At one time, the French Academy of Sciences, having studied the evidence of stones falling from the sky and the stones themselves, decided that stones could not fall from the sky, because they had nowhere to take and many museums, not wanting to be ridiculed by superstition, rushed to get rid of the collections of heavenly stones . But the facts are a stubborn thing, stones from the sky did not obey scientists and continued to fall, and soon scientists changed their mind about meteorites, and they themselves became perhaps the most desirable object to be studied. After all, it was then the only way to look beyond the boundaries of the earth's atmosphere.
Thunder Stones and Lavoisier
The legends of the terrible signs of heaven, accompanied by the crash and fall of stones from the sky, have been known since antiquity. Perhaps the earliest documented descriptions of the fall of meteorites are found in Chinese chronicles: in 616 BC, ten people were killed by a stone rain, and another such event, which also led to the death of people, occurred in 588 BC. There is evidence of meteorites falling both in the Old Testament, in the Mahabharata, and in the Russian chronicles.
And in 1768 an event occurred that made scientists pay attention to meteorites. A stone from the sky (then scientists more often called them aerolites
) fell on the evening of September 13 in a field near the town of Lyuse. He fell with a crash and a whistle, buried in the soft plowed land and was very hot. The peasants who witnessed the fall scattered in horror, and then, returning, found the stone peacefully lying, black and cold. The phenomenon of the heavenly stone, of course, was considered a divine miracle, about which rumors and rumors immediately spread among the people, and the time was not the most suitable for them: it was the Age of Enlightenment, when such rumors were not honored, but society needed a scientific explanation happened
The French Academy of Sciences assigned commissions of three scientists — Claude Louis Cadet, a chemist-pharmacist, Olivier Fougereau, a mineralogist — and the not-so-famous Antoine Lavoisier. And, after careful study of the stone and questioning of witnesses, they had no choice but to wring their hands. And Lavoisier presented a report in which the possibility that the stone fell from the sky was completely rejected. After all, ideas about the heavenly firmament have long been rejected by science. There were two options: either the stone condensed in the atmosphere, or was thrown away by a distant volcano and fell to the ground. And both options were deemed impossible. But the fact that stones just can fly in space, scientists are not that they did not guess, but space itself seemed more like a philosophical category than a place where stones can fly. In addition, the very hypothesis of the “heavenly origin” of the stone was based only on the confused testimony of exalted and poorly educated witnesses to the miracle, who perceived it only as a divine phenomenon. That is, everything was too much like superstition. And with the superstitions, the French Academy of Sciences considered it their duty to fight.
It is a myth that this report imposed a ban on the study of meteorites and discussion of their extraterrestrial origin for many years. In fact, this is not the case, and this report itself did not have the character of a historical document and was published only in 1772 with the comment of the “permanent secretary” of the Academy of Sciences J. de Fouci about the need for further research on such phenomena. Meteorites, meanwhile, continued to fall, and by the time the Czech scientist Ernest Hladni (or Chladny, as someone who likes it more), who became the “father of meteoritics”, undertook to study them, several meteorites had time to fall in Europe.
Kladni, being not a naturalist, but a lawyer, took up this case as a detective, and began methodically to interview hundreds of witnesses, comparing their testimony.They had a lot of fiction, involuntary beatings, articulating pictures, outright lies and driving by the nose. But in the midst of all this rubbish there were details that invariably coincided. And these details did not allow to doubt: the stones really fell from the sky. And he clearly established the connection of falling stones with fireballs. By that time something was already known about the race cars. March 19, 1718 a bright race car raced over London and this flight was watched by Halley - the same one whose name is called a comet. And these observations were not only descriptive - they were able to determine the distance to the car and the height of its flight, which turned out to be very great. It all said: the stones come from outside the earth's atmosphere.
Recent prejudices against meteorites were dispelled by a study of meteorite rain, which occurred on April 26, 1803 near the city of Aigle in Normandy, 160 km from Paris. This was done by Jean-Baptiste Biot, who confirmed the cosmic origin of the fallen stones.
I wanted to be an outside witness, not biased in my opinion, and tried to state the facts as they were without giving any hypotheses ... I hope that I proved the obviousness of the most unusual phenomenon that people have ever seen ... the achievements of science in order to properly investigate this phenomenon, regarding which we do not have a satisfactory explanation; only a knowledgeable person is capable of such courage. With all the dubious questions, the ignorant are ready to blindly believe, dropouts solve everything, and only true scientists are able to investigate everything.
Meteoroids, meteors, meteorites
The word meteorite
is known to everyone, but is often used incorrectly. You can hear the phrase “Look, the meteor is flying!”, Read that craters on the Moon were formed as a result of meteorites falling to the Moon, etc. However, a meteorite is a body of cosmic origin that has fallen on the surface of a large celestial object.
. This is the result of a fall that has already taken place
The body itself, before it hit the Earth (or another planet, satellite, asteroid or planetoid) is called meteoroid
. And the atmospheric phenomenon caused by a meteoroid passing through it is called meteor
if it has the character of a "falling star" or bolide
if it is a larger phenomenon that looks like a blinding fireball with smoke plume, often accompanied by a whistle, roar and other sound phenomena.
And the meteorite is that which fell and remained as an independent solid body. By the way, the fall of a meteorite is a phenomenon characteristic only of planets with a fairly dense atmosphere and, for example, there are no meteorites on the Moon, despite the abundance of craters. The fact is that the mutual speed of a meteoroid and a planet in a collision is always quite large and almost always exceeds 10 km/s. And at such a speed, the kinetic energy of a kilogram body is 50 MJ, which is several times higher than the heat of evaporation of iron. So, as a result of a collision with the surface of a atmosphereless planet or satellite, the meteoroid completely ceases to exist - the energy that has become heat during the collision evaporates it and a certain amount of target rock
. The expansion of the superheated highly compressed vapor formed during this process is a powerful explosion, the result of which is the formation of an shock crater
And the former meteorite substance, condensing in the form of tiny dust particles, falls around.
In fact, the minimum speed of a meteoroid falling to the moon is 2.4 km/s (second cosmic), and on small planets it is even less. At this rate of energy release is not enough for complete evaporation of the meteorite. However, the probability of falling at such a speed is extremely small, so the chances of finding a meteorite on the surface of the moon or an asteroid are small.
When falling on a planet with an atmosphere, the picture changes completely. Air molecules, whose speed relative to a meteorite corresponds to a temperature of tens and hundreds of thousands of kelvin, bombard the surface of the body, transferring its energy to it, and, bouncing off it, rush towards the oncoming flow, compressing it and transforming the energy of translational motion of molecules into chaotic, thermal energy. A shock wave is formed, in which air heats up to monstrous temperatures, turning into a plasma. Its radiation warms the surface of the meteoroid, melting it and converting it to steam, which is immediately carried off by the oncoming flow, along with drops of melt. This process, which is called ablation
, leads to an intense ablation of matter from the surface of a meteorite. Due to the very high temperature, the air and meteorite vapors are largely ionized, that is, they represent a plasma.
As a result of ablation, a small meteoroid just completely evaporates. A larger body does not have time to evaporate and its residue, often not exceeding several percent of the original mass, has time to lose speed to “safe”, after which it loses it in dense layers up to the speed of free fall and falls to the ground at the feet of frightened observers ( much more often - in some wilderness, in a swamp, in the ocean or on the ice of Antarctica).
But the meteorite is exposed not only to ablation, but also to the colossal aerodynamic forces of the oncoming superfast velocity. Only very strong objects can withstand them. Otherwise, it turns out that's an interesting thing. The space stone is falling apart, but this does not reduce the loads acting on its fragments. On the contrary, they grow together with an increase in the density of the atmosphere, so the fragmentation continues and acquires an avalanche-like character: instead of the original body, we now have a swarm of increasingly shredded debris with an avalanche-like area of drag. As a result, the whole swarm at once, in a short time, is completely inhibited and its kinetic energy goes into heat.
The result is about the same as when a meteorite crater was formed: this thermal energy turns all meteorite matter into a hot plasma, the expansion of which is an explosion, the power of which is sometimes comparable to or even surpasses nuclear explosions. Only this explosion is not ground, but air. Apparently, such an explosion occurred during the famous Tunguska event of 1908. A rather large and fragile body, perhaps, was an ice cometary nucleus or loose carbonaceous chondrite, and possessed kinetic energy of the same order as emitted during the explosion of Tsar-Bomba (or Kuzkina Mother). It did not reach the earth’s surface, but having collapsed like an avalanche, it released this energy in the form of an air explosion, the equal of which mankind had not seen for half a century. The explosion of the Chelyabinsk meteorite has a similar nature. The difference between them is that the meteoroid entering the atmosphere over Chelyabinsk was smaller and more robust. Therefore, its destruction was not as complete as it was over Podkamennaya Tunguska. In the case of the fall of iron meteoroids, the crushing of a meteoroid into fragments begins after a significant loss of speed and does not have time to end with an explosion and their complete or almost complete destruction, as a result of which fragments fall to Earth with abundant meteoric rain like Sikhote-Alinsky.
Large meteoroids and asteroids do not have time to significantly slow down the Earth's atmosphere and crash into it with cosmic speed, which leads to a huge explosion and the formation of a meteorite crater. In this case, the meteoritic substance is also completely converted into steam, and then into characteristic microparticles, silicate and magnetite beads.This results in the futility of searching for meteorites in impact craters, with the exception of small, meter-sized craters caused by low-speed impacts, which are not accompanied by significant energy release and explosions.
Dimensions and mass of meteorites and meteoroids
The size of bodies flying from space into the earth’s atmosphere varies from tiny micron particles to asteroids across and tens of kilometers across. Absolutely microscopic particles, invading the atmosphere, do not produce any visible effects, but contribute to the ionization of its upper layers, and larger micrometeoroids, the size of a grain and milligrams, burn in the upper layers of the atmosphere, brightly flashing at altitudes of 80-120 km, which is observed as meteors - "shooting stars". A pebble bigger, with a fist size, will become a long and bright fireball, which will brightly illuminate the night landscape and give distinct shadows, while the meteoroid fireball will already compete for a couple of seconds with the noonday sun. But all these stones will not reach the ground. They burn without residue, turning first into plasma, which then, cooled and condensed, will become meteoric dust - countless tiny balls of iron oxide and silicate glass, which represents the lion’s share of the hundreds of thousands of tons of cosmic matter that falls on the earth's surface every year. Only a space alien with a mass of a hundred tons can already reach the Earth. Rather, it will reach its tiny residue, weighing in grams, or at best - kilograms. The loss of mass of a meteorite during its passage through the Earth’s atmosphere depends on its speed and composition.
The largest meteorites found on Earth weigh tens of tons. The first place is the iron meteorite Goba weighing 60 tons, found in Namibia (pictured below). In general, the largest meteorites, as a rule, are iron. Only strong iron is able to withstand not only the strongest thermal effects, but also terrible mechanical stresses tearing a meteorite in flight, as happened with the Sikhote-Alin meteorite, the total mass of which fragments found reach 27 tons.
The mass of meteorites is limited from above not only by their fragmentation in flight, but also by the fact that a large body does not have time to slow down in the atmosphere and will retain cosmic velocity by the time of collision with the Earth.
Iron and Stone Guests
Even a non-expert can divide all meteorites into two large groups - iron and stone meteorites. Iron meteorites are known from ancient times - they are not only very different from earthly stones, but also have a completely utilitarian meaning: meteorite iron was the first metal that was held in the hands of man. They also remain longer, and they are easier to find.
Iron meteorites are mainly a fusion of two metals - iron and nickel with small admixtures of other elements. They form two minerals - kamacite, containing 5-6% nickel and tenite, rich in them (from 20% up to pure nickel). Kamasit is
is iron, and the tenite is
-phase - austenite. There is also a tetragonal modification of the tenite - tetratenite (kamasite and tenite - cubic). Kamasit and tenite do not dissolve in each other below 500 ° C, and the extremely slow cooling of intermediate-grade iron from higher temperatures led to the formation of a characteristic structure in the form of mutually germinating each other crystals of these two crystalline phases.These are the so-called Vidmanshtettenovy figures (photo from Wikipedia), which become clearly visible on the polished and polished surface of most iron meteorites after its etching. And you know that is incredibly surprising? The fact that at the cut of an iron meteorite of meter dimensions Vidmanshtetenovye figures often retain a single orientation! This means that prior to their formation, this whole giant piece of iron was a ... single crystal. And maybe the whole iron asteroid? Widmannstatten figures are characteristic only for meteorites and are not reproduced in any artificial sample of iron.
The meteorites in which the Widmanstätten figures are observed are called octahedrites. They contain 7-15% nickel. There are iron meteorites that do not give a characteristic pattern during etching — they are richer in nickel than octahedrons and the kamasite – tenitic structure of decay in them has microscopic scales or is completely absent. With a lower nickel content than in octahedra, meteorites consist entirely of kamasite and are called hexahedrites.
But most of the meteorites falling are stone.
The structure of most stone meteorites is also unusual and not similar to terrestrial rocks. A characteristic element of their structure is the so-called chondras (from which this type of meteorites got its name - chondrites), rounded formations of a millimeter (sometimes several millimeters) in size, which are frozen drops of silicate melt and immersed in the fine-grained, close-grained mass, composition ( in particular, the high water content) and the structure of which suggests that it has not undergone significant heating and melting. Their chemical composition is also unusual: they usually contain metallic iron and other native metals that are not found under terrestrial conditions. The age of chondrites, determined by radioisotope dating, is more than 4.5 billion years old, that is, it is the same as that of the Sun, that is, these are samples of the oldest substance that can be held in hands.
The most interesting are the so-called carbonaceous chondrites. They contain a lot of water (of course, in a bound form, in the form of hydrosilicates), a large amount of carbon in the form of soot and ... organic compounds. Apparently, these meteorites are practically unchanged primary matter of the protoplanetary disk, from which the solar system was formed. Ordinary and enstatite chondrites compared with them were subjected during their “life” to much more intense heating, which changed their structure.
Some chondra stone meteorites do not contain and are similar in structure to ordinary rocks, which indicates that in their history they were heated to full melting. Such meteorites are called achondrites. At least some of them are fragments of the rocks of the Moon and Mars, ejected as a result of asteroids colliding with them.
Established Martian origin of 34 samples of achondrites. In connection with one of the Martian meteorites, which is known under the number ALH 84001, disputes still remain in the scientific community. This meteorite has two features. The first is that it is a sample of rocks from the epoch of “wet Mars”. The second is that strange structures resembling fossilized biological objects are found in it. And it turned out that they contain traces of organic matter! However, it is still not possible to establish: whether these structures are not traces of the Martian, but the terrestrial part of the history of this meteorite. However, these “fossilized bacteria” have nothing in common with terrestrial microorganisms: their sizes are too small for any terrestrial cellular life forms.
Stone meteorites often contain some amount of native iron. If its content is large, then such meteorites are called ironstone. Maybe not the most interesting from a scientific point of view, but certainly the most beautiful of them are Pallasites. Large, often transparent olivine crystals are immersed in them in the mass of iron.Some pallasites also contain pyroxene crystals.
In addition to the unusual composition and structure, all terrestrial meteorites have two features associated with the passage through the atmosphere. This is the crust of melting and the characteristic shape and sculpture of the surface associated with its aerodynamic processing. The raging flame of high-temperature plasma surrounding a meteorite warms its surface to 1600–3000 ° C, it melts and boils, and the melt and steam are immediately blown away by the incoming flow. This whole process lasts literally seconds, so the internal part of the meteorite practically does not have time to heat up (especially in stone meteorites, whose thermal conductivity is small), and the thickness of the molten layer does not exceed a millimeter. This layer is then transformed into the melting crust. Usually it is smooth and black, often matte, sometimes shiny. In stone meteorites, it is a silicate glass, painted with iron in a dark color, in iron - it consists of oxides of iron and nickel. In fossil meteorites, the crust is usually oxidized, acquiring a red-brown color, and is often lost on part or all of the surface.
Ablation while flying in the atmosphere sometimes gives a meteorite the characteristic shape of a cone with fissures radiating radially from the summit. In other cases, the shape of the meteorite remains irregular, and the predominant blowing of the melt from the depressions leads to the appearance of a characteristic cellular structure on the surface — regmaglypt. They look like dents made with fingers in soft clay or plasticine, and, like melting bark, are a very characteristic feature of the meteorite origin of a piece of iron.
Meteorites and Pseudo-Meteorites
Despite the fact that the fall of meteorites - not such a rare event on a global scale, not everyone was lucky enough to watch it. And even fewer people were lucky to find a fallen meteorite. And, as for any rarity, not only scientists hunt for meteorites, but also businessmen and speculators of various kinds, and even fraudsters.
Their meteorites are a very expensive commodity. For a small, a few grams, fragment of a not rare Sikhote-Alin meteorite, the total weight of which fragments is tens of tons, several thousand rubles are asked for on the semiprecious ruins, for which, together with the meteorite, they will give a beautiful, but not having any legal force, that this is really
a meteorite. And the price they have requested for fragments of a recently fallen meteorite Chelyabinsk is already off scale for tens of thousands of rubles, and the total number of these fragments, which are used by credulous rarities seekers, clearly exceeds a reasonable amount.
And here they go on the scene - they are pseudo-meteorites.
For a meteorite take (or try to give) a variety of things things. Slag of various kinds, splashes of metal from electric welding, metallurgical wastes, aluminum melted in a bonfire, melted brick fragments, and even pieces of anthracite that were not completely burnt in a furnace, issued for the rarest carbonaceous chondrite.
Sometimes there are really difficult cases. Achondrite can only be distinguished from a piece of terrestrial rock by the melting crust and thin
features of the composition (including isotopic), but most meteorites and pseudometeorites are recognized by experts from the very first sight.
Here are signs that surely is not a meteorite:
- "Meteorite" is a coarse-grained rock like granite.The vast majority of stone meteorites have a fine-grained structure, sometimes with small fragments, cemented by compressed dust, but only a few types contain large crystals of non-metallic minerals (this is the exception of palassites);
- it has signs of deep penetration, consists entirely or partially of vitreous matter (with the exception of tektites, which, not being meteorites in the full sense, still represent meteoritic matter);
- inside it there are pores and shells. Meteorites, as a rule, do not contain pores;
- meteorites never give a red or brown line on unglazed porcelain (with the exception of weathered melting crust);
- unequivocally rejects the meteoric origin of the presence of signs of sedimentary rock - stratification, carbonate minerals (calcite, etc.) in the composition, fossils.
And here are signs that indicate that you have a real meteorite in your hands.
The un-weathered crust of real stone meteorites is usually thin and black, even if the rock is light. Melted industrial waste is often the other way around, covered with light melting bark, which sometimes reaches several millimeters thick.
Most stone meteorites contain metallic iron in one quantity or another. On the surface of the thin section this iron is clearly visible in the form of sparkles. It is easy to distinguish these glitters from sulphides that are often found in terrestrial rocks (pyrite, chalcopyrite, galena, etc.) by magnetism and malleability.
Iron meteorites and iron in the composition of stone and iron-stone meteorites always contain quite a lot of nickel, and industrial metallurgical wastes (except stainless steel) and often shells fragments taken as iron meteorites practically do not contain it. The simplest method of its detection, which does not require any equipment at all, but only a few drops of two reagents - a solution of dimethylglyoxime in alcohol and an ammonia solution (pharmacy ammonia). Apply a drop of ammonia solution to the test surface and, after a few minutes, transfer it to a piece of filter paper, and add a solution of dimethylglyoxime to the same place. Nickel finds itself pink or red.
But in order to accurately establish the meteoric origin, it is advisable to seek help from a specialized scientific institution. In Moscow it is GEOKHI RAS. Meteorites are examined by Anna Yakovlevna Skrypnyk from the meteoritics laboratory. As far as I am aware of the current situation, the examination is carried out free of charge, cutting off from each received sample of present
meteorite 20% to replenish the collection. You can send a sample for examination by mail (without declared value) or bring it in person after meeting an appointment. By the way, the “meteorite hunters” simply hate our institute: here they (like the collectors deceived by them) are immediately explained why this stone or piece of iron is not a meteorite.
* * *
The earthly atmosphere is not only the air we breathe, it is also protection. Meteorites - this is the little that broke through this shield. Flying through the earth's atmosphere for a meteorite is a serious test, and it can only be completed by losing almost everything. But the little that remains, surprisingly, keeps its internal structure unchanged.
If the meteoric subject interests Habr, the next article will be about meteorite craters.