Space meteors. The passage of cosmic rays through the earth's atmosphere. How much does it cost to write your paper?

Phenomena observed in the form of short-term flashes that occur during the combustion of small meteoric objects (for example, fragments of comets or asteroids) in the earth's atmosphere. Meteors streak across the sky, sometimes leaving behind a narrow glowing trail for a few seconds before disappearing. In everyday life they are often called shooting stars. For a long time, meteors were considered a common atmospheric phenomenon such as lightning. Only at the very end of the 18th century, thanks to observations of the same meteors from different points, their altitudes and speeds were first determined. It turned out that meteors are cosmic bodies that enter the Earth’s atmosphere from the outside at speeds from 11 km/sec to 72 km/sec, and burn up in it at an altitude of about 80 km. Astronomers began to seriously study meteors only in the 20th century.

The distribution across the sky and the frequency of occurrence of meteors are often not uniform. So-called meteor showers, meteors which appear in approximately the same part of the sky over a certain period of time (usually several nights). Such streams are given the names of constellations. For example, the meteor shower that occurs annually from approximately July 20 to August 20 is called the Perseids. The Lyrid (mid-April) and Leonid (mid-November) meteor showers take their names from the constellations Lyra and Leo, respectively. IN different years meteorite showers exhibit different activities. The change in the activity of meteor showers is explained by the uneven distribution of meteor particles in the streams along the elliptical orbit intersecting the earth's.

Rice. 2. Perseid meteor shower ()

Meteors that do not belong to showers are called sporadic. On average, about 108 meteors brighter than 5th magnitude flare up in the Earth's atmosphere during the day. Bright meteors occur less frequently, weak ones more often. Fireballs(very bright meteors) can be visible even during the day. Sometimes fireballs are accompanied by meteorite falls. Often the appearance of a fireball is accompanied by a fairly powerful shock wave, sound phenomena, and the formation of a smoke tail. The origin and physical structure of the large bodies observed as fireballs are likely to be quite different compared to the particles that cause meteoric phenomena.

It is necessary to distinguish between meteors and meteorites. A meteor is not the object itself (that is, the meteor body), but the phenomenon, that is, its luminous trail. This phenomenon will be called a meteor, regardless of whether the meteoroid flies away from the atmosphere into outer space, burns up in it, or falls to Earth in the form of a meteorite.

Physical meteorology is the science that studies the passage of a meteorite through the layers of the atmosphere.

Meteor astronomy is the science that studies the origin and evolution of meteorites

Meteor geophysics is the science that studies the effects of meteors on the Earth's atmosphere.

- body cosmic origin, fallen onto the surface of a large celestial object.

In my own way chemical composition and structure meteorites are divided into three large groups: stone, or aerolites, iron-stone, or siderolites, and iron - siderites. The opinion of most researchers agrees that stone meteorites predominate in outer space (80-90% of the total), although more iron meteorites have been collected than stone ones. Relative quantity various types Meteorites are quite difficult to identify, since iron meteorites are easier to find than stone ones. In addition, stony meteorites are usually destroyed when passing through the atmosphere. When a meteorite enters the dense layers of the atmosphere, its surface becomes so hot that it begins to melt and evaporate. Jets of air blow away large drops of molten matter from iron meteorites, while traces of this blowing remain and can be observed in the form of characteristic notches. Rocky meteorites often break up, scattering a shower of fragments of various sizes onto the Earth's surface. Iron meteorites are more durable, but they sometimes break into separate pieces. One of the largest iron meteorites, which fell on February 12, 1947 in the Sikhote-Alin region, was discovered in the form of a large number of individual fragments, the total weight of which is 23 tons, and, of course, not all the fragments were found. The largest known meteorite, Goba (in South-West Africa), is a block weighing 60 tons.

Rice. 3. Goba - the largest meteorite found ()

Large meteorites burrow to a considerable depth when they hit the Earth. In this case, in the Earth's atmosphere at a certain altitude, the cosmic velocity of a meteorite is usually extinguished, after which, having slowed down, it falls according to the laws of free fall. What will happen in a collision with Earth? large meteorite, for example, weighing 105-108 tons? Such a gigantic object would pass through the atmosphere almost unhindered, and when it fell, a powerful explosion would occur with the formation of a funnel (crater). If such catastrophic events ever occurred, we should find meteorite craters on the surface of the Earth. Such craters really exist. Thus, the funnel of the largest, Arizona, crater has a diameter of 1200 m and a depth of about 200 m. According to a rough estimate, its age is about 5 thousand years. Not long ago, several more ancient and destroyed meteorite craters were discovered.

Rice. 4. Arizona meteorite crater ()

Shock crater(meteor crater) - a depression on the surface cosmic body, the result of the fall of another smaller body.

Most often, a meteor shower of high intensity (with a zenith hour number of up to a thousand meteors per hour) is called a star or meteor shower.

Rice. 5. Star rain ()

1. Melchakov L.F., Skatnik M.N. Natural history: textbook. for 3.5 grades avg. school - 8th ed. - M.: Education, 1992. - 240 pp.: ill.

2. Bakhchieva O.A., Klyuchnikova N.M., Pyatunina S.K., et al. Natural history 5. - M.: Educational literature.

3. Eskov K.Yu. and others. Natural history 5 / Ed. Vakhrusheva A.A. - M.: Balass

1. Melchakov L.F., Skatnik M.N. Natural history: textbook. for 3.5 grades avg. school - 8th ed. - M.: Education, 1992. - p. 165, tasks and question. 3.

2. How are meteor showers named?

3. How does a meteorite differ from a meteor?

4. * Imagine that you have discovered a meteorite and want to write an article about it for a magazine. What would this article look like?

The Earth, like other planets, regularly experiences collisions with cosmic bodies. Usually their size is small, no more than a grain of sand, but over 4.6 billion years of evolution there have been significant impacts; their traces are visible on the surface of the Earth and other planets. On the one hand, this causes natural concern and a desire to foresee a possible catastrophe, and on the other, curiosity and thirst to explore the substance that has fallen to Earth: who knows from what cosmic depths it came? Fear and curiosity have accompanied man since his appearance on the planet. The fruit of curiosity is usually liberation from fear.

"FALLING STARS" - METEORS AND BOLIDES

Interplanetary objects whose size does not exceed several hundred meters are usually called meteoroids, or meteoroids. Flying at cosmic speed into the planet’s atmosphere, due to collisions with gas molecules, they become very hot, crush, melt, evaporate and leave behind them a glowing trail for a second or two in flight. This atmospheric phenomenon is called meteor. Meteors are usually spotted against a clear night sky, which is why they are popularly called “shooting stars.” The apparent brightness of meteors is expressed in the same way as the brightness of other celestial objects - in stellar magnitude, based on the subjective impression that the meteor leaves on the observer.

If the brightness of a meteor exceeds -4 m (i.e. the brightness of Venus), then it is called fireball. The brightest fireballs are visible even during the day; their flight is sometimes accompanied by bright flashes, a trail of smoke, and sometimes powerful sounds. At a brightness of more than -6 m, a solid residue usually falls on the Earth's surface - meteorite. The most likely candidates for a meteorite fall are slow fireballs that do not exhibit a sharp flash at the end of their trajectory, indicating destruction.

If several independent observers report accurate data on the fireball's trajectory, then there is a chance of detecting a fallen meteorite. Of particular value are photo and video recordings of fireballs, accurate sketches of their trajectories relative to the stars, indicating the time and place of observation. This information should be sent to the RAS Committee on Meteorites:
117975 Moscow, st. Kosygina, 19; tel. 939-02-05, [email protected] or in
International Meteor Organization (IMO) Fireball Data Center (Saarbrucker Str. 8, D-40476 Duesseldorf, Germany; [email protected].
Additional information can be found on the Internet (http://www.imo.net).

Starfalls - meteor showers

Sometimes you can watch meteor shower- a breathtaking spectacle of almost simultaneous mass entry into the atmosphere of meteoroids moving along parallel trajectories. Unlike a meteor shower, meteor shower refers to multiple occurrences of meteors in approximately the same area of ​​the sky over a longer period of time, for example, over several nights. If the visible paths of these meteors are continued backward, they will intersect near one point in the sky, called radiant meteor shower.

Many meteor showers can be observed periodically, on the same days of the year, against the backdrop of the same constellation. On this basis, meteor showers are given names derived from the Latin names of the constellations in which their radiants lie. Many people are familiar with such “starfalls” as the Perseids (in August), the Leonids (in November) and some others. For example, the Leonid shower, observed in the area of ​​the constellation Leo, has been known since 902.

The section “Comets” states that the vast majority of meteor showers were formed as a result of the disintegration of comet nuclei, which lost the most volatile compounds during repeated approaches to the Sun. Therefore, the names of some meteor showers use the names of those comets with which they have been found to be associated (Bielids, Jacobinids, etc.).

Beginning of meteorite research

As the famous chemist of the St. Petersburg Academy of Sciences Ivan Mukhin rightly wrote in 1819, “the beginning of legends about stones and iron blocks falling from the air is lost in the deepest darkness of centuries that have passed.”

Meteorites have been known to man for many thousands of years. Tools of primitive people made from meteorite iron have been discovered. When people accidentally found meteorites, they hardly knew about their special origin. The exception was the discovery of “heavenly stones” immediately after the grandiose spectacle of their fall. Then meteorites became objects of religious worship. Legends were written about them, they were described in chronicles, they were feared and even chained so that they would not fly away to heaven again.

Information has been preserved that Anaxagoras (see, for example, the book by I.D. Rozhansky "Anaxagoras", pp. 93-94) considered meteorites to be fragments of the Earth or solid celestial bodies, and other ancient Greek thinkers - fragments of the firmament. These, in principle, correct ideas lasted as long as people still believed in the existence of the firmament or solid celestial bodies. Then, for a long time, they were replaced by completely different ideas that explained the origin of meteorites by any reasons, but not celestial ones.

The foundations of scientific meteoritics were laid by Ernst Chladni (1756-1827), already a fairly well-known German acoustic physicist by that time. On the advice of his friend, physicist G.Kh. Lichtenberg, he began collecting and studying descriptions of fireballs and comparing this information with what was known about the stones found. As a result of this work, Chladni in 1794 published the book “On the origin of the iron masses found by Pallas and other similar iron masses and on some related natural phenomena.” In it, in particular, a mysterious sample of “native iron” was discussed, discovered in 1772 by the expedition of Academician Peter Pallas and subsequently brought to St. Petersburg from Siberia. As it turned out, this mass was found back in 1749 by local blacksmith Yakov Medvedev and initially weighed about 42 pounds (about 700 kg). Analysis showed that it consists of a mixture of iron with rocky inclusions and is a rare type of meteorite. In honor of Pallas, meteorites of this type were called pallasites. Chladni's book convincingly proves that Pallas's iron and many other stones that "fell from the sky" are of cosmic origin.

Meteorites are divided into “fallen” and “found”. If someone has seen a meteorite fall through the atmosphere and is then actually discovered on earth (a rare event), then the meteorite is called a "fallen" meteorite. If it was found by chance and identified as a “space alien” (which is typical for iron meteorites), then it is called “found”. Meteorites are named after the places where they were found.

Cases of meteorite falls in Russia

The oldest record of a meteorite falling on Russian territory was found in the Laurentian Chronicle of 1091, but it is not very detailed. But in the 20th century, a number of major meteorite events occurred in Russia. First of all (not only chronologically, but also in terms of the scale of the phenomenon) is the fall of the Tunguska meteorite, which occurred on June 30, 1908 (new style) in the area of ​​the Podkamennaya Tunguska River. The collision of this body with the Earth led to a powerful explosion in the atmosphere at an altitude of about 8 km. Its energy (~10 16 J) was equivalent to an explosion of 1000 atomic bombs, similar to the one dropped on Hiroshima in 1945. The resulting shock wave circled the globe several times, and in the area of ​​the explosion felled trees within a radius of up to 40 km from the epicenter and led to death of a large number of deer. Fortunately, this enormous phenomenon occurred in a deserted area of ​​Siberia and almost no people were injured.

Unfortunately, due to wars and revolutions, the study of the Tunguska explosion area began only 20 years later. To the surprise of scientists, they did not find any, even the most insignificant, debris at the epicenter fallen body. After repeated and thorough studies of the Tunguska event, most experts believe that it was associated with the fall of the nucleus of a small comet to Earth.

A shower of stone meteorites fell on December 6, 1922 near the village of Tsarev (now Volgograd region). But traces of it were discovered only in the summer of 1979. 80 fragments with a total weight of 1.6 tons were collected over an area of ​​about 15 square meters. km. The weight of the largest fragment was 284 kg. This is the largest stone meteorite by mass found in Russia, and the third in the world.

Among the largest meteorites observed during the fall is the Sikhote-Alinsky one. It fell on February 12, 1947 in the Far East in the vicinity of the Sikhote-Alin ridge. The dazzling fireball it caused was observed in the daytime (about 11 a.m.) in Khabarovsk and other places within a radius of 400 km. After the fireball disappeared, there was a roar and a rumble, air shocks occurred, and the remaining dust trail slowly dissipated for about two hours. The place where the meteorite fell was quickly discovered based on information about the observation of the fireball from different points. An expedition of the USSR Academy of Sciences under the leadership of Academician immediately set off there. V.G. Fesenkova and E.L. Krinova - famous researchers of meteorites and small bodies of the Solar system. Traces of the fall were clearly visible against the background of the snow cover: 24 craters with a diameter of 9 to 27 m and many small craters. It turned out that the meteorite disintegrated while still in the air and fell in the form of “iron rain” over an area of ​​about 3 square meters. km. All 3,500 fragments found consisted of iron with small inclusions of silicates. The largest fragment of the meteorite has a mass of 1745 kg, and the total mass of the entire substance found was 27 tons. According to calculations, the initial mass of the meteoroid was close to 70 tons, and its size was about 2.5 m. By a lucky coincidence, this meteorite also fell in an uninhabited area, and no harm done.

And finally, about the latest events. One of them also occurred on the territory of Russia, in Bashkiria, near the city of Sterlitamak. A very bright fireball was observed on May 17, 1990 at 23:20. Eyewitnesses reported that for a few seconds it became as bright as day, thunder, crackling and noise were heard, which made the window panes rattle. Immediately after this, a crater with a diameter of 10 m and a depth of 5 m was discovered in a country field, but only two relatively small fragments of an iron meteorite (weighing 6 and 3 kg) and many small ones were found. Unfortunately, when excavating this crater with an excavator, a larger fragment of this meteorite was missed. And only a year later, the children discovered the main part of the meteorite weighing 315 kg in the dumps of soil removed by an excavator from the crater.

On June 20, 1998, at about 5 p.m. in Turkmenistan, near the city of Kunya-Urgench, a chondritic meteorite fell during the day in clear weather. Before this, a very bright fireball was observed, and at an altitude of 10-15 km there was a flash comparable in brightness to the Sun, there was a sound of an explosion, a roar and a crack that could be heard at a distance of up to 100 km. The main part of the meteorite, weighing 820 kg, fell on a cotton field just a few tens of meters from the people working in it, forming a crater with a diameter of 5 m and a depth of 3.5 m.

Physical phenomena caused by the flight of a meteoroid in the atmosphere

The speed of a body falling to the Earth from afar, near its surface, always exceeds the second cosmic speed (11.2 km/s). But it can be much more. The speed of the Earth's orbit is 30 km/s. When crossing the Earth's orbit, solar system objects can have speeds of up to 42 km/s (= 2 1/2 x 30 km/s).

Therefore, on opposite trajectories, the meteoroid can collide with the Earth at a speed of up to 72 km/s.

When a meteoroid enters the earth's atmosphere, many interesting phenomena occur, which we will only mention. Initially, the body interacts with a very rarefied upper atmosphere, where the distances between gas molecules are greater than the size of the meteoroid. If the body is massive, then this does not affect its condition and movement in any way. But if the mass of the body is not much greater than the mass of the molecule, then it can completely slow down already in upper layers atmosphere and will slowly settle to the earth's surface under the influence of gravity. It turns out that in this way, that is, in the form of dust, the bulk of solid cosmic matter falls on Earth. It is estimated that about 100 tons of extraterrestrial matter enter the Earth every day, but only 1% of this mass is represented by large bodies that have the ability to reach the surface.

Noticeable deceleration of large objects begins in dense layers of the atmosphere, at altitudes less than 100 km. The motion of a solid body in a gaseous environment is characterized by the Mach number (M) - the ratio of the speed of the body to the speed of sound in the gas. The Mach number for a meteoroid varies with altitude, but usually does not exceed M = 50. A shock wave is formed in front of the meteoroid in the form of highly compressed and heated atmospheric gas. Interacting with it, the surface of the body heats up to melting and even evaporation. The incoming gas jets spray and carry away molten and sometimes solid crushed material from the surface. This process is called ablation.

Hot gases behind the shock wave front, as well as droplets and particles of matter carried away from the surface of the body, glow and create the phenomenon of a meteor or fireball. With a large body mass, the phenomenon of a fireball is accompanied not only by a bright glow, but sometimes also by sound effects: a loud bang, as if from supersonic aircraft, rumbles of thunder, hissing, etc. If the mass of the body is not too large, and its speed is in the range from 11 km/s to 22 km/s (this is possible on trajectories “catching up” with the Earth), then it manages to slow down in atmosphere. After this, the meteoroid moves at such a speed at which ablation is no longer effective, and it can reach the earth's surface unchanged. Braking in the atmosphere can completely extinguish the horizontal speed of the meteoroid, and its further fall will occur almost vertically at a speed of 50-150 m/s, at which the force of gravity is compared with air resistance. Most meteorites fell to Earth at such speeds.

With a very large mass (more than 100 tons), the meteoroid does not have time to either burn up or slow down significantly; it hits the surface at cosmic speed. An explosion occurs, caused by the conversion of large kinetic energy of the body into thermal energy, and an explosion crater is formed on the earth's surface (Fig. 1). As a result, a significant part of the meteorite and surrounding rocks melt and evaporate.

Loss is often observed meteor showers. They are formed from fragments of meteoroids that are destroyed when they fall. An example is the Sikhote-Alin meteor shower. As calculations show, when a solid body descends in the dense layers of the earth's atmosphere, enormous aerodynamic loads act on it. For example, for a body moving at a speed of 20 km/s, the pressure difference on its front and rear surfaces varies from 100 atm. at an altitude of 30 km up to 1000 atm. at an altitude of 15 km. Such loads are capable of destroying the vast majority of falling bodies. Only the most durable monolithic metal or stone meteorites are able to withstand them and reach the earth's surface.

For several decades, there have been so-called fireball networks - systems of observation posts equipped with special cameras for recording meteors and fireballs. From these images, the coordinates of a possible meteorite impact site are quickly calculated and searched for. Such networks were created in the USA, Canada, Europe and the USSR and cover territories of approximately 10 6 square meters. km.

About meteorite craters and other consequences of meteorite falls

Encounters of the Earth with large meteoroids create a danger for people and everything they create, as well as for the earth's flora and fauna. Moreover, catastrophic events like Tunguska could pose a threat to the entire human civilization. Of course, this can only happen in a collision with a sufficiently large body, such as an asteroid or comet nucleus. The Earth's surface stores traces of such collisions in the form of large craters - the so-called "astroblemes" (i.e., "stellar wounds"). More than 230 of them have already been discovered. The diameters of the largest of them exceed 200 km (Fig. 1). One of the well-preserved craters (due to its relative youth) is Devil's Canyon in Arizona (USA). Its diameter is 1240 m and its depth is 170 m. In 1906, geologist D. Barringer proved that this crater is of impact origin. During its study, about 12 tons of meteorite substance were discovered and it was established that it arose approximately 50 thousand years ago during the fall of an iron-nickel meteorite about 60 m in size, flying at a speed of 20 km/s.

Rice. 1. Barringer crater of impact origin with a diameter of 1240 m and a depth of 170 m, which was formed about 50,000 thousand years ago by the fall of an iron meteorite measuring 30-50 m in size. The crater is located near the city of Winslow (Arizona, USA):

a) general view of the crater from an airplane;

b) panorama of the crater.

b) panorama of the crater.

Due to atmospheric and water erosion, there are practically no ancient craters less than 1 km in size left on Earth. Meteor craters on the Moon, Mercury, Mars and other planets and satellites with a thin atmosphere or without it at all are preserved much better and longer. As calculations show, during the first 100 million years after its formation, the Earth “scraped out” almost all the solid matter moving in the vicinity of its orbit. However, the Earth continues to encounter dust, stones and even kilometer-sized blocks on its path. Where do they come from? We will answer this question, but first we will get acquainted with the composition and structure of meteorite matter.

Composition and structure of meteorite matter

Among the meteorite matter falling to the Earth, by the number of falls, approximately 92% are stone meteorites, 6% are iron and 2% are iron-stone (or, respectively, 85, 10 and 5% by mass).

The atmosphere serves as the first “filter” through which meteorite matter must pass. The more refractory and durable it is, the more likely it is to reach the earth’s surface. Another filter can be considered the selection of meteorites when they are found. The more a meteorite stands out against the background of the earth's surface, the easier it is to find. Thirty years ago, Japanese scientists discovered that the best place to find meteorites was Antarctica. Firstly, the meteorite is easy to spot against the background white ice. Secondly, they are better preserved in ice. Meteorites that fall in other places on Earth are exposed to atmospheric weathering, water erosion and other destructive factors; That's why they either decompose or end up buried.

The main components of the meteorite substance are iron-magnesium silicates and nickel iron. Sometimes iron sulfides (troilite, etc.) are also abundant. Common minerals included in the silicates of meteorite matter are olivines (Fe, Mg) 2 SiO 4 (from fayalite Fe 2 SiO 4 to forsterite Mg 2 SiO 4) and pyroxenes (Fe, Mg) SiO 3 (from ferrosilite FeSiO 3 to enstatite MgSiO 3) of different composition. They are present in silicates either in the form of small crystals or glass, or as a mixture in varying proportions. To date, about 300 different minerals have been discovered in meteorite matter. And although their number is gradually increasing in the process of researching new meteorites, it is still more than an order of magnitude smaller than the number of known terrestrial minerals.

Chondrites

The most numerous stony meteorites are divided into two groups: chondrites and achondrites. Chondrites are so named because of the presence of unusual inclusions of spherical or elliptical shape - chondrules, included in a darker substance - the matrix (Fig. 2). Chondrules can be seen on the surface of a meteorite's fracture, but they are best seen on the polished surface of its cut. The size of chondrules ranges from microscopic to centimeter. Sometimes they occupy up to 50% of the volume of the meteorite. The chondrules and matrix practically do not differ in composition and consist mainly of fine-crystalline iron-magnesium silicates and glasses. But the structure of chondrules is mainly crystalline. On this basis, some experts believe that chondrules crystallized from a melt. The content of nickel iron in chondrites does not exceed 30%, and it is present in the form of small particles of irregular or spherical shape. In general, the substance of chondrites is relatively dense (2.0 - 3.7 g/cm3), but fragile. A little effort is enough to crush a chondritic meteorite in your hands. It is surprising that chondrules have so far only been found in meteorites. Their origin still remains a mystery, since the mechanisms of their occurrence are unknown.

Rice. 2. An ordinary chondrite found in the Alan Hills region of Antarctica.

Rice. 2. An ordinary chondrite found in the Alan Hills region of Antarctica. The substance of this meteorite contains spheroidal inclusions of millimeter size - chondrules, consisting of a mixture of crushed minerals, formed in a protoplanetary nebula about 4.55 billion years ago.

Another important feature of chondrites is their extremely simple elemental composition. If we do not take into account the most volatile elements (H, He, O and some others), it turns out that the composition of chondrites is very close to the elemental composition of the Sun. Moreover, such proximity can be traced not only in the main elements, but also in the impurity elements, which also serve as important indicators. Impurity elements are divided into three groups: lithophile (Se, Sr, Rb, Ba, Ce, Cs, Th, U, etc.), chalcophile (Cu, Zn, Sn, Pb, Ag, Hg, Cd, In, etc.) and siderophile (Ga, Ge, Ru, Pt, Pd, Os, Ir, Rh, etc.); they show an affinity for minerals rich in oxygen, sulfur and iron, respectively. In particular, the rocks of the Earth that have undergone magmatic differentiation contain mainly lithophilic trace elements. Chalcophile elements are found on the earth's surface only in limited areas of ore deposits, and siderophile elements are practically absent. It turned out that in chondritic meteorites, trace elements different groups are present in the same proportions (with minor variations) as in the Sun. This means that chondrites were formed from matter solar composition and did not undergo differentiation. At the same time, it is obvious that they were occasionally subjected to heating, although not very strong, and therefore some structural and mineralogical changes occurred in them, called thermal metamorphism.

Chondrites are clearly divided into three large classes according to the form of iron content, more precisely according to the degree of its oxidation. Chondrites of these classes were given the following names and designations: enstatite (E), ordinary (O) and carbonaceous (C). In the same order, the content of oxidized (divalent and trivalent) iron increases in them. All chondrites are divided into six petrological types, in which the structural and mineralogical manifestations of thermal metamorphism gradually increase (from type 1 to type 6).

Carbonaceous chondrites

Carbonaceous chondrites (denoted by the letter "C", from the English carbonaceous - carbonaceous) are the darkest, which justifies their name. They contain a lot of iron, but it is almost entirely bound in silicates. The dark color of carbonaceous chondrites is mainly due to the mineral magnetite (Fe 3 O 4), as well as small amounts of graphite, soot and organic compounds. These meteorites also contain a significant proportion of hydrous minerals or hydrosilicates (serpentine, chlorite, montmorillonite and a number of others).

J. Wasson proposed in the 1970s to divide carbonaceous chondrites into four groups (CI, CM, CO and CV) based on the gradual change in their properties. Each group has a typical, standard meteorite, the first letter of whose name is added to the index "C" when designating the group. Typical representatives in the mentioned groups are the Ivuna, Migei (found in Ukraine, Nikolaev region), Ornans and Vigarano meteorites. Somewhat earlier, in 1956, G. Wiik proposed dividing carbonaceous chondrites into three groups (CI, CII and CIII), references to which can sometimes be found in the literature. Wasson's groups CI and CM correspond fully to Wiick's groups CI and CII, and groups CO and CV can be considered as components of group CIII.

In CI chondrites, hydrated silicates occupy most of the volume. Their X-ray studies showed that the predominant silicate is septechlorite (the general formula of septechlorites is Y 6 (Z 4 O 10)(OH) 8, where Y = Fe 2+, Mg; Z = Si, Al, Fe 3+). Moreover, all hydrosilicates are in amorphous form, that is, in the form of glass. There are no dehydrated silicates (pyroxenes, olivines, etc., which appear at temperatures above 100°C) here. CI meteorites are an exception among chondrites, since their substance does not contain chondrules at all, but consists of a single matrix. This supports the idea that chondrules crystallized from molten material, since studies show that the material of CI chondrites did not undergo melting. It is considered the most unchanged, essentially the primary matter of the Solar System, preserved from the moment of condensation of the protoplanetary cloud. This explains the high interest of scientists in CI meteorites.

CM chondrites contain only 10-15% bound water (in the composition of hydrosilicates), and 10-30% pyroxene and olivine are present in the form of chondrules.

CO and CV chondrites contain only 1% bound water and are dominated by pyroxenes, olivines and other dehydrated silicates. They also contain nickel iron in small quantities. The presence of hydrosilicates noticeably reduces the density of carbonaceous chondrites: from 3.2 g/cm 3 in CV to 2.2 g/cm 3 in CI meteorites.

Ordinary chondrites

Ordinary chondrites are so named because they are the most common in meteorite collections (Figure 2). They include three chemical groups: H, L and LL (H - from English high, high; L - from low, low). Meteorites of these groups are similar in a number of properties, but differ in the total content of iron and siderophile elements (H > L > LL) and in the ratio of oxidized iron to metallic (LL > L > H). Group H chondrites span petrologic types 3 to 6, while group L and LL chondrites span petrologic types 3 to 7.

Structural and mineralogical features of O-chondrites indicate that these meteorites experienced thermal metamorphism at temperatures ranging from approximately 400°C (for low petrological type 3) to over 950°C (for type 7) and at shock pressures up to 1000 atm. (increasing with increasing temperature). Compared to the more “regular” chondrules of carbonaceous chondrites, ordinary chondrules are more often irregular in shape and filled with detrital material. The total iron content in O-chondrites varies by group within the following limits: 18-22% (LL), 19-24% (L), 25-30% (H). The amount of metallic iron also increases from group LL to L and further to H.

Enstatitechondrites

In enstatite (E) chondrites, iron is found mainly in the metallic phase, that is, in a free state (at zero valence). At the same time, their silicate compounds contain very little iron. Almost all of the pyroxene in them is presented in the form of enstatite (hence the name of this class). Structural and mineralogical features of enstatite chondrites indicate that they experienced thermal metamorphism at maximum temperatures (for chondrites), ranging from approximately 600°C to 1000°C. As a consequence, E-chondrites, compared to other chondrites, are the most reduced and contain the least amount of volatile compounds.

In this group, 3 petrological types are distinguished (E4, E5 and E6), in which an increase in signs of thermal metamorphism can be traced. E-chondrites have also been found to have wide variations in iron and sulfur content depending on the petrological type. On this basis, some scientists further divide them into types I (which includes E4 and E5) and II (E6). Chondrules in enstatite chondrites are embedded in a dark, fine matrix, have irregular outlines, and are filled with detrital material.

Differentiated meteorites

Achondrites

Less large group stony meteorites (about 10%) are achondrites. They do not contain chondrules and are not chemically similar to chondrites because they have a non-solar composition. Achondrites range from nearly monomineral olivine or pyroxene rocks to objects similar in structure and chemical composition to terrestrial and lunar basalts. They are poor in iron and siderophile impurity elements; they have slightly different contents of Fe, Mg and Ca. In general, these meteorites are similar to igneous rocks of the Earth and the Moon that have undergone magmatic differentiation.

It is assumed that achondrites were formed from parent material of chondritic composition in the same differentiation process that also produced iron meteorites. Achondrites are divided into groups according to their mineral composition. The name of each group corresponds either to the name of the main mineral or to the name of the meteorite, which can be considered a typical representative of this group: aubrites (97% by weight is orthoenstatite), ureilites (85% olivine), diogenites (95% orthopyroxene), howardites (40- 80% orthopyroxene) and eucrite (40-80% pigeonite).

Iron and stony-iron meteorites

In addition to achondrites, differentiated meteorites are also iron and stony-iron meteorites. They attract significant interest not only because they fall to Earth less often than chondrites. They also represent a different stage in the evolution of matter in the Solar System. While chondrites record the history of the accumulation of matter in the preplanetary cloud and during the formation of planetesimals, differentiated meteorites record the sequence of processes that occurred in the parent bodies of meteorites and their internal structure. Iron meteorites were previously thought to be part of the destroyed core of one large parent body, the size of the Moon or larger.

But now it is known that they represent many chemical groups, which in most cases indicate the crystallization of the substance of these meteorites in the cores of different parent bodies of asteroidal sizes (on the order of several hundred kilometers). Others of these meteorites may be samples of individual clumps of metal that were dispersed in their parent bodies. There are also those that bear evidence of incomplete separation of metal and silicates, such as stony-iron meteorites.

Stone-iron meteorites

Stone-iron meteorites are divided into two types, differing in chemical and structural properties: palacites and mesosiderites. Pallasites are those meteorites whose silicates consist of crystals of magnesian olivine or their fragments enclosed in a continuous matrix of nickel iron. Mesosiderites are called iron-stony meteorites, the silicates of which are mainly recrystallized mixtures of different silicates, which are also included in the metal cells.

Iron meteorites

Iron meteorites are composed almost entirely of nickel iron and contain small amounts of minerals in the form of inclusions. Nickel iron (FeNi) is a solid solution of nickel in iron. With a high nickel content (30-50%), nickel iron is found mainly in the form of taenite (g-phase) - a mineral with a face-centered crystal lattice cell; with a low (6-7%) nickel content in the meteorite, nickel iron consists almost of kamacite ( a-phase) - a mineral with a body-centered lattice cell.

Most iron meteorites have a surprising structure: they consist of four systems of parallel kamacite plates (differently oriented) with interlayers consisting of taenite, against a background of a fine-grained mixture of kamacite and taenite. The thickness of the kamacite plates can vary from fractions of a millimeter to a centimeter, but each meteorite has its own plate thickness.

If the polished cut surface of an iron meteorite is etched with an acid solution, its characteristic internal structure in the form of “Widmanstätten figures” (Fig. 3). They are named in honor of A. de Widmanstätten, who was the first to observe them in 1808. Such figures are found only in meteorites and are associated with the unusually slow (over millions of years) cooling process of nickel iron and phase transformations in its single crystals.

Rice. 3. Iron meteorite (octahedrite IIIA) "Baghdad".

Rice. 3. Iron meteorite (octahedrite IIIA) “Baghdad”, found in Arizona (USA) in 1959. A large Widmanschätten structure is visible on the cut of the meteorite.

Until the early 1950s. iron meteorites were classified solely by their structure. Meteorites with Widmanstätten figures began to be called octahedrites, since the kamacite plates that make up these figures are located in planes forming an octahedron.

Depending on the thickness L of kamacite plates (which is related to the total nickel content), octahedrites are divided into the following structural subgroups: very coarse-structured (L > 3.3 mm), coarse-structured (1.3< L < 3,3), среднеструкткрные (0,5 < L < 1,3), тонкоструктурные (0,2 < L < 0,5), весьма тонкоструктурные (L < 0,2), плесситовые (L < 0,2).

Some iron meteorites with low nickel content (6-8%) do not exhibit Widmanstätten patterns. Such meteorites seem to consist of a single kamacite single crystal. They are called hexahedrites because they have a mostly cubic crystal lattice. Sometimes meteorites with an intermediate type of structure are found, called hexaoctahedrites. There are also iron meteorites that do not have an ordered structure at all - ataxites (translated as “lacking order”), in which the nickel content can vary widely: from 6 to 60%.

The accumulation of data on the content of siderophile elements in iron meteorites also made it possible to create their chemical classification. If in n-dimensional space, the axes of which are the contents of different siderophile elements (Ga, Ge, Ir, Os, Pd, etc.), mark the positions of different iron meteorites with points, then the concentrations of these points (clusters) will correspond to such chemical groups. Among the almost 500 currently known iron meteorites, 16 chemical groups are clearly distinguished by the content of Ni, Ga, Ge and Ir (IA, IB, IC, IIA, IIB, IIC, IID, IIE, IIIA, IIIB, IIIC, IIID, IIIE, IIIF, IVA, IVB). Since 73 meteorites in this classification turned out to be anomalous (they are classified as unclassified), there is an opinion that there are other chemical groups, perhaps more than 50, but they are not yet sufficiently represented in collections.

The chemical and structural groups of iron meteorites are ambiguously related. But meteorites from the same chemical group, as a rule, have a similar structure and some characteristic thickness of kamacite plates. It is likely that meteorites of each chemical group were formed under similar temperature conditions, perhaps even in the same parent body.

Methods for studying meteorites and their results

When pure crystalline iron is heated, the temperature of the phase transformation kamacite (a -phase) R taenite (g -phase) is 910°C. At typical average nickel concentrations in iron meteorites (7-14%), g R a -transformation begins at lower temperatures (650-750°C). When the temperature drops, kamacite appears in taenite in the form of thin sheets, or plates, oriented along the faces of the octahedron - four planes with an equivalent arrangement of atoms. Therefore, iron meteorites in the process of g R a transformation acquire an octahedrite structure, reflecting the directions of preferential growth of kamacite plates.

Depending on the direction of cutting of the meteorite in relation to the octahedrite orientation of its plates, the Widmanschätten figures have a different pattern. The plates themselves in cross-section look like beams. The lower the nickel content in the initial taenite, the higher the temperature at which the phase transformation begins and the longer the growth of kamacite plates lasts, and the thicker they are at the end of growth. This explains why meteorites with a high nickel content are fine-structured, and meteorites with a low nickel content are coarse-structured, up to the formation of a solid kamacite single crystal up to 50 cm thick, like hexahedrites.

At the end of the 1950s. In iron meteorites, Soviet researchers discovered using electron microprobing a specific M-shaped profile of nickel distribution in the cross section of taenite layers located between kamacite layers. In the 1960s J. Golsteyn, W. Buchwald and others showed that this profile is also formed during g R a transformations in nickel iron during its cooling. It arises due to the different diffusion rates of nickel in kamacite and taenite (it is 100 times greater in kamacite) and the lower solubility of nickel in kamacite than in taenite. This discovery gave astronomers a new method for reconstructing the history of meteorites.

By calculating the profiles of nickel in tenite at different initial contents and comparing them with the measured characteristics in meteorites, it was possible to estimate the cooling rates of the material of iron meteorites in the depths of the parent bodies, and, consequently, the sizes of these bodies. J. Wood proposed another method for estimating the cooling rate - based on the width of the taenite plate and the concentration of nickel in its center in relation to the average nickel content in the meteorite. Both of these methods gave matching results. It turned out that the substance of octahedrites in the temperature range of 600-400 ° C cooled at a rate of 1-10 ° C per million years, and sometimes slower. A similar result was obtained for iron-stone meteorites, the metal of which also has an octahedrite structure.

Moreover, the study of metal particles present in meteorites of other classes showed that they also contain taenite and kamacite. J. Wood applied his technique, developed for iron meteorites, to chondrites and estimated their cooling rate. Unexpectedly, it turned out that most chondrites cooled at about the same rate as iron meteorites: about 10 ° C per million years in the temperature range 550-450 ° C. Such a long cooling of the substance of a variety of meteorites means that during the heating period and tens to hundreds of millions of years after that, it was deep in the depths of the parent bodies.

Calculations have shown that to ensure such a slow cooling, the thickness of the protective layer, even with very low thermal conductivity (like rocky material with a chondritic composition), should be 70-200 km. This means that the minimum diameter of the primary parent bodies of meteorites of different classes was about 140-400 km, and this exactly corresponds to the size of large asteroids.

So, the parent bodies of most meteorites were large asteroids, and some had molten cores, which required a temperature of at least 1200-1400°C (for a substance of chondritic composition). The source of heating of the asteroids could be either radioactive elements (for example, the Al 26 isotope, which with a half-life of 760 thousand years turns into Mg 26, releasing a lot of energy), or inductive currents that could be excited in the asteroids by the powerful stellar wind of the young Sun. But so far these are hypotheses that have not received reliable confirmation. In addition, a number of meteorites from scientific collections do not show signs of being in the depths of their parent bodies.

The epoch of secondary heating of some meteorites was determined using the helium-argon method. It is based on measuring the content of He and Ar, which arise in a substance during the radioactive decay of Th and K 40, respectively. At low temperatures, these gases are retained by the substance, but at high temperatures they begin to leak out of it (diffuse). Moreover, helium diffusion begins at temperatures above 200°C, and argon - above 300°C. The parent bodies of meteorites or the meteoroids themselves could be heated to such temperatures not only by the energy of radioactive decay, but also by collisions with other bodies or approach to the Sun. This time for some enstatite chondrites is about 600 million years, which is consistent with the long period of their cooling from high temperatures. This is another confirmation (besides petrological) of a long period of cooling of chondritic meteorites from high temperatures.

It is also possible to estimate the period of independent existence of the meteoroid that gave rise to a particular meteorite, that is, the time interval from the fragmentation of the parent body to the fall of the meteorite to Earth. This space age A meteorite is determined by the density of tracks left in its substance by cosmic particles of solar or galactic origin. They do not penetrate deeply, but linger in a layer about 1 m thick. If a fragment breaks off from the parent body and lives independently in interplanetary space for some time, then its cosmic age is determined by the age of its most “fresh” side. It turned out that the cosmic ages of meteorites differ different classes. In particular, for enstatite chondrites it was possible to measure two fairly young ages: 7 and 20 million years. And some iron-nickel “cosmic” clocks are much older: they are about 700 million years old. However, it cannot be excluded that the surface of chondrites, which is most saturated with tracks of cosmic particles, is partially destroyed when passing through the Earth’s atmosphere, which can lead to a false assessment of the difference in their age compared to more durable iron meteorites. The absolute age of meteorites is determined by the rubidium-strontium method: the decay of the long-lived isotope Rb 87 produces stable Sr 87; By measuring its content in relation to the stable isotope Sr 86, the age of the meteorite is found. It turns out to be within the range of 4.5-4.7 billion years, just like for terrestrial rocks.

The complex history of meteorite matter

There is another important argument in favor of the asteroid origin of most meteorites. The substance of meteorites in many cases represents a complex conglomerate of materials that could have arisen in different, sometimes even incompatible, conditions. Carbonaceous chondrites, which are often primitive in composition, contain inclusions of materials characteristic of ordinary, enstatite, or even iron meteorites, and vice versa. An amazing example of such a substance is the Kaidun meteorite, weighing 850 g, which fell on December 3, 1980 on the territory of a Soviet military base in Yemen. Particles of three types of carbonaceous chondrites, an ordinary chondrite, two enstatite chondrites, as well as water-altered particles of metallic iron were found in it. This is probably a fragment of a body that had a very complex history.

This structure of meteorites was difficult to explain until the 1970s. But when studying samples of lunar soil delivered to Earth, it turned out that they are often mixtures of substances from different areas of the lunar surface. The lunar soil is repeatedly mixed by the impacts of meteorites bombarding the Moon. The same should happen with the matter on the surface of asteroids. Satellite images of asteroids 951 Gaspra, 243 Ida, 253 Matilda and 433 Eros confirm that their shape is irregular and the surface is covered with many craters. Obviously, this is the result of collisions of asteroids with each other and with more small bodies. For this reason, the surface of asteroids, like the lunar surface, is covered with a layer of crushed matter - regolith. In the present era, the average relative speed of asteroids in the main belt, determined by the nature of their orbits, is about 5 km/s. At this speed, each kilogram of matter carries kinetic energy of about 10 7 J. At the moment of collision most of This energy turns into heat, which leads to explosion, melting and evaporation of a significant part of the substance of the colliding bodies. At this impact speed, the explosion pressure reaches 1.5 Mbar. A significant part of the energy turns into mechanical energy of shock waves and goes to crushing, scattering or, conversely, compacting (depending on the direction and distance from the explosion site) the surrounding matter of the asteroid.

There was a period in the history of the Solar System when the relatively calm, with relative velocities of less than 1 km/s, movement of the main belt asteroids was subject to strong disturbances from the growing Jupiter, and these bodies themselves, which had different compositions at different heliocentric distances, were strongly “mixed” . Asteroids of different types with significantly different compositions of matter ended up in neighboring or intersecting orbits. During their collisions and fragmentation, materials that arose under different physicochemical conditions accumulated in the surface layers of many asteroids. The parent body of the Kaidun meteorite, for example, could move along a highly elongated orbit, colliding with bodies of different compositions along the way and, as it were, “collecting” samples of their matter. It is possible that this parent body was not an asteroid with an anomalous orbit, but a comet nucleus that had exhausted its supply of volatile compounds.

Calculations show that when a large crater is formed on an asteroid about 200 km in size, approximately 85% of the substance ejected by the explosion is not able to overcome the gravity of the asteroid (although the escape velocity from its surface is only 50 m/s). The birth of an impact crater on an asteroid is accompanied by the formation of a short-term “atmosphere” of stones and dust, which after some time settles and covers its entire surface. The thickness of this layer depends on the force of the impact and, accordingly, the volume of ejected substance. Cracks that appear as more bodies fall onto an asteroid can gradually fragment it (if it is large enough) and subsequent falls of bodies will already occur into fragmented material. The more the asteroid is fragmented and loosened, the faster the vibrations in it die out. In this case, the energy of the falling body is absorbed in a smaller volume, accompanied by more powerful effects. Most likely, with such an impact “compaction” of heterogeneous matter on the surfaces of asteroids over tens and hundreds of millions of years, some samples were formed that fell as meteorites to Earth.

Debris from other planets?

What is described in this paragraph would seem to contradict what was just said about the “softness” of meteorite impacts. It turns out that space bombardment can not only “gently mix” the soil of planets and asteroids, but also throw it into space, transferring it from one planet to another. There is still little clarity on these issues, but the results of unexpected discoveries make us take them very seriously.

To overcome the Earth's gravity (even without taking into account atmospheric resistance), a speed of more than 11.2 km/s is required; for Mars it is 5 km/s, and for the Moon 2.4 km/s. Only at this or greater starting speed can fragments of planets fall into outer space and, wandering there, be captured by other planets. Until recently, such a process seemed impossible. But it seems that astronomers underestimated nature's imagination. Now many experts are confident that fragments of the Moon and Mars have been found on Earth. Perhaps the impacts of large meteorites can actually “launch” particles of planets into space.

Lunar and Martian meteorites

When comparing samples of the Moon delivered to Earth with a group of meteorites similar to them, it turned out that they were practically the same substance. Today there is no longer any doubt that long before space flights, samples of lunar soil were gathering dust in meteorite collections. True, to prove this, it was necessary to fly to the moon.

In addition, among meteorites, a group was identified that differs sharply in characteristics from others, but its members are similar to each other. This group was named SNC, after the first letters of the names of their typical representatives - the Shergotty, Nakhla and Chassigny meteorites. Now 12 such meteorites are known and it is believed that they came to Earth from Mars. This is indicated by the chemical and, very importantly, isotopic composition of microscopic gas bubbles in one of the meteorites of this group, EETA 79001, which coincides with the composition of the atmosphere of Mars measured by the Viking probes in 1976 (see more in the chapter “Mars” .)

Fossils of ancient Martian life?

One of the “Martian” meteorites, ALH 84001 weighing 1.9 kg, found in Antarctica in the Alan Hills region and assigned to the SNC group, caused a real sensation (Fig. 4). Studying the substance ALH 84001 revealed its most interesting history. The substance of this meteorite arose from liquid magma 4.5 billion years ago, when Mars was still forming. Then, 3.9 billion years ago, the substance underwent a strong impact, leaving numerous cracks. An even more powerful blow 16 million years ago threw it from the surface of Mars into space, where it was located before meeting the Earth. And finally, 13 thousand years ago, a meteorite fell on the ice of Antarctica, where it remained to this day.

Rice. 4. Meteorite ALH 84001 of Martian origin.

Rice. 4. The ALH 84001 meteorite is of Martian origin, including, as some scientists believe, fossilized waste products of Martian bacteria.

But this is not the most interesting thing: after 1.5 years of research, a group of American scientists reported in August 1996 that this meteorite may contain ancient fossils of unearthly biological origin. Near the surface of the meteorite, many oval formations were discovered, similar to fossilized colonies of the oldest terrestrial bacteria. But their size (10-100 nm) is 100-1000 times smaller than that of typical terrestrial bacteria.

For several years, this meteorite was carefully studied by specialists from various sciences. Many arguments have appeared both for and against the “biological” hypothesis (see more in the chapter “Mars”). These studies forced scientists to take a fresh look at the idea of ​​panspermia (the spread of microscopic embryos of life throughout the Universe), which had been criticized for many years. Maybe meteorites are the very carriers of life that brought it from somewhere to Earth?

About unresolved problems

Discussions are still ongoing about the correspondence of meteorites of different classes to asteroids of different types. In particular, why the optical characteristics of the most numerous S-type asteroids do not coincide with the same characteristics of the most frequently falling chondrites to Earth.

But most importantly, the celestial-mechanical problem of transporting matter from the asteroid belt to the Earth’s orbit has not yet been confidently solved. It is believed that the most likely sources of meteorites are asteroids approaching the Earth - Atonians, Apollonians and Amurians (see chapter "Asteroids"). However, they are all small: the largest of them, 1036 Ganymede and 433 Eros, have average diameters of 38.5 and 22 km. In general, the population of near-Earth asteroids has not yet been studied enough to consider them the main source of meteorite matter.

The direct study of planets and asteroids by space probes, which has begun today, will make it possible to link their properties with the properties of meteorites studied in detail in the laboratory. This will make meteorites an even more valuable witness to the history of our planetary system, and perhaps other worlds.

BIBLIOGRAPHY:

Rozhansky I.D. Anaxagoras. M: Nauka, 1972

Getman V.S. Grandchildren of the Sun. M: Nauka, 1989.

Fleisher M. Dictionary of mineral species. M: "Mir", 1990, 204 p.

Simonenko A.N. Meteorites are fragments of asteroids. M: Nauka, 1979.