station “Messenger” in 2008, a rule was added for naming grooves that receive the names of great architectural structures. For example: Pantheon on the Plain of Heat.

As soon as the automatic station Mariner 10, sent from Earth, finally reached the almost unexplored planet Mercury and began photographing it, it became clear that big surprises awaited earthlings here, one of which was the extraordinary, striking similarity of the surface of Mercury with the Moon. The results of further research plunged the researchers into even greater amazement: it turned out that Mercury has much more in common with the Earth than with its eternal satellite.

Illusory kinship

From the first images transmitted by Mariner 10, scientists were indeed looking at the familiar Moon, or at least its twin; there were many craters on the surface of Mercury that, at first glance, looked completely identical to the lunar ones. And only careful examination of the images made it possible to establish that the hilly areas around the lunar craters, composed of material ejected during the crater-forming explosion, are one and a half times wider than those on Mercury, with the same size of craters. This is explained by the fact that the greater gravity on Mercury prevented the soil from spreading further. It turned out that on Mercury, like on the Moon, there are two main types of terrain - analogues of lunar continents and seas.

Continental regions are the most ancient geological formations of Mercury, consisting of cratered areas, intercrater plains, mountainous and hilly formations, as well as lined areas covered with numerous narrow ridges.

Analogues of the lunar seas are considered to be the smooth plains of Mercury, which are younger in age than the continents and somewhat darker than the continental formations, but still not as dark as the lunar seas. Such areas on Mercury are concentrated in the area of ​​the Zhary Plain, a unique and largest ring structure on the planet with a diameter of 1,300 km. The plain received its name not by chance; the meridian 180° west passes through it. etc., it is he (or the meridian 0° opposite to it) that is located in the center of the hemisphere of Mercury that faces the Sun when the planet is at the minimum distance from the Sun. At this time, the surface of the planet heats up most strongly in the areas of these meridians, and in particular in the area of ​​the Zhary Plain. It is surrounded by a mountainous ring that borders a huge circular depression formed early in Mercury's geological history. Subsequently, this depression, as well as the areas adjacent to it, were flooded by lavas, during the solidification of which smooth plains arose.

On the other side of the planet, exactly opposite the depression in which the Zhara plain is located, there is another unique formation - a hilly-linear terrain. It consists of numerous large hills (5 x 10 km in diameter and up to 1 x 2 km in height) and is crossed by several large straight valleys, clearly formed along fault lines in the planet’s crust. The location of this area in the area opposite the Zhara plain served as the basis for the hypothesis that the hilly-linear relief was formed due to the focusing of seismic energy from the impact of the asteroid that formed the Zhara depression. This hypothesis received indirect confirmation when areas with a similar relief were soon discovered on the Moon, located diametrically opposite to the Mare Monsii and the Mare Orientalis, the two largest ring formations of the Moon.

The structural pattern of Mercury's crust is determined to a large extent, like that of the Moon, by large impact craters, around which systems of radial-concentric faults are developed, dividing Mercury's crust into blocks. The largest craters have not one, but two ring-shaped concentric shafts, which also resembles the lunar structure. On the filmed half of the planet, 36 such craters were identified.

Despite the general similarity of the Mercury and lunar landscapes, completely unique geological structures were discovered on Mercury that had not previously been observed on any of the planetary bodies. They were called lobe-shaped ledges, since their outlines on the map are typical of rounded protrusions - “lobes” up to several tens of kilometers in diameter. The height of the ledges is from 0.5 to 3 km, while the largest of them reach 500 km in length. These ledges are quite steep, but unlike lunar tectonic ledges, which have a sharp downward bend in the slope, the Mercurian lobe-shaped ones have a smoothed line of surface inflection in their upper part.

These ledges are located in the ancient continental regions of the planet. All their features give reason to consider them a superficial expression of compression of the upper layers of the planet’s crust.

Calculations of the compression value, carried out using the measured parameters of all ledges on the filmed half of Mercury, indicate a reduction in the crustal area by 100 thousand km 2, which corresponds to a decrease in the radius of the planet by 1 x 2 km. Such a decrease could be caused by the cooling and solidification of the planet’s interior, in particular its core, which continued even after the surface had already become solid.

Calculations showed that the iron core should have a mass of 0.6 x 0.7 of the mass of Mercury (for the Earth the same value is 0.36). If all the iron is concentrated in the Mercury core, then its radius will be 3/4 of the radius of the planet. Thus, if the radius of the core is approximately 1,800 km, then it turns out that inside Mercury there is a giant iron ball the size of the Moon. The two outer rocky shells, the mantle and the crust, account for only about 800 km. This internal structure is very similar to the structure of the Earth, although the dimensions of Mercury’s shells are determined only in the most general terms: even the thickness of the crust is unknown, it is assumed that it could be 50 x 100 km, then a layer about 700 km thick remains on the mantle. On Earth, the mantle occupies the predominant part of the radius.

Relief details. The giant Discovery Escarpment, 350 km long, intersects two craters with a diameter of 35 and 55 km. The maximum height of the ledge is 3 km. It was formed by thrusting the upper layers of Mercury's crust from left to right. This happened due to warping of the planet's crust during compression of the metal core caused by its cooling. The ledge was named after James Cook's ship.

Photo map of the largest ring structure on Mercury, the Zhara Plain, surrounded by the Zhara Mountains. The diameter of this structure is 1300 km. Only its eastern part is visible, and the central and western parts, not illuminated in this image, have not yet been studied. Meridian area 180° W. d. this is the most strongly heated region of Mercury by the Sun, which is reflected in the names of the plain and mountains. The two main types of terrain on Mercury - ancient heavily cratered areas (dark yellow on the map) and younger smooth plains (brown on the map) - reflect the two main periods of the planet's geological history - the period of massive falls of large meteorites and the subsequent period of outpouring of highly mobile, presumably basaltic lavas.

Giant craters with a diameter of 130 and 200 km with an additional shaft at the bottom, concentric to the main ring shaft.

The winding Santa Maria Escarpment, named for Christopher Columbus's ship, crosses ancient craters and later flat terrain.

Hilly-linear terrain is a unique section of the surface of Mercury in its structure. There are almost no small craters here, but many clusters of low hills crossed by straight tectonic faults.

Names on the map. The names of the relief features of Mercury identified in the Mariner 10 images were given by the International Astronomical Union. The craters are named after figures of world culture - famous writers, poets, artists, sculptors, composers. To designate the plains (except for the Zhara plain), the names of the planet Mercury were used in different languages. Extended linear depressions, tectonic valleys, were named after radio observatories that contributed to the study of planets, and two ridges, large linear hills, were named after the astronomers Schiaparelli and Antoniadi, who made many visual observations. The largest lobe-shaped ledges received the names of sea ships on which the most significant voyages in the history of mankind were made.

Iron Heart

A surprise was also other data obtained by Mariner 10, which showed that Mercury has an extremely weak magnetic field, the value of which is only about 1% of the Earth's. This seemingly insignificant circumstance was extremely important for scientists, since of all the planetary bodies of the terrestrial group, only the Earth and Mercury have a global magnetosphere. And the only most plausible explanation for the nature of Mercury’s magnetic field may be the presence in the depths of the planet of a partially molten metallic core, again similar to the Earth’s. Apparently, Mercury has a very large core, as evidenced by the planet’s high density (5.4 g/cm3), which suggests that Mercury contains a lot of iron, the only heavy element widely distributed in nature.

To date, several possible explanations have been put forward for the high density of Mercury given its relatively small diameter. According to the modern theory of planet formation, it is believed that in the preplanetary dust cloud the temperature of the region adjacent to the Sun was higher than in its outlying parts, therefore light (so-called volatile) chemical elements were carried to distant, colder parts of the cloud. As a result, in the circumsolar region (where Mercury is now located), a predominance of heavier elements was created, the most common of which is iron.

Other explanations attribute Mercury's high density to the chemical reduction of oxides of light elements to their heavier, metallic forms under the influence of very strong solar radiation, or to the gradual evaporation and volatilization of the outer layer of the planet's original crust into space under the influence of solar heating, or with the fact that a significant part of the “stone” shell of Mercury was lost as a result of explosions and ejections of matter into outer space during collisions with smaller celestial bodies, such as asteroids.

In terms of average density, Mercury stands apart from all other terrestrial planets, including the Moon. Its average density (5.4 g/cm3) is second only to the density of the Earth (5.5 g/cm3), and if we keep in mind that the Earth’s density is affected by stronger compression of matter due to the larger size of our planet, then it turns out that with equal sizes of the planets, the density of the Mercury substance would be the greatest, exceeding the Earth's by 30%.

Hot ice

Judging by the available data, the surface of Mercury, which receives huge amounts of solar energy, is a real inferno. Judge for yourself: the average temperature at the time of Mercury noon is about +350°C. Moreover, when Mercury is at a minimum distance from the Sun, it rises to +430°C, while at its maximum distance it drops to only +280°C. However, it has also been established that immediately after sunset the temperature in the equatorial region drops sharply to 100°C, and by midnight it generally reaches 170°C, but after dawn the surface quickly warms up to +230°C. Radio measurements taken from Earth showed that inside the soil at shallow depths the temperature does not depend at all on the time of day. This indicates the high thermal insulation properties of the surface layer, but since daylight hours last on Mercury for 88 Earth days, during this time all areas of the surface have time to warm up well, albeit to a small depth.

It would seem that talking about the possibility of ice existing in such conditions on Mercury is at least absurd. But in 1992, during radar observations from the Earth near the north and south poles of the planet, areas that very strongly reflect radio waves were discovered for the first time. It was these data that were interpreted as evidence of the presence of ice in the near-surface Mercury layer. Radar from the Arecibo radio observatory located on the island of Puerto Rico, as well as from NASA's Deep Space Communications Center in Goldstone (California), revealed about 20 round spots several tens of kilometers across with increased radio reflection. Presumably these are craters, into which, due to their close location to the poles of the planet, the sun's rays fall only briefly or not at all. Such craters, called permanently shadowed ones, are also present on the Moon; measurements from satellites have revealed the presence of some amount of water ice in them. Calculations have shown that in the depressions of permanently shadowed craters at the poles of Mercury it can be cold enough (175 ° C) for ice to exist there for a long time. Even in flat areas near the poles, the estimated daily temperature does not exceed 105°C. There are still no direct measurements of the surface temperature of the polar regions of the planet.

Despite observations and calculations, the existence of ice on the surface of Mercury or at a small depth beneath it has not yet received unambiguous evidence, since rocks containing compounds of metals with sulfur and possible metal condensates on the surface of the planet, such as ions, also have increased radio reflection sodium deposited on it as a result of the constant “bombardment” of Mercury by solar wind particles.

But here the question arises: why is the distribution of areas that strongly reflect radio signals clearly confined specifically to the polar regions of Mercury? Maybe the rest of the territory is protected from the solar wind by the planet's magnetic field? Hopes for clarifying the mystery of ice in the kingdom of heat are connected only with the flight to Mercury of new automatic space stations equipped with measuring instruments that make it possible to determine the chemical composition of the planet's surface. Two such stations, Messenger and Bepi Colombo, are already being prepared for flight.

Schiaparelli's fallacy. Astronomers call Mercury a difficult object to observe, because in our sky it moves away from the Sun no more than 28° and it must always be observed low above the horizon, through the atmospheric haze against the background of dawn (in autumn) or in the evenings immediately after sunset (in spring ). In the 1880s, the Italian astronomer Giovanni Schiaparelli, based on his observations of Mercury, concluded that this planet makes one revolution around its axis in exactly the same time as one revolution around the Sun, that is, “days” on it are equal to “ year." Consequently, the same hemisphere always faces the Sun, the surface of which is constantly hot, but on the opposite side of the planet eternal darkness and cold reign. And since Schiaparelli’s authority as a scientist was great, and the conditions for observing Mercury were difficult, this position was not questioned for almost a hundred years. And only in 1965, using radar observations using the largest Arecibo radio telescope, American scientists G. Pettengill and R. Dice for the first time reliably determined that Mercury makes one revolution around its axis in approximately 59 Earth days. This was the largest discovery in planetary astronomy of our time, which literally shook the foundations of ideas about Mercury. And this was followed by another discovery - Professor of the University of Padua D. Colombo noticed that the time of Mercury's revolution around its axis corresponds to 2/3 of the time of its revolution around the Sun. This was interpreted as the presence of a resonance between the two rotations, which arose due to the gravitational effect of the Sun on Mercury. In 1974, the American automatic station Mariner 10, flying near the planet for the first time, confirmed that a day on Mercury lasts more than a year. Today, despite the development of space and radar research of planets, observations of Mercury using traditional methods of optical astronomy continue, albeit with the use of new instruments and computer data processing methods. Recently, at the Abastumani Astrophysical Observatory (Georgia), together with the Space Research Institute of the Russian Academy of Sciences, a study of the photometric characteristics of the surface of Mercury was carried out, which provided new information about the microstructure of the upper soil layer.

Around the sun. The planet Mercury closest to the Sun moves in a highly elongated orbit, sometimes approaching the Sun at a distance of 46 million km, sometimes moving away from it by 70 million km. The highly elongated orbit differs sharply from the almost circular orbits of the other terrestrial planets - Venus, Earth and Mars. Mercury's rotation axis is perpendicular to the plane of its orbit. One revolution in orbit around the Sun (Mercurian year) lasts 88, and one revolution around the axis lasts 58.65 Earth days. The planet rotates around its axis in the forward direction, that is, in the same direction as it moves in orbit. As a result of the addition of these two movements, the length of a solar day on Mercury is 176 Earth days. Among the nine planets of the Solar System, Mercury, whose diameter is 4,880 km, is in penultimate place in size, only Pluto is smaller. The gravity on Mercury is 0.4 that of the Earth, and the surface area (75 million km 2) is twice that of the Moon.

Coming Messengers

NASA plans to launch the second automatic station in history heading to Mercury, “Messenger”, in 2004. After launch, the station must fly close to Venus twice (in 2004 and 2006), the gravitational field of which will bend the trajectory so that the station exactly reaches Mercury. The research is planned to be carried out in two phases: first, familiarization from the flight trajectory during two encounters with the planet (in 2007 and 2008), and then (in 2009–2010) detailed from the orbit of the artificial satellite of Mercury, work on which will take place during one earthly year.

During a flyby of Mercury in 2007, the eastern half of the planet's unexplored hemisphere should be photographed, and a year later the western half. Thus, for the first time a global photographic map of this planet will be obtained, and this alone would be enough to consider this flight quite successful, but the Messenger program of work is much more extensive. During two planned flights, the planet’s gravitational field will “slow down” the station so that at the next, third meeting, it could move into the orbit of the artificial satellite of Mercury with a minimum distance from the planet of 200 km and a maximum of 15,200 km. The orbit will be located at an angle of 80° to the equator of the planet. The low area will be located over its northern hemisphere, which will allow for a detailed study of both the largest plain on the planet, Zhara, and the supposed “cold traps” in craters near the North Pole, which do not receive the light of the Sun and where the presence of ice is assumed.

During the operation of the station in orbit around the planet, it is planned in the first 6 months to carry out a detailed survey of its entire surface in various spectral ranges, including color images of the area, determination of the chemical and mineralogical composition of surface rocks, measurement of the content of volatile elements in the near-surface layer to search for places of ice concentration.

Over the next 6 months, very detailed studies of individual terrain objects will be carried out, the most important for understanding the history of the geological development of the planet. Such objects will be selected based on the results of the global survey carried out at the first stage. Also, a laser altimeter will measure the heights of surface features to obtain overview topographic maps. The magnetometer, located far from the station on a 3.6 m long pole (to avoid interference from instruments), will determine the characteristics of the planet’s magnetic field and possible magnetic anomalies on Mercury itself.

The joint project of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) BepiColombo is called upon to take over the baton from Messenger and begin studying Mercury using three stations in 2012. Here, exploration work is planned to be carried out using simultaneously two artificial satellites, as well as a landing apparatus. In the planned flight, the orbital planes of both satellites will pass through the poles of the planet, which will make it possible to cover the entire surface of Mercury with observations.

The main satellite, in the form of a low prism weighing 360 kg, will move in a slightly elongated orbit, sometimes approaching the planet up to 400 km, sometimes moving away from it by 1,500 km. This satellite will house a whole range of instruments: 2 television cameras for overview and detailed imaging of the surface, 4 spectrometers for studying chi-bands (infrared, ultraviolet, gamma, x-ray), as well as a neutron spectrometer designed to detect water and ice. In addition, the main satellite will be equipped with a laser altimeter, with the help of which a map of the surface heights of the entire planet should be compiled for the first time, as well as a telescope to search for potentially dangerous asteroids that enter the inner regions of the Solar System, crossing the Earth's orbit.

Overheating by the Sun, from which 11 times more heat comes to Mercury than to Earth, can lead to failure of electronics operating at room temperature; one half of the Messenger station will be covered with a semi-cylindrical heat-insulating screen made of special Nextel ceramic fabric.

An auxiliary satellite in the form of a flat cylinder weighing 165 kg, called magnetospheric, is planned to be placed in a highly elongated orbit with a minimum distance from Mercury of 400 km and a maximum of 12,000 km. Working in tandem with the main satellite, it will measure the parameters of remote areas of the planet’s magnetic field, while the main one will observe the magnetosphere near Mercury. Such joint measurements will make it possible to construct a three-dimensional picture of the magnetosphere and its changes over time when interacting with fluxes of charged solar wind particles that change in intensity. A television camera will also be installed on the auxiliary satellite to photograph the surface of Mercury. The magnetospheric satellite is being created in Japan, and the main one is being developed by scientists from European countries.

The Research Center named after G.N. is involved in the design of the landing apparatus. Babakin at the NPO named after S.A. Lavochkin, as well as companies from Germany and France. The launch of BepiColombo is planned for 2009-2010. In this regard, two options are being considered: either a single launch of all three spacecraft by an Ariane-5 rocket from the Kourou cosmodrome in French Guiana (South America), or two separate launches from the Baikonur cosmodrome in Kazakhstan by Russian Soyuz Fregat rockets (on one is the main satellite, the other is a landing vehicle and a magnetospheric satellite). It is assumed that the flight to Mercury will last 23 years, during which the device must fly relatively close to the Moon and Venus, the gravitational influence of which will “correct” its trajectory, giving the direction and speed necessary to reach the immediate vicinity of Mercury in 2012.

As already mentioned, satellite research is planned to be carried out within one earthly year. As for the landing unit, it will be able to work for a very short time; the strong heating it must undergo on the surface of the planet will inevitably lead to the failure of its radio-electronic devices. During the interplanetary flight, a small disk-shaped landing vehicle (diameter 90 cm, weight 44 kg) will be “on the back” of the magnetospheric satellite. After their separation near Mercury, the lander will be launched into an artificial satellite orbit with an altitude of 10 km above the surface of the planet.

Another maneuver will put it on a descent trajectory. When 120 m remain from the surface of Mercury, the speed of the landing block should decrease to zero. At this moment, it will begin a free fall onto the planet, during which plastic bags will be filled with compressed air; they will cover the device on all sides and soften its impact on the surface of Mercury, which it will touch at a speed of 30 m/s (108 km/h).

To reduce the negative impact of solar heat and radiation, it is planned to land on Mercury in the polar region on the night side, not far from the dividing line of the dark and illuminated parts of the planet, so that after about 7 Earth days the device will “see” the dawn and rising above the horizon Sun. In order for the on-board television camera to obtain images of the area, it is planned to equip the landing block with a kind of spotlight. Using two spectrometers, it will be determined what chemical elements and minerals are contained at the landing point. A small probe, nicknamed the “mole,” will penetrate deep into the soil to measure the mechanical and thermal characteristics of the soil. They will try to register possible “mercuryquakes” with a seismometer, which, by the way, are very probable.

It is also planned that a miniature planetary rover will descend from the lander to the surface to study the properties of the soil in the surrounding area. Despite the grandeur of the plans, detailed study of Mercury is just beginning. And the fact that earthlings intend to spend a lot of effort and money on this is by no means accidental. Mercury is the only celestial body whose internal structure is so similar to that of the earth, therefore it is of exceptional interest for comparative planetology. Perhaps research on this distant planet will shed light on the mysteries hidden in the biography of our Earth.

The BepiColombo mission over the surface of Mercury: in the foreground the main orbital satellite, in the background the magnetospheric module.

Lonely guest. Mariner 10 is the only spacecraft to explore Mercury. The information he received 30 years ago remains the best source of information about this planet. The Mariner 10 flight is considered extremely successful; instead of the planned one time, it explored the planet three times. All modern maps of Mercury and the vast majority of data on its physical characteristics are based on the information he obtained during the flight. Having reported all possible information about Mercury, Mariner 10 has exhausted its “life activity” resource, but still continues to silently move along its previous trajectory, meeting Mercury every 176 Earth days - exactly after two revolutions of the planet around the Sun and after three revolutions of it around its axis. Because of this synchronicity of movement, it always flies over the same area of ​​the planet, illuminated by the Sun, at exactly the same angle as during its very first flyby.

Sun dancing. The most impressive sight in the Mercury sky is the Sun. There it looks 23 times larger than in the earthly sky. The peculiarities of the combination of the speed of rotation of the planet around its axis and around the Sun, as well as the strong elongation of its orbit, lead to the fact that the apparent movement of the Sun across the black Mercury sky is not at all the same as on Earth. Moreover, the path of the Sun looks different at different longitudes of the planet. So, in the areas of meridians 0 and 180° W. e. early in the morning in the eastern part of the sky above the horizon, an imaginary observer could see a “small” (but 2 times larger than in the Earth’s sky), very quickly rising above the horizon Sun, whose speed gradually slows down as it approaches the zenith, and itself it becomes brighter and hotter, increasing in size by 1.5 times this is Mercury approaching its highly elongated orbit closer to the Sun. Having barely passed the zenith point, the Sun freezes, moves back a little for 23 Earth days, freezes again, and then begins to go down with an ever-increasing speed and noticeably decreasing in size this is Mercury moving away from the Sun, going into the elongated part of its orbit and disappears at high speed behind the horizon in the west.

The daily course of the Sun looks completely different near 90 and 270° W. d. Here the Sun performs absolutely amazing pirouettes - three sunrises and three sunsets occur per day. In the morning, a bright luminous disk of enormous size (3 times larger than in the earth’s sky) very slowly appears from behind the horizon in the east; it rises slightly above the horizon, stops, and then goes down and disappears briefly behind the horizon.

Soon a second rise follows, after which the Sun begins to slowly creep upward across the sky, gradually accelerating its pace and at the same time quickly decreasing in size and dimming. At the zenith point, this “small” Sun flies by at high speed, and then slows down, grows in size and slowly disappears behind the evening horizon. Soon after the first sunset, the Sun rises again to a small height, freezes in place for a short time, and then descends again to the horizon and sets completely.

Such “zigzags” of the solar course occur because in a short segment of the orbit, when passing perihelion (the minimum distance from the Sun), the angular velocity of Mercury’s motion in its orbit around the Sun becomes greater than the angular velocity of its rotation around its axis, which leads to the movement of the Sun in the firmament of the planet for a short period of time (about two earthly days) reversing its normal course. But the stars in the sky of Mercury move three times faster than the Sun. A star that appears simultaneously with the Sun above the morning horizon will set in the west before noon, that is, before the Sun reaches its zenith, and will have time to rise again in the east before the Sun has set.

The sky above Mercury is black both day and night, and all because there is practically no atmosphere there. Mercury is surrounded only by the so-called exosphere, a space so rarefied that its constituent neutral atoms never collide. In it, according to observations through a telescope from Earth, as well as during the flights of the Mariner 10 station around the planet, atoms of helium (they predominate), hydrogen, oxygen, neon, sodium and potassium were discovered. The atoms that make up the exosphere are “knocked out” from the surface of Mercury by photons and ions, particles arriving from the Sun, as well as micrometeorites. The absence of an atmosphere leads to the fact that there are no sounds on Mercury, since there is no elastic medium - air, transmitting sound waves.

Georgy Burba, Candidate of Geographical Sciences