As the planet closest to the Sun, Mercury receives central luminary significantly greater energy than, for example, the Earth (on average 10 times). Due to the elongation of the orbit, the energy flux from the Sun varies by approximately two times. The long duration of day and night leads to the fact that brightness temperatures (measured by infrared radiation in accordance with Planck’s law of thermal radiation) on the “day” and “night” sides of the surface of Mercury at an average distance from the Sun can vary from approximately 90 K to 700 K (-180 o C to +430 o C). At the same time, the temperature in the polar region reaches - 210 o C at night, and during the day under the scorching rays of the Sun in the equatorial zone + 500 o C. But already at a depth of several tens of centimeters there are no significant temperature fluctuations, which is a consequence of the very low thermal conductivity of rocks. Mercury's polar regions may have water ice. The sun never illuminates the interior areas of the craters located there, and the temperature there can remain around -210°C. Mercury's albedo is extremely low, about 0.11. In 1970, T. Murdock and E. Ney from the University of Minnesota found that the average temperature of the night hemisphere is -162 ° C (111 K). On the other hand, the temperature of the subsolar point at the average distance of Mercury from the Sun is +347°C.
In 1992, during radar observations from Earth near the planet's north and south poles, areas that highly reflect radio waves were first discovered. It was these data that were interpreted as evidence of the presence of ice in the near-surface layer of Mercury. 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, are also present on the Moon; measurements from satellites revealed the presence of a certain amount of water ice. Calculations have shown that the depressions of permanently shadowed craters near Mercury's poles 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.
Mercury's surface is reminiscent of the moon, covered with thousands of craters formed from collisions with meteorites and rocks that formed when the young core cooled and contracted, pulling together the planet's crust, as well as crushed basalt-type material, and is quite dark. During research carried out 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. There are mountains on Mercury, the highest ones reach 2-4 km. In a number of areas of the planet, valleys and craterless plains are visible on the surface. Judging by observations from Earth and photographs from spacecraft, it is generally similar to the surface of the Moon, although the contrast between dark and light areas is less pronounced. Along with craters (usually shallower than those on the Moon) there are hills and valleys. The largest crater on Mercury is named after the great German composer Beethoven, its diameter is 625 km.
Up to 70% of the studied area is occupied by an ancient, heavily cratered surface. The most significant feature is the Zhara Plain (Caloris Basin), a huge impact crater with a diameter of 1300 km (a quarter of the planet's diameter). The depression was filled with lava and relatively smoothed, with the same type of surface also covering part of the ejecta region. The impact occurred 3800 million years ago, causing a temporary revival of volcanic activity that had largely ceased 100 million years earlier. This led to a smoothing of the areas in and around the depression. In that area of Mercury's surface, which is diametrically opposite to the point of impact, a surprisingly chaotic structure is observed, apparently created by the shock wave.
Characteristic features found on Mercury are rugged cliffs (lobe-shaped ledges - scarps), which take the form of cliffs. They were called ledges because their outlines on the map are characterized by rounded protrusions - “blades” 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 pronounced downward bend in the slope, the Mercurian lobe-shaped ones have a smoothed line of inflection of the surface in their upper part. These ledges are located in the ancient continental regions of the planet. They are believed to have formed during compression of the planetary crust during the cooling process. In some places they cross the walls of craters. Calculations of the compression value indicate a reduction in the area of the crust by 100 thousand sq km, which corresponds to a decrease in the radius of the planet by 1-2 km. (cooling and solidification of the planet’s interior). Radar observations of Mercury at the end of 2001 showed the presence of a large crater with a diameter of 85 km on its surface. It is similar in structure to the Tycho crater on the lunar surface, but may be significantly younger than the 109-million-year-old lunar formation.
The first data from a study of the elemental composition of the surface using the X-ray fluorescence spectrometer of the Messenger apparatus showed that it is poor in aluminum and calcium compared to 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.
Mercury is the first planet solar system. In relation to the star, it is located closer than other planets terrestrial group and even more so gas giants. This cosmic body has many of its own individual characteristics, which mainly concern its orbit. It makes a complete revolution around the Sun in 87.969 Earth days, and around its own axis in 58.646 Earth days.
Now comes the fun part. Multiply 87.969 by 2 and then 58.646 by 3. You'll get the same value, only slightly different by thousandths. That is, Mercury makes 3 revolutions around its own axis in 2 revolutions around the Sun. This is a unique phenomenon in the solar system. It's called orbital resonance, and its ratio in this case corresponds to 3:2.
Orbital resonance directly depends on the tidal forces of the Sun. And it so happened that in one Mercury year the first planet rotates one and a half revolutions. That is, if at perihelion (the closest point of the orbit relative to the Sun) some area on the surface of Mercury faces the Sun, then at the next perihelion passage this area will be on the opposite side of the planet. And in another year the Sun will again be above this area. As a result, a solar day on the planet corresponds to 176 Earth days.
But we got distracted and completely forgot that our main question is distance from Earth to Mercury. Here we must immediately say that the first planet is significantly smaller in size than our native blue planet. Its mass is 18 times less than the earth’s, and its volume is correspondingly 17.8 times less. That is, we can affectionately call Mercury baby. And this baby is rushing around the Sun in an elongated orbit, the eccentricity of which is 0.205636.
At aphelion, its distance to the Sun is 69.817 million km, and at perihelion it is 46 million km. The spread is more than 20 million km, as indicated by the rather high eccentricity. In relation to the Earth, not everything is smooth either, and the reason again lies in the high orbital eccentricity of the planet.
Mercury approaches its minimum distance to Earth every 116 Earth days. But this is on average. This interval ranges from 105 to 129 Earth days due to eccentricity. Currently the minimum distance from Earth to Mercury is 82.2 million km. But it should be noted that it is slowly decreasing. In 2679 its value will be 82.1 million km, and in 4487 it will be equal to 82 million km.
Further calculations show that after 27 thousand years the distance will become 80 million km. And after this, the baby’s distance from the blue planet should begin. These are the surprises the orbit of the first planet in the solar system will bring us in the future. As for maximum distance from Earth to Mercury, then it is equal to 217 million km.
The first planet of the solar system is subject to such an astronomical effect as passing across the disk of the Sun when observed from Earth. The first such telescopic observation was made in 1631 by the French astronomer Pierre Gassendi.
This event is rare, but even rarer is Mercury passing across the disk of the Sun at the same time as Venus. One such occultation of Mercury by Venus was observed on May 28, 1737 by the English astronomer John Bevis. This is the only case in the entire history of world astronomy. The next one will occur only on December 3, 2133.
Vladislav Ivanov
planet Mercury
General information about the planet Mercury. Mysterious planet
Fig.1 Mercury. The image is compiled from MESSENGER photographs dated January 30, 2008. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury is the planet closest to the Sun and the smallest in the Solar System, both in terms of mass and diameter. In addition, Mercury has the smallest albedo. However, in terms of average density, Mercury is ahead of almost all planets, with the exception of Earth. In addition, this is one of the most mysterious planets of the solar planet, despite the fact that Mercury lies only 90 million km from Earth. It seems that the figure is quite large, but if you remember that Mars lies at the same distance from our planet - studied no worse than the Earth, then it becomes clear that there are only 2 (!) flights of spacecraft to the “nearest neighbor of the Sun” (of the known ones) - the figure is undoubtedly small and therefore it is natural that the process of studying Mercury is a very exciting activity that can captivate no less than studying any ancient manuscripts.
These are just some questions regarding the planet Mercury that still do not have an exact answer.
The first unresolved question. As mentioned above, in terms of average density, Mercury is only slightly inferior to Earth. However, in all other respects it is very similar to the Earth's natural satellite - the Moon. So high density Mercury may be caused by the loss of light rocks due to some catastrophe at an early stage of formation. But did such a catastrophe really take place or is it just an assumption - unknown?
Question number two. There are no traces of iron found on the surface of Mercury, which is the main element in its core. What caused this is still unclear.
Another question is related to the previous one: the presence of a liquid core on Mercury. It would seem that what’s surprising about this, because the Earth’s outer core is also liquid. But the thing is that the mass of Mercury is very small (0.055 the mass of the Earth), therefore, even despite the very high temperature of its surface, reaching 400°C, its interior had to cool and harden very quickly. And in favor of the fact that Mercury still has a liquid (albeit not completely) core speaks of the presence of a weak magnetic field, as well as the results of research by astronomers in the USA and Russia. But how this liquid core of the planet Mercury was preserved is a big question.
As can be seen from this far from complete list, the planet Mercury is full of mysteries, and any person who is interested in this can try to solve them. And to make this difficult task easier, I suggest you familiarize yourself with the information that is already known about the planet Mercury. And it’s natural to start by considering its position in the sky.
Observing the planet Mercury from Earth
Mercury is a difficult object to observe from Earth. This is due to the fact that it never visibly moves away from the Sun by more than 28.3°, i.e. has a very small angular distance - elongation. Other planets that can be observed from Earth with the naked eye are not only larger than the planet Mercury, but also lie higher above the horizon, and are visible almost every day. Mercury always has to be observed against the background of the evening or morning dawn low above the horizon, and for a very short period of time: no later than 2 hours before dawn and no later than 2 hours after sunset. However, much more often the observation time is much shorter and is only 20-30 minutes.
Fig.2 Change of phases of Mercury. Credit: website
Observing Mercury, you can notice that relative to the Sun it moves first to the right of it, then to the left, taking the form of either a narrow crescent or a small bright round spot. These visible changes associated with Mercury's reflection of sunlight are called phases and are similar to those of the Moon, with the only difference being that the size of the crescent changes noticeably over time due to changes in the distance between the Earth and Mercury.
The planet Mercury is best visible at the moments of superior conjunctions (point 5 in the figure), when it is hidden in the rays of the Sun and has a minimum diameter. At this moment, Mercury takes on the appearance of a small bright spot without any details on its surface.
Continuing its path in orbit, Mercury begins to approach the Earth and therefore the size of its disk increases. The area sanctified by the Sun begins to shrink. After some time, Mercury is no longer a round spot. And after another 36 days, only half of Mercury remains visible. The phase of the planet (i.e., the angle at the planet between the directions to the Sun and the Earth) at this moment is close to 90°.
Soon, namely after 22 days, the area sanctified by the Sun decreases even more and Mercury becomes like a thin sickle.
Fig.3 Transit of Mercury across the disk of the Sun. Image from the SOHO spacecraft and the TRACE telescope from May 7, 2003. Credit: NASA Goddard Space Flight Center
Moving further, the planet Mercury finds itself on the same side of the sun, as the Earth (the so-called inferior conjunction), and becomes invisible to the observer. This is due to the fact that Mercury at this moment is turned towards the Earth with its unsanctified, dark side, although the size of its disk at this moment is maximum. However, once every 3-13 years it happens that Mercury passes directly between the Sun and the Earth and becomes visible as a dim spot on the disk of the Sun.
Then the phases begin to change in the reverse order: first a thin crescent appears, which begins to grow, and now half of the planet becomes visible; Another short period of time passes and Mercury is completely sanctified.
Between the appearances of the planet in the west and in the east of the Sun, from 106 to 130 days pass (on average - 116); the large difference is explained by the significant elongation of Mercury's orbit. By the way, when Mercury is clockwise ahead of the Sun (points 3-7) it is visible in the morning; when behind the Sun (points 1, 2, 8) - it is visible in the evening.
The magnitude of Mercury during observations from Earth is small and ranges from -2 to 5.5. At the same time, it is the fourth brightest planet in the sky; at its maximum brightness, when Mercury reaches -1 magnitude, it shines almost like star Sirius, and among the planets it is second only to Venus, Mars and Jupiter.
You can see the planet Mercury with the naked eye, not to mention observations through binoculars or a telescope. But observations should be made only at a certain time of the day: this, as mentioned above, is twilight. With the help of a telescope, Mercury can be seen in the daytime, and it is practically impossible to recognize any details on it. However, observation should be carried out very carefully, because Mercury never moves far from the Sun, and if the telescope is handled ineptly, this can lead to bad consequences caused by the powerful radiation of the star closest to us.
More or less productive study of Mercury is possible only in mountain observatories or at low latitudes. This is due both to the shorter duration of twilight and to the presence of conditions suitable for observations: more clean air than on the plains, cloudless skies, etc..
It should be noted that it was precisely on the basis of observations from Earth that it was established that: Mercury is devoid of an atmosphere (found out on the basis of the low reflectivity of Mercury, determined by the low albedo value (0.07)), the surface of its side facing the Sun is subjected to strong heating, while as the opposite shadow side cools down greatly. And with the help of the most modern telescopes, images of the planet were obtained with a resolution sufficient to examine the largest details of the Mercury surface. However, about physical properties, until recently very little was known about the nature of its rotation around its axis.
Now a lot has changed and people know almost everything about the planet Mercury. Read below about how such an amazing result was achieved...
History of exploration of the planet Mercury
The first people to observe the planet Mercury were the Sumerians from the Tigris-Euphrates region, who recorded their observations in cuneiform texts, and pastoral tribes from the Lower Nile Valley. It was 5 thousand years ago.
However, due to the complexity of observations, people for a long time they thought that Mercury observed in the morning was one planet, and in the evening it was completely different.
Therefore, Mercury had two names. Thus, the Egyptians called him Set and Horus, the Indians - Buddha and Roginea, and the ancient Greeks - Apollo and Stilbon (starting from 200 BC - Hermes). In Chinese, Japanese, Vietnamese and Korean, Mercury is called the Water Star, in Hebrew - “Kohav Hama” - “ solar planet", and the inhabitants of Ancient Babylon came up with the name Nabu for Mercury, in honor of their god.
Familiar for modern man The name of the planet was given by the Romans. It was they who named Mercury Mercury, in honor of the god of travelers and traders, who among the Greeks was named Hermes. And the stylized image of the divine staff - the caduceus - served as the prototype of the astronomical sign of this planet.
By this time, people already knew that morning Mercury and evening Mercury were the same planet and were actively studying it. True, this study was reduced mainly to observations of the planet against the background of the morning or evening dawn.
The first astronomer to observe Mercury through a telescope was the great Italian astronomer Galileo Galilei. A few years later - in 1639, the Italian Giovanni Battista Zupi, when observing the first planet from the Sun, noticed that the sanctity of Mercury changes over time, i.e. There is a change of Mercury phases. This observation proved that the planet Mercury is a satellite of the Sun.
Another great astronomer of the Middle Ages, Johannes Kepler, who discovered the three laws of motion of the planets of the solar system, predicted the passage of Mercury across the disk of the Sun, which was observed by the Frenchman Pierre Gassendi on November 7, 1631.
After this event, so significant in the astronomical chronicle, there was a lull in astronomical observations for almost 250 years...
And only in late XIX centuries, astronomers again began to observe Mercury, while trying to create maps of its surface. The first such attempts were made by the Italian J. Schiaparelli and the American P. Lovell. And in 1934, the French astronomer Eugene Michel Antoniadi, when compiling his map of Mercury, proposed a system for naming dark and light surface features associated with the god Hermes. According to this system, the dark areas were called deserts (solitudo), while the light areas had their own names.
However, it should be noted that all of the maps listed above had one significant drawback: they were compiled only for one hemisphere. The reason for this was the assumption of the Italian astronomer Giovanni Schiaparelli, who, based on his astronomical observations, concluded that Mercury is constantly turned to the Sun with one side, like the Moon to the Earth..
Only in 1965, radar methods measured the exact period of the planet’s rotation around its axis, which turned out to be equal to 58.6 days. It also turned out that Mercury rotates asynchronously, making one revolution around its axis faster than one revolution around the Sun, and previously compiled maps and astronomy textbooks had to be rewritten.
It was then that the automatic interplanetary station (AMS) Mariner 10 was launched to Mercury, which, approaching the surface of the planet on March 29, 1974 at a distance of 704 km, made it possible to take a series of detailed photographs, revealing the similarity of the Mercury surface with the lunar one.
The same numerous meteorite craters (as a rule, less deep than on the Moon), hills and valleys, mountains, smooth rounded plains, which, due to their similarity to the lunar “seas,” were called basins. The largest of them, Caloris, has a diameter of 1350 km.
The difference between the surface of Mercury and the Moon was the presence of such specific relief forms as scarps - protrusions 2-3 km high that separate two areas of the surface. The scarps are believed to have formed as shear faults during the early compression of the planet.
But most important difference Mercury from the Moon turned out to have water, or rather water ice. Such ice is found at the bottom of craters in the polar regions of the planet. The walls of the crater protect the ice from the rays of the Sun and it never melts...
In addition to filming the surface of the AMS, a plasma shock wave and a magnetic field were detected near Mercury. It was possible to clarify the value of the planet’s radius and its mass.
A few months later, on September 21, 1974, the Mariner 10 spacecraft again flew up to Mercury. At a fairly large distance - more than 48 thousand kilometers, using temperature sensors it was found that during a day, the duration of which is 88 Earth days, the brightness temperatures of the planet's surface (measured by infrared radiation in accordance with Planck's law of thermal radiation) rise to 600K , and at night they drop to 100K (-210°C). Using a radiometer, the heat flux emitted by the surface was determined; Against the background of heated areas consisting of loose rocks, colder ones were identified, which are silicate rocks close to terrestrial basalts. This circumstance once again confirmed the similarity of the Mercury and lunar surfaces.
During its third and final flyby of Mercury, which took place on March 16, 1975 at a distance of 327 km from the surface of the planet, Mariner 10 confirmed that the magnetic field discovered a little earlier indeed belongs to the planet. Its strength is about 1/100 of the strength of the earth's magnetic field.
In addition to measuring physical fields, the station took 3 thousand photographs with a resolution of up to 50 m, which, together with photographs taken during two previous flights, covering 45% of the surface of Mercury, made it possible to compile a detailed map of its surface, although only in the western hemisphere. the eastern hemisphere remained unexplored.
Objects on the compiled map: craters, plains, ledges, received their own names. Craters - in honor of humanitarian figures: writers, poets, artists, sculptors, composers, many of whom are Russian; plains - in honor of the gods who played a role similar to the god Mercury in various mythologies, and some - after the names of the planet on different languages; the ledges are given the names of research vessels; valleys - radio observatories. Of course, there are exceptions: this is how the Northern Plain got its name from its location, and the Heat Plain - because of the high temperatures within its territory. The mountains bordering this plain bear the same name. Two more Mercury ridges are named after the astronomers Antoniadi and Schiaparelli, who compiled the first maps of this planet.
A small crater with a diameter of 1.5 km, located near the equator, was taken as a reference object for measuring longitudes in the coordinate system on the surface of Mercury. This crater is named Hun Kal, which in the language of the ancient Mayans means “twenty” (they based their counting system on this number). The 20° meridian passes through the Hun Kal crater. Longitudes on Mercury are measured from 0° to 360° west of the prime meridian.
On March 24, 1975, Mariner 10 ran out of fuel and could no longer be controlled from Earth. His mission has come to an end. But, astronomers suspect, Mariner 10 is still orbiting the Sun, sometimes passing near the planet Mercury.
Fig.5 MESSENGER. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
After the completion of the Mariner 10 mission, there were no flights to Mercury for almost thirty years. Only on August 3, 2004, from Cape Canaveral in Florida, the United States launched the Messenger spacecraft, which finally flew up to the surface of the planet on January 14, 2008. By the way, it was very difficult to do this. And here’s why: in order to move from a near-Earth orbit to a near-Mercurian orbit, it is necessary to extinguish a significant part of the Earth’s orbital speed, which is ~30 km/s, and for this it is necessary to perform a series of gravitational maneuvers. During its mission, Messenger will perform 6 such maneuvers, 5 of which have already been completed: on August 2, 2005, the device passed at an altitude of 2347 km from the Earth’s surface, on October 24, 2006, the first flight near Venus took place at a minimum altitude of 2992 km, on June 5 2007 Messenger made a second flyby of Venus, this time much lower: along the top of the clouds. 8 months later - on January 14, 2008, Messenger finally flew up to Mercury. This event was eagerly awaited not only by NASA specialists, but also by all progressive humanity. And for good reason!
Messenger took detailed images of Mercury's surface, including the planet's far side (which we previously knew nothing about).
The images transmitted to Earth made it possible to establish that quite intense tectonic activity took place on the planet Mercury, traces of which, in the form of huge flat plains, are especially noticeable in the eastern hemisphere. Also during the first approach, the magnetosphere and atmosphere of Mercury were studied in more detail.
A few months later, on October 6 of the same year, Messenger again flew up to Mercury. A series of detailed photographs of the planet were taken, which revealed strange points of dark matter abundantly scattered across the surface. Astronomers believe this is the result of meteorite impacts.
In addition, as a result of the second flyby, a heterogeneous structure of the surface of Mercury was discovered, the nature of which is not completely clear, and a measurement of the Mercury landscape, which showed that the height of the measured landscape remains surprisingly constant: 30% more even than the landscape of the opposite region. No less amazing discoveries awaited astronomers under the surface of Mercury: a sharp drop in height of as much as 600 m was discovered in Mercury’s crust, which may be a “scar” left on the planet as a result of its compression during a period of rapid cooling.
On September 29, 2009, Messenger performed its last gravity assist maneuver before entering a highly elliptical polar orbit around the planet on March 18, 2011, becoming its first artificial satellite. According to the plan, after this the probe will have to work for at least two Mercury days, which is slightly less than an Earth year...
Fig.6 Global map of Mercury, compiled on the basis of images taken by Mariner 10 and Messenger. Credit: NASA
During the last flyby of the planet Mercury to date, Messenger took a number of images of hitherto unexplored areas (6% of the entire surface of the planet), conducted a study of the Mercury atmosphere and discovered traces of recent volcanic eruptions. Thus, to date, more than 98% of the surface of Mercury has been explored and photographed. The remaining 2% of the surface is polar regions, which scientists hope to explore in 2011.
Fig.7 BepiColombo. Credit: ESA
Currently, the European Space Agency (ESA), together with the Japanese Aerospace Exploration Agency (JAXA), is developing the BepiColombo mission (in honor of the scientist Giuseppe Colombo, who developed the theory of gravitational maneuver), consisting of two spacecraft Mercury Planetary Orbiter (MPO) and 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. In addition to directly studying the planet, both spacecraft hope to use the study area's proximity to the Sun to test general theory relativity.
The launch of BepiColombo is planned for 2013, and in 2019, after performing a series of gravity assist maneuvers, it will reach the orbit of Mercury, where it will split into two components. The BepiColombo mission to Mercury is expected to last approximately one Earth year.
It should be noted that the study of the planet Mercury is also carried out from Earth, using CCD radiation receivers and subsequent computer processing of images. This became possible thanks to the development of electronics and computer science.
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 information.
Observations of Mercury were also carried out at the Abastumani Astrophysical Observatory on November 3, 2001, as well as at the Skinakas Observatory of the University of Heraklion on May 1-2, 2002. After processing the observation results using the correlation combination method, a resolved image of the planet was obtained, similar to the Mariner-10 photomosaic. This is how a map of Mercury was compiled for longitudes 210-350°.
This is where the story of Mercury exploration ends for now. But not for long. After all, already in 2011 Messenger will fly to the planet, which will probably make many more interesting discoveries. Then BepiColombo will study Mercury...
Orbital motion and rotation of the planet Mercury
Fig.8 Distance from the terrestrial planets to the Sun. Credit: Lunar and Planetary Institute
Mercury is the planet closest to the Sun. It moves around the star in a highly elongated orbit, at an average distance of 0.387 AU. (59.1 million km) At perihelion this distance decreases to 46 million km, at aphelion it increases to 69.8 million km. Thus the orbital eccentricity (e) is 0.206.
The inclination of the Mercury orbit (i) to the ecliptic plane is 7°.
In orbit, the planet Mercury not only moves, but literally flies: at a speed of about 48 km/sec, being by this indicator the fastest planet in the solar system. The entire orbital journey takes Mercury 88 days - this is the length of the Mercury year.
Unlike the crazy movement in orbit around its axis, almost perpendicularly inclined to the plane of the planetary orbit, Mercury rotates slowly, making a full revolution in 59 (58.65) Earth days, which is 2/3 of the planet’s orbital period. For several centuries, this coincidence misled astronomers, who believed that the period of Mercury's rotation around its axis and the period of its orbit around the Sun coincided. The reason for the misconception was 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 period of Mercury’s rotation around its axis (352 days), so astronomers observed approximately the same area of the surface planets. On the other hand, some of them believed that the Mercury day was approximately equal to the Earth's. Only in 1965 was the inconsistency of both hypotheses established, and the true time of rotation of the planet closest to the Sun was determined.
Fig.9 Arecibo Observatory. Credit: courtesy of the NAIC - Arecibo Observatory, a facility of the NSF
That year, the three-hundred-meter radio telescope at the Arecibo Observatory (Puerto Rico) sent a powerful radio pulse towards the planet Mercury. The radio pulse was reflected in a small “beam” from the central region of the planet and rushed in all directions, including to the antenna of the radar that sent it. Following the first radio pulse, a second one was sent to Mercury, which was reflected in a narrow ring around the place where the first radio pulse was reflected. And in turn there was already a third, then a fourth ring, and so on until the last one, limiting the disk of the planet (in fact, the entire process of sending a radio signal was continuous). The side of the planet farthest from the radar was in the radio shadow, and therefore nothing was reflected from it.
Because the planet rotates, the pulses reflected by each ring are not entirely uniform. The frequency at which the signal was received does not match the frequency of the sent pulse. Since in their movement around the Sun the Earth and Mercury either move away from each other or come closer, the Doppler effect occurs and the frequency shifts.
For Mercury, the largest offset of the radar signal, which operates at a wavelength of 10 cm, is 500 kHz. Also Mercury. like any other planet, it rotates, and therefore its western (left) side moves towards the impulse, causing an additional positive Doppler shift, while the eastern (right) side moves away from it and gives a negative Doppler shift. These shifts, called residual differences, at the equator near Mercury are 32 Hz.
Knowing the shifts and linear distance between the opposite edges of the planet, astronomers R. Dice and G. Pettengil, working at the Arecibo Observatory, measured the speed of Mercury’s rotation around its axis, determining it as 59 ± 5 days.
A little later, in 1971, the American scientist R. Goldstein clarified the rotation speed of Mercury. It turned out to be 58.65±0.25 days. After 3 years, the first spacecraft Mariner 10 flew to Mercury, which only corrected Goldstein’s data to 58.646 days.
Having found out the time of Mercury's rotation around its axis and the time of its rotation in orbit and comparing them, scientists were able to calculate the length of the solar day. They turned out to be equal to 176 Earth days or 2 Mercury years. During this time, the Mercury day lasts 88 earthly days and the Mercury night lasts exactly the same amount.
The synchronization of Mercury's orbit and the period of its rotation around its axis is the result of the tidal influence of the Sun. 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 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 on which, because of this, an extremely high temperature is observed, even by Mercury standards - 440-500 ° C.
By the way, the Sun in the Mercury sky behaves very unusually for an earthly observer. It rises in the east, rises extremely slowly (on average one degree per twelve hours), gradually increasing in size, then reaches its highest culmination (zenith at the equator), stops, changes direction, stops again, and slowly sets. With all this doomsday, the stars would move across the sky three times faster.
Sometimes the Sun behaves even more strangely in the sky of Mercury: it rises, reaches its highest culmination, stops, and then begins to move in the opposite direction, setting at the same point where it rose. After several earthly days, the Sun rises again at the same point, for a long time. This behavior of the Sun is typical for longitudes 0° and 180°. At longitudes 90° away from the “hot longitudes,” the Sun rises and sets twice. On the meridians 90° and 270° you can see three sunsets and three sunrises in one solar day, which last 176 Earth days.
The effect of the Sun's behavior in the sky of Mercury is sometimes called the Joshua effect, named after biblical hero, able to stop the movement of the Sun.
The surprising behavior of the Sun in the Mercury sky is caused by the fact that the speed of Mercury's orbital motion is constantly changing, in contrast to the speed of rotation around its axis, which is constant. So, in the section of the orbit near perihelion, for about 8 days, the speed of orbital motion exceeds the speed rotational movement.
By the way, strange as it may sound, Mercury is the closest planet to Earth most time.
Internal structure planet Mercury
Mercury is one of the densest planets in the solar system. Its average density - 5.515 g/cm 3 is only slightly inferior to the average density of the Earth, 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, it turns out that with equal sizes of planets, the density of Mercurian matter would exceed the earth's by 30%.
According to modern theory During the formation of planets, it is believed that in the protoplanetary dust cloud, the temperature of the region adjacent to the Sun was higher than in its outlying parts, which is why light chemical elements were carried to distant, cold parts of the cloud. As a result, in the circumsolar region where the planet Mercury is located, there is a noticeable predominance of heavy elements, the most common of which is iron.
Some scientists believe that Mercury's high density is caused by very strong solar radiation. Radiation causes the chemical reduction of oxides to their heavier, metallic form. Perhaps the Sun contributed to the evaporation and, as a result, the evaporation of the outer layer of the original Mercury crust of the planet into space, heating it to critical temperatures.
Fig. 10 Internal structure of Mercury. Credit: NASA
Affects the average density of the planet Mercury and its massive planetary core. Representing a huge ball, comparable in size to the Moon (radius 1800 km), it concentrates up to 80% of the mass of the entire planet. The average density of Mercury's core according to calculations by S.V. Kozlovskaya - 9.8 g/cm3. It is a partially molten iron-nickel substance with an admixture of sulfur, and consists of an outer liquid and an inner solid core. This assumption was put forward after the flight of the Mariner 10 probe and further radar observations of Mercury by the group of Jean-Luc Margot in 2007. Mariner discovered a weak magnetic field on the planet, and Margot's group studied variations in its rotation around its axis.
The presence of even a partially molten core on Mercury has plunged scientists into deep thought.
The fact is that, although its surface has a very high surface temperature, reaching 400 ° C, its mass is very small, and therefore the planet must have cooled and hardened very quickly. Therefore, astronomers had no doubt that such a small planet as Mercury should have a solid core. The discovery of Mariner 10 led astronomers to talk about the possibility of Mercury having at least a partially molten core, like Earth.
Thirty years after the Mariner flight, Jean-Luc Margot's group, which brought together astronomers from Cornell University (Ithaca, New York, USA) and other institutions in the United States and Russia, based on five years of radar studies of Mercury carried out using 3 ground-based radio telescopes , proved that variations associated with the rotation of Mercury are indeed characteristic of a celestial body with a molten core.
By combining all this data, physicists were able to detect periodic disruptions in Mercury's rotation caused by tidal interactions with the Sun.
The influence of the Sun, by the way, affects the rotation of the planets differently depending on their composition. This is similar to the well-known method for identifying hard-boiled eggs: a fully hardened egg rotates quickly and for a long time, while a soft-boiled egg rotates slowly and oscillates.
The results of Margot's group's measurements were published in one of the latest issues of the journal Science. The new work also adds weight to the theory that Mercury, like Earth, generates its own magnetic field through a hydromagnetic dynamo mechanism - that is, through convection of a liquid, electrically conductive metal core.
Above the core of Mercury lies a silicate shell - the mantle, 600 km thick, which is 3 times less dense than the core - 3.3 g / cm 3. At the boundary between the mantle and the core, the temperature reaches 10 3 K.
The third shell of solid Mercury is its crust, the thickness of which is 100-300 km.
Based on an analysis of photographs of Mercury, American geologists P. Schultz and D. Gault proposed a scheme for the evolution of its surface.
According to this scheme, after the process of accumulation and formation of the planet was completed, its surface was smooth.
Fig. 11 Caloris Basin on Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington. Image reproduced courtesy of Science/AAAS
Next came the process of intensive bombardment of the planet by the remnants of the preplanetary swarm, during which Caloris-type pools were formed, as well as Copernicus-type craters on the Moon. At the same time, the enrichment of the Mercury core with iron apparently occurred as a result of a collision with a large cosmic body- planetesimal. As a result, Mercury lost up to 60% of its original mass, part of the mantle and planetary crust.
The next period was characterized by intense volcanism and the release of lava flows that filled large basins. These processes occurred as a result of the cooling of Mercury over time. The volume of the planet decreased, and its outer rocky shell - the crust, which cooled and hardened earlier than the interior, was forced to shrink. This led to cracking of the rock shell of Mercury, pushing one edge of the cracks over the other with the formation of a kind of thrusts, in which one layer of rocks is pushed over another. The upper layer, overlaid on the lower one, has a convex profile, resembling a frozen stone wave.
During this period, the so-called “spider” appeared, which is a system of more than a hundred wide grabens radiating out from a small crater in the center of the Caloris Basin. According to the hypothesis, huge masses of magma rose from the depths of Mercury to the surface of the planet, bending upward the Mercury crust.
In some places, the crust burst, and molten deep rocks poured into the resulting cracks, forming the observed grooves. But astronomers do not know how the central crater itself was formed. Apparently, it could have accidentally hit the center of Caloris, or it could have caused its formation by striking hard enough for the crust to spring back over such a huge area. So far, it is only clear that the Caloris basin was filled with lava approximately 3.8-3.9 billion years ago.
Approximately 3 billion years ago, the described period ended. It was replaced by a period of relative calm, when volcanic activity weakened or stopped completely (this issue is not completely clear, perhaps the Messenger AMS will be resolved), and meteorite bombardments became less frequent. This period continues to this day...
Surface of the planet Mercury
In terms of size, Mercury is the smallest planet in the solar system. Its radius is 2440 km, which is 0.38 of the Earth's radius. Surface area - 74.8 million km 2.
Fig. 12 Comparison of the planets of the solar system. Credit: website
When Mariner 10 flew past Mercury in 1974 and transmitted the images it took to Earth, astronomers were amazed: it looked so much like the Moon. The same flat plains, incl. unique - straight, numerous steep cliffs and a lifeless desert densely strewn with craters. Even the minerals scattered across the surface of the planet Mercury in the form of tiny particles are similar to those of the moon and are called silicates. But the main similarity between the Mercury and lunar surfaces lies in the presence of two main types of terrain: continents and seas.
Continents are the most ancient geological formations on the planet, covered with craters, plains, hills, mountains and canyons crossing them. Unlike the continents, the Mercurian seas are younger formations, representing vast smooth plains formed as a result of the outpouring of lavas onto the Mercurian surface and the deposition of material ejected during the formation of craters. They appear darker than the Mercury continents, but lighter than the lunar seas.
Most of the seas are within the so-called. Zhara plains (lat. “Caloris Planitia” or Caloris basin) - a giant ring structure with a diameter of 1300 km, surrounded by a mountainous range. The Zhara Plain received its name because of its location: the 180° meridian passes through it, which, together with its opposite prime meridian included in the so-called “hot longitudes” - those facing the Sun during Mercury’s minimum approach to it.
It is believed that the Heat Plain was formed as a result of the collision of Mercury with a large celestial body with 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 of the surface, led to the formation here of a kind of rugged “chaotic” landscape, a system of numerous large hills with a diameter of about a hundred kilometers, intersected by several large rectilinear valleys, clearly formed along the lines fractures in the planet's crust.
Unlike all other areas of Mercury, there are almost no small craters, so common on objects in the Solar System, almost or completely devoid of atmosphere. The presence of impact craters on all these objects was predicted in 1947 by Soviet astronomers Vsevolod Fedynsky and Kirill Stanyukovich.
Around some of the Mercury craters, radial-concentric faults were discovered - rays dividing the Mercurian crust into separate blocks, which indicates the geological youth of the craters, and shafts of surface rocks ejected during the impact. The largest craters, with a diameter of more than 200 km, have not one, but two such shafts, and unlike the lunar ones, they are one and a half times narrower and lower due to the greater gravity of Mercury. It should be noted that the brightness of the rays emanating from the craters regularly increases towards the full moon, and then weakens again. This phenomenon is due to the fact that the bottom of small craters reflects light mainly in the same direction from which the sun's rays come.
Fig. 13 "Spider" within the Caloris basin. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
One of the most interesting Mercury surface features is the so-called Messenger spacecraft discovered. "Spider". The Spider is located in the center of another crater - the largest Caloris basin and is a system of hundreds of grabens radiating from a small crater in the center.
Speaking of grabens. This is a purely Mercurian relief detail, representing long narrow depressions with a flat bottom. Grabens are located in the ancient continental regions of the planet and were formed during compression and cracking of the crust of Mercury during its cooling, as a result of which the surface of the planet decreased by 1% or 100 thousand km 2.
Except grabens characteristic feature The surface of Mercury are scarps - lobe-shaped ledges, up to several tens of kilometers in diameter. The height of the scarps is up to 3 km, and the length of the largest of them can reach 500 km.
The most famous scarps are the Santa Maria Escarpment, named after the ship of Christopher Columbus, the 450 km long Antoniadi Escarpment, named after the French astronomer, and the 350 km long Discovery Escarpment, named after the ship of James Cook. It should be noted that all the ledges on Mercury are named after sea ships on which the most significant voyages in the history of mankind were made, and two are named after the astronomers Schiaparelli and Antoniadi, who made many visual observations.
Fig. 14 Craters on the surface of Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury craters, often large: more than 100 km. in diameter, and selectively smaller ones, are given the names of figures of world culture - famous writers, poets, artists, sculptors, composers. To designate the plains (except for the Zhara Plain and the Northern Plain), the names of the planet Mercury were used in different languages. Extended tectonic valleys were named after radio observatories that contributed to the study of planets. The names of the relief features on Mercury were given by the International Astronomical Union, an organization uniting astronomical communities around the world.
As mentioned above, the surface of Mercury is heavily cratered. There are few large craters and many of them have smaller, and therefore younger, craters on their surface. The bottom of large craters is filled with lava flows that poured onto the surface, which subsequently solidified, forming a smooth surface similar to the Mercurian seas. At the bottom of most small craters, central hills are visible, well known to astronomers from lunar landscapes.
Among the most notable Mercury craters are Beethoven - the largest on Mercury with a diameter of 625 km, Tolstoy - with a diameter of 400 km, Dostoevsky - its diameter is 390 km, Raphael, Shakespeare, Goethe, Homer and others...
By the way, comparing from photographs the environs of the North Pole of Mercury with the environs of the South Pole, astronomers noticed significant differences between them, namely the predominance of a smooth, flat surface around the North Pole, versus a heavily cratered one around the South Pole.
The atmosphere of the planet Mercury. Physical conditions on Mercury
The atmosphere of Mercury was discovered by the Mariner 10 spacecraft, thereby raising a lot of questions among astronomers, and primarily due to its existence. Mercury, being close to the Sun and having a small mass, could not have it in principle. After all, what is needed for the existence of an atmosphere?
Firstly, greater gravity: the more massive the planet and the smaller its radius, the more reliably it holds even very light gases, such as hydrogen, helium, etc. On the planet Mercury, gravity is about three times less than on Earth's surface, i.e. it is not able to hold even gases heavier than hydrogen.
The second condition for a planet to have an atmosphere is the temperature, both of the surface and of the atmosphere itself. The energy of chaotic thermal motion of gas atoms and molecules depends on temperature. The higher it is, the higher particle speed, therefore, having reached the limiting value, namely the second cosmic velocity, gas particles leave the planets forever, and light gases are the first to escape into outer space.
On Mercury, the temperature of the surface layers can reach 420°-450°C, which is one of the record values among the planets of the Solar System. At such extreme temperatures, helium is the first to “escape.” However, contrary to all the arguments listed above, helium was found in the atmosphere of Mercury. What is the reason for the presence of this gas, which in theory should have evaporated from the atmosphere of the planet closest to the Sun billions of years ago. And this is connected precisely with the position of Mercury in a certain place in outer space.
Lying in close proximity to the Sun, Mercury is constantly fed with helium, which is supplied to it by the solar wind - a stream of electrons, protons and helium nuclei flowing from the solar corona. Without this replenishment, all the helium in the Mercury atmosphere would have evaporated into outer space within 200 days.
In addition to helium, the presence of hydrogen, oxygen and sodium was discovered in the atmosphere of Mercury, but in very small quantities, as well as the presence of traces carbon dioxide and alkali metal atoms. So the number of helium molecules in a column of “air” above 1 cm 2 of the Mercury surface is only 400 trillion, the number of molecules of other gases is an order of magnitude less. The total number of gas molecules in the column of Mercury's atmosphere is 2x10 14 over 1 cm 2 of surface area.
The small amount of gases in the planet’s atmosphere indicates its extreme rarefaction: the pressure of all Mercury gases per 1 cm 2 of the planet’s surface area of half a billion is less than the pressure at the surface of the Earth. In addition, the rarefied atmosphere, as well as the low thermal conductivity of the surface layer of Mercury, is not able to equalize the temperature, which leads to its sharp daily fluctuations. So the average temperature of the day side of Mercury is 623K, and the night side is only 103K. However, at a depth of several tens of centimeters, the temperature is approximately constant and stays at around 70-90°C.
Despite extremely high daytime temperatures, the presence of water ice is allowed in the polar regions of Mercury. This conclusion was made on the basis of data from a radar study, which showed the presence of a substance that strongly reflects radio waves, which, apparently, is water ice. The existence of ice is possible only at the bottom of deep craters, where sunlight never penetrates.
Mercury's magnetic field. Magnetosphere of the planet Mercury
In 1974, the Mariner 10 spacecraft discovered that the planet Mercury has a weak magnetic field. Its strength is 100-300 times less than the strength of the Earth's magnetic field and increases as it moves towards the poles.
Fig. 15 Magnetosphere of Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury's magnetic field is global, has a dipole structure, is stable and symmetrical: its axis deviates by only 2° from the planet's rotation axis. In addition to the dipole, Mercury has fields with four and eight poles.
Scientists believe that Mercury's magnetic field is formed by the rotation of the substance of its liquid outer core. By the way, the rotation, or better yet, the movement of matter in the core of Mercury occurs in a very interesting way, which scientists from two American universities described in their article: Illinois and the Western Reserve Region.
To better understand the physical state in Mercury's core, scientists used a heavy-duty press to study the behavior of a mixture of iron and sulfur under conditions of high pressure and temperature. In each experiment, samples of a mixture of iron and sulfur were subjected to a certain pressure and heated to a certain temperature. The samples were then cooled, cut in half, and examined under an electron microscope and an electron microanalyzer.
Rapid cooling preserved the structure of the samples, which showed separation into solid and liquid phase, and sulfur was contained in each of them, says lead author of the study, Illinois graduate student Bin Chen. Based on the data from our experiment, we can draw conclusions about what is happening in the core of Mercury, he adds.
As the molten mixture of iron and sulfur cools in the outer layers of the core, iron atoms condense into "snowflakes" that fall toward the center of the planet. As the cold iron "snow" sinks and the light, sulfur-rich liquid rises, convective currents create a giant dynamo that creates the planet's relatively weak magnetic field.
In addition to the magnetic field, the planet Mercury has an extensive magnetosphere, which is strongly compressed from the side of the Sun under the influence of the solar wind.
Mercury is a hot planet
1. Mercury is the first planet in our solar system
2. Planetary movement
3. Physical characteristics
4. Surface
5. Atmosphere and physical fields
6. Research
7. Colonization of Mercury
8. Mercury in numbers
Mercury- the first from the Sun, the innermost and smallest planet in the Solar System, revolving around the Sun in 88 days. Mercury's apparent magnitude ranges from −2.0 to 5.5 and is not easily visible due to its very small angular distance from the Sun (maximum 28.3°). 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). Relatively little is known about the planet yet. The Mariner 10 spacecraft, which studied Mercury in 1974-1975, managed to map only 40-45% of the surface. In January 2008, MESSENGER flew past Mercury and 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 a source of a magnetic field in its totality constituting 0.1 of the Earth's. The temperature on the surface of Mercury ranges from 90 to 700 K (−180...430 °C). The sunflower side heats up much more than the polar regions and the far side of the planet.
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, an analogue of the Greek Hermes and Babylonian Naboo. The astronomical symbol of Mercury is a stylized image of the winged helmet of the god Mercury from his caduceus. 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 is two separate objects: one is visible only at sunrise, the other only in the evening at sunset. In ancient India, Mercury was called Buddha and Roginea. In Chinese, Japanese, Vietnamese and Korean, Mercury is called the Water Star (水星) (in accordance with the concept of the Five Elements). In Hebrew, the name of Mercury is Kokhav Hama (כוכב חמה) ("Solar Planet"). Despite smaller radius, Mercury still exceeds in mass such satellites of the giant planets as Ganymede and Titan.
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, at aphelion - 69.7 million km. 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 is a unique phenomenon for the Solar System. The phenomenon of commensurability between the periods of rotation and revolution of Mercury is apparently explained by the fact that the tidal influence of the Sun carried away the angular momentum and slowed down the rotation, which was initially faster, until both periods were in 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 effect of Joshua, after the biblical hero 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's also interesting that while Mars and Venus are the closest planets to Earth in terms of orbits, Mercury is the closest planet to Earth most of the time than any other (since the others move further away without being as "attached" to the Sun).
physical characteristics
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. Acceleration free fall on Mercury it is 3.70 m/s². The second escape velocity is 4.3 km/s.
The proximity to the Sun and the rather slow rotation of the planet, as well as the lack of an atmosphere, lead to the fact that Mercury experiences the most dramatic temperature changes in the Solar System. The average temperature of its daytime surface is 623 K, and at night it is only 103 K. The minimum temperature on Mercury is 90 K, and the maximum, reached at noon at “hot longitudes”, is 700 K.
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 a substance there that strongly reflects radio waves, the most likely candidate for which is 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 for an almost unlimited time.
Until recently, it was assumed that in the bowels of Mercury there is a metallic core with a radius of 1800-1900 km containing 60% of the planet’s mass, surrounded by a silicate shell 500-600 km thick, since the Mariner 10 spacecraft discovered a weak magnetic field, and it was believed that a planet with such small size cannot have a liquid core. But in 2007, Jean-Luc Margot's group summed up the results of five years of radar observations of Mercury, during which variations in the planet's rotation were noticed that were too large for a model with a solid core.
Surface
The surface of Mercury is in many ways reminiscent of the Moon - it is dotted with many craters. 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 German composer Beethoven, its diameter is 625 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 stretching for hundreds of kilometers - 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 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 has been 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 carried out by the MESSENGER probe, over 80% of the surface of Mercury was photographed and it was revealed that it is homogeneous, which distinguishes Mercury from the Moon or Mars, in which one hemisphere is sharply different from the other.
One of the most noticeable features of the surface of Mercury is the Plain of Heat (“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 × 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. 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 rotation axis, which imposes a significant limitation on the range of theories explaining its origin.
Research
Mercury is the least studied terrestrial planet. Only two devices were sent to study it. The first was Mariner 10, which in 1974-1975. flew past Mercury three times; 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.
NASA is currently conducting a second mission to Mercury called MESSENGER. The device was launched on August 3, 2004. On January 14, 2008, the device made its first flyby of its target - Mercury. To enter orbit around the planet on March 18, 2011, the device will have to perform two more gravity assist maneuvers past Mercury on October 6, 2008 and September 29, 2009. MESSENGER also completed one flyby of Earth in 2005 (February 8), and two flybys of Venus: October 24, 2006 and June 5, 2007, during which he checked the equipment.
The European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) are developing the BepiColombo 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 reach the orbit of 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 receivers 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 information. The reduction began to be applied at the Abastumani Astrophysical Observatory to series of photographs of Mercury received 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 Mariner-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°.
Colonization of Mercury
Like the Moon, Mercury does not have a dense atmosphere, is located relatively close to the Sun and rotates slowly around its axis, which has a very slight tilt. Therefore, due to the relatively large similarities, it is believed that the colonization of Mercury can be carried out using essentially the same technologies, approaches and equipment as the colonization of the Moon.
Despite its proximity to the Sun, the existence of ice caps at Mercury's poles was theoretically predicted. This makes the poles the most suitable place to establish a colony. In addition, in the region of the poles, temperature fluctuations during the change of day and night will not be as noticeable as in any other place on the surface of Mercury.
Being the closest planet to the Sun, Mercury has enormous reserves of solar energy. The amount of incoming solar energy per unit area here is 9.13 kW/m² (for the Earth and the Moon - 1.36 kW/m²). Since the inclination of Mercury's axis to the ecliptic axis is insignificant (approximately 0.01°), there is a possibility that there are peaks of eternal light at the heights of the poles. Even if they are not there, they can be obtained on high towers. In addition, it is possible to build a closed ring of solar power plants in the area of the poles, capable of providing a continuous supply of energy.
It is believed that Mercury's soil contains a large reserve of helium-3, which could become an important source of clean energy on Earth and a decisive factor in the development of the solar system's economy in the future. Additionally, Mercury may have large deposits of rich ore available for mining. This ore can later be used to build space stations.
Mercury is larger than the Moon (Mercury's diameter is 4879 km, the Moon's is 3476 km) and has a higher density due to its massive iron core. As a result, the acceleration due to gravity on Mercury is 0.377 g, which is more than twice that on the Moon (0.1654 g) and is equal to the acceleration due to gravity on the surface of Mars. Due to the fact that prolonged exposure to reduced gravity has a detrimental effect on human health, Mercury is more attractive as an object for long-term stay than the Moon
The almost complete absence of an atmosphere, extreme proximity to the Sun and a long day (176 Earth days) can become serious obstacles to the settlement of Mercury. Even with ice at the planet's poles, the presence of light elements necessary for life seems very unlikely. They will need to be imported. In addition, in the vicinity of Mercury, the gravitational force of the Sun is very strong, which requires more power to travel to and from Mercury than for other planets. The gravitational influence of Venus can be used to reach Mercury.
Mercury in numbers
Orbital characteristics
Aphelion - 69,816,927 km
Perihelion - 46,001,210 km
Semi-major axis - 57,909,068 km
Orbital eccentricity - 0.20530294
Sidereal period - 87.969098 days
Synodic period - 115.88 days
Orbital speed - 47.87 km/s
Average anomaly - 174.795884°
Inclination - 3.38° (o.c.e.)
Longitude of the ascending node - 48.330541°
Periapsis argument - 29.124279°
Satellites - no
physical characteristics
Average radius - 2439.7 ± 1.0 km
Surface area - 7.48×107 km²
Compression< 0,0006
Volume 6.083×1010 km³
Weight 3.3022×1023 kg
Average density 5.427 g/cm³
Gravity at the equator 3.7 m/s²
Second escape velocity 4.25 km/s
Rotation period (around its axis) 58.646 days
Rotation speed at the equator 10.892 km/h
The inclination of the planet's rotation axis is 0.01°
Right ascension at the North Pole 18 hours 44 minutes 2 seconds
Declination 61.45°
Albedo 0.119 (Bond)
Temperature
Minimum 100 K (-173 °C)
Average 340 K (67 °C)
Maximum 700 K (427 °C)
Atmospheric composition
Compound:
31.7% Potassium
24.9% Sodium
9.5%, A. Oxygen
7.0% Argon
5.9% Helium
5.6%, M. Oxygen
5.2% Nitrogen
3.6% Carbon dioxide
3.4% Water
0.147 Earth
0.056 Earth
0.055 Earth
0.984 Earth
0,38
281.01°
0.106 (geom. albedo)
24.9% sodium
9.5%, A. oxygen
7.0% argon
5.9% helium
5.6%, M. oxygen
5.2% nitrogen
3.6% carbon dioxide
3.4% water
3.2% hydrogen
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 the 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 intersected “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 at 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: Facts & Figures" NASA. Summary physical characteristics of the planet.
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