About stones from the sky
At one time, the French Academy of Sciences, having studied the evidence of stones falling from the sky and the stones themselves, decided: stones cannot fall from the sky, because many museums have nowhere to come from, and, not wanting to be ridiculed for superstition, they hastened to get rid of the collections of heavenly stones . But the facts are stubborn things, 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 desired object for study. After all, it was then the only way to look beyond the boundaries of the earth's atmosphere.
Thunder stones and Lavoisier
Traditions about terrible heavenly signs, accompanied by the roar and falling of stones from the sky, have been known since antiquity. Perhaps the earliest documented descriptions of meteorite fall are in the Chinese annals: in 616 BC, ten people were killed in stone rain, and another similar event, which also led to deaths, occurred in 588 BC. There is evidence of the fall of meteorites in the Old Testament, and in the Mahabharata, and in Russian chronicles.
And in 1768 the very event occurred that made scientists pay attention to meteorites. Stone from the sky (then scientists often called them aerolites) fell in the evening of September 13 in a field near the town of Luce. Fell with a roar and whistle, buried in the soft plowed earth and was very hot. The peasants who witnessed the fall fled 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, and the society needed a scientific explanation what happened.
To understand what happened, the French Academy of Sciences commissioned a commission of three scientists - Claude Louis Cadet, a chemist-pharmacist, Olivier Fougerot, a mineralogist - and then not so famous Antoine Lavoisier. And, after a careful study of the stone and the questioning of witnesses, they had no choice but to shrug. And Lavoisier presented a report in which the possibility that this stone fell from the sky was completely rejected. After all, ideas about the firmament of heaven have long been discarded by science. Two options remained: either the stone condensed in the atmosphere, or was thrown out by a distant volcano and collapsed to the ground. And both options were considered impossible. And the fact that stones just can fly in space, scientists didn’t even guess, but space itself seemed more like a philosophical category than a place in which stones can fly. Moreover, the very hypothesis about the “heavenly origin” of the stone was based only on confused testimonies of exalted and poorly educated witnesses of the miracle, who perceived it as a divine phenomenon. That is, everything was too much like superstition. And the French Academy of Sciences considered superstition to be a duty to fight.
They can be understood. After all, in addition to reports of meteorites, they received the following:
In 1123, under the reign of Henry 1, as Geoffroa de Breuil writes, an aircraft resembling a sea ship appeared over London and anchored in the center of the English capital. People walked down the rope ladder. Londoners, considering them the messengers of the devil, drowned the aliens in the Thames. Those remaining on the ship chopped off the rope and flew away. For many years, the church in Bristol, if you follow the chronicle, had on its doors a unique lattice made of a "heavenly anchor". He was lowered on a rope from an “airship” in 1214 during a religious holiday and firmly caught on a pile of stones.
The myth is widespread that for many years this report banned the study of meteorites and discussion of their extraterrestrial origin. In fact, this is not so, and this report itself did not have the character of a historical document and was published only in 1772 with a comment by J. de Fouchi, the "permanent secretary" of the Academy of Sciences, on the need for further investigation of such phenomena. Meteorites, meanwhile, continued to fall, and by the time the Czech scientist Ernest Hladni (or Hladny, whoever likes it), who became the “father of meteorics,” had begun to fall, several meteorites had fallen in Europe.
Hladni, being not a naturalist, but a lawyer, took up this matter as a detective, and began methodically interviewing hundreds of witnesses, comparing their testimonies. There was a lot of fiction in them, involuntary repainting, thinking of a picture, outright lies and driving by the nose. But among all this garbage there were parts that invariably coincided. And these details did not allow to doubt: the stones really fell from the sky. And he clearly established a connection between the fall of stones and fireballs. By that time, something was already known about the fireballs. On March 19, 1718, a bright car drove over London and this flight was observed by Halley - the very one whose name the comet is named after. 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 large. All this said:
The last prejudice against meteorites was dispelled by the study of meteorite rain that occurred on April 26, 1803 near the city of Aigle in Normandy, 160 km from Paris. This was done by Jean-Baptiste Bio, 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 have proved the full evidence of the most unusual phenomenon that has ever been observed by people ... It will take great scientific achievements for in order to properly investigate this phenomenon, for which we have no satisfactory explanation; only a person rich in knowledge is capable of such courage. With all the dubious questions of the ignoramus, they are ready to blindly believe, half-educated people decide 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, a meteorite is flying!”, Read that craters on the moon were formed as a result of meteorites falling on the moon, etc. However, a meteorite is a body of cosmic origin that has fallen to the surface of a large celestial object. This is the result of an already falling fall .
The body itself, before it fell to the Earth (or another planet, satellite, asteroid or planetoid) is called a meteoroid . And the atmospheric phenomenon caused by the passage of a meteoroid through it is called a meteor , if it has the character of a “shooting star” or a carif this is a larger-scale phenomenon that looks like a dazzling fireball with a smoke plume, often accompanied by a whistle, a roar and other sound phenomena.
And a meteorite is that which fell and was preserved as an independent solid. By the way, a meteorite fall is a phenomenon characteristic only of planets that have 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 the meteoroid and the planet during a collision is always quite large and almost always exceeds 10 km / s. And at this speed, the kinetic energy of a kilogram body is 50 MJ, which is several times higher than the heat of vaporization of iron. So, as a result of a collision with the surface of an atmosphereless planet or satellite, the meteoroid completely ceases to exist - the energy converted into heat during the collision evaporates it and a number of target rocks. The expansion of the superheated highly compressed steam formed during this is a powerful explosion, the result of which is the formation of an impact crater . And the former substance of the meteorite, having condensed in the form of the smallest dust particles, falls around.
In fact, the minimum rate of incidence of a meteoroid on the Moon is 2.4 km / s (second space), and on smaller planets it is even less. At this rate of energy release is not enough for the complete evaporation of the meteorite. Nevertheless, 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 onto a planet with the atmosphere, the picture completely changes. Air molecules, whose speed relative to the meteorite corresponds to a temperature of tens and hundreds of thousands of Kelvin, bombard the surface of the body, transferring its energy to it. Bouncing off of it, they rush towards the oncoming flow, compressing it and transforming the energy of the translational motion of molecules into the energy of chaotic, thermal. A shock wave is formed in which air is heated to monstrous temperatures, turning into a plasma. Its radiation heats the surface of the meteoroid, melting it and converting it into steam, which is immediately carried away by the oncoming stream, along with drops of melt. This process is called ablation., leads to intense ablation of the substance from the surface of the meteorite. Due to the very high temperature, air and meteorite vapors are largely ionized, that is, they are plasma.
As a result of ablation, a small meteoroid simply completely evaporates. The larger body does not have time to evaporate and its remainder, often not exceeding several percent of the initial 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 Earth at the feet of terrified observers (a much more often - in some wilderness, in a swamp, in the ocean or on the ice of Antarctica).
But the meteorite is subjected not only to ablation, but also to the colossal aerodynamic forces of the incoming superhigh-speed stream. They can withstand only very durable objects. Otherwise, it turns out what an interesting thing. The cosmic rock is falling apart, but this does not reduce the loads acting on its debris. On the contrary, they grow together with an increase in atmospheric density, so that the crushing continues and acquires an avalanche-like character: instead of the initial body, we now have a swarm of increasingly crushed fragments with an avalanche-like increasing frontal 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 approximately the same as during the formation of a meteorite crater: this thermal energy turns all the meteorite material into a hot plasma, the expansion of which is an explosion, the power of which is sometimes comparable to or even exceeds 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, at the same time, fragile body, it might have been an icy cometary nucleus or loose carbonaceous chondrite, and possessed the kinetic energy of the same order that was released during the explosion of Tsar-Bomba (or “Kuzkin’s 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 that flew into the atmosphere over Chelyabinsk was smaller and more durable. Therefore, its destruction was not as complete as over the Podkamennaya Tunguska. In the case of a fall of iron meteoroids, the meteoroid fragmentation into fragments begins after a significant loss of speed and does not have time to complete with an explosion and their complete or almost complete destruction, as a result of which the fragments fall to the Earth with heavy meteor shower, similar to Sikhote-Alinsky.
Large meteoroids and asteroids do not have enough time to slow down noticeably with the Earth’s atmosphere and crash into it at space speed, which leads to a huge explosion and the formation of a meteorite crater. In this case, the meteorite material also completely turns into steam, and then into characteristic microparticles - silicate and magnetite balls. This implies the futility of the search for meteorites in impact craters, with the exception of small, meter-sized funnels caused by low-speed impacts, which are not accompanied by significant energy release and explosions.
Dimensions and masses of meteorites and meteoroids
The sizes of bodies flying from space into the Earth’s atmosphere range from tiny micron particles to asteroids with a diameter of kilometers and tens of kilometers. 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 of sand and a mass of 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 larger pebble with a fist size will become a long and bright car, which will brightly illuminate the night landscape and give distinct shadows, and a car from a watermelon-sized meteoroid will compete for a couple of seconds with the midday sun. But all these stones will not reach the ground. They burn up without a trace, turning first into a plasma, which then, cooling and condensing will become meteor dust - countless tiny balls of iron oxide and silicate glass, which represent the lion's share of the hundreds of thousands of tons of cosmic matter that fall on the earth's surface every year. Only a space alien weighing a hundred tons can already reach the Earth. Rather, its tiny remainder will fly, weighing in grams or, at best, kilograms. The loss of mass of a meteorite when flying through the Earth’s atmosphere depends on its speed and composition. its tiny residue will fly, weighing in grams or, at best, kilograms. The loss of mass of a meteorite when flying through the Earth’s atmosphere depends on its speed and composition. its tiny residue will fly, weighing in grams or, at best, kilograms. The loss of mass of a meteorite when flying through the Earth’s atmosphere depends on its speed and composition.
The largest meteorites found on Earth weigh tens of tons. In the first place is the Goba iron meteorite weighing 60 tons, found in Namibia (pictured below). In general, the largest meteorites are usually iron. Only strong iron is able to withstand not only the strongest thermal effects, but also terrible mechanical stresses that tear apart a meteorite in flight, as happened with the Sikhote-Alin meteorite, whose total mass of debris reaches 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 by the time of collision with the Earth will maintain cosmic velocity, forming a shock crater.
Iron and stone guests
Even a layman will be able to divide all meteorites into two large groups - iron and stone meteorites. Iron meteorites have been known since ancient times - they not only differ sharply from earthly stones, but also have a completely utilitarian value: meteorite iron was the first metal that man held in his hands. They also last longer and are easier to find.
Iron meteorites are mainly an alloy of two metals - iron and nickel with small impurities of other elements. They form two minerals - kamasit, containing 5-6% nickel and tenite, rich in it (from 20% up to pure nickel). Kamasit is airon, and theite is phase - austenite. There is also a tetragonal modification of tenit - tetrathenite (kamasit and tenit - cubic). Kamasite and tenite do not dissolve in each other below 500 ° C, and the extremely slow cooling of intermediate-composition iron from higher temperatures has led to the formation of a characteristic structure in the form of crystals of these two crystalline phases mutually sprouting into each other. These are the so-called Widmannstetten figures (Wikipedia photo), which become clearly visible on the polished and polished surface of most iron meteorites after etching. And you know what is immensely surprising? The fact that on a section of an iron meteorite of a meter size the Widmannstetten figures often maintain a single orientation! This means that before they formed, this whole gigantic piece of iron was ... a single crystal. Or maybe the whole iron asteroid? Widmannstetten figures are characteristic only of meteorites and are not reproduced in any artificial sample of iron.
Meteorites in which the Widmannstetten figures are observed are called octahedrites. They contain 7-15% nickel. There are iron meteorites that do not give a characteristic picture during etching - they are richer in nickel than the octahedrites and the kamaite-tenite decay structure in them has microscopic scales or is completely absent. With a lower nickel content than octahedrons, meteorites are composed entirely of kamasite and are called hexahedrites.
But most of the falling meteorites are stone.
The structure of most stone meteorites is also unusual and does not resemble terrestrial rocks. A characteristic element of their structure is the so-called chondras (from which this type of meteorites was called chondrites), rounded formations of a fraction of a millimeter (sometimes several millimeters) in size, which are frozen drops of a silicate melt and immersed in a fine-grained fine-grained mass, composition ( in particular, a high water content) and the structure of which indicates that it was not subjected to significant heating and melting. Their chemical composition is also unusual: they usually contain metallic iron and other native metals that are not found in terrestrial conditions. The age of chondrites, determined by radioisotope dating is more than 4.5 billion years, that is, the same as that of the Sun,
The most interesting are the so-called carbonaceous chondrites. They contain a lot of water (of course, in bound form, in the form of hydrosilicates), a large amount of carbon in the form of soot and ... organic compounds. Apparently, these meteorites are almost unchanged primary substance of the protoplanetary disk, from which the solar system was formed. Ordinary and enstatite chondrites, in comparison with them, underwent significantly stronger heating during their “life”, 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, thrown out as a result of collisions of asteroids with them.
The Martian origin of 34 samples of achondrites is established. In connection with one of the Martian meteorites, which is known under the number ALH 84001, disputes in the scientific community still do not subside. This meteorite has two features. First - it is a sample of rocks of the "wet Mars" era. The second - strange structures resembling petrified biological objects were 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 earthly part of the history of this meteorite. However, these “fossilized bacteria” have nothing to do with terrestrial microorganisms: their sizes are too small for any terrestrial cellular life forms.
Stone meteorites most often contain a certain amount of native iron. If its content is high, then such meteorites are called iron-stone. Maybe not the most interesting from a scientific point of view, but certainly the most beautiful of them are pallasites. In them, large, often transparent, olivine crystals are immersed in an iron mass. 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 melting crust and the characteristic shape and surface sculpture associated with its aerodynamic treatment. The raging flame of the high-temperature plasma surrounding the meteorite heats its surface to 1600-3000 ° C, it melts and boils, and the melt and steam are immediately blown away by the oncoming flow. The whole process lasts literally seconds, so the inside of the meteorite practically does not have time to heat up (especially for stone meteorites, whose thermal conductivity is small), and the thickness of the molten layer does not exceed a millimeter. This layer then turns into a melting crust. Usually it is smooth and black, often matte, sometimes shiny. In stone meteorites, it is a silicate glass, painted with iron in dark color, in iron - consists of iron and nickel oxides. In fossil meteorites, the crust is usually oxidized, acquiring a reddish-brown color, and is often lost on part or all of the surface.
Ablation during flight in the atmosphere sometimes gives the meteorite the characteristic shape of a cone with furrows radiantly diverging from the top. 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 - regmaglipts. They look like dents made by fingers in soft clay or plasticine, and, like the melting crust, are a very characteristic sign of the meteorite origin of a piece of iron.
Meteorites and pseudo-meteorites
Despite the fact that the fall of meteorites is not such a rare event on a global scale, not everyone was lucky enough to observe it. And even fewer people were lucky enough to find a fallen meteorite. And, as with all rarity, meteorites are hunted not only by scientists, but also by various kinds of businessmen and speculators, and even simply scammers.
Meteorites are a very expensive commodity. For a small, a few grams, fragment of the by no means rare Sikhote-Alin meteorite, the total weight of the fragments of which is tens of tons, they ask for several thousand rubles on semiprecious ruins, for which together with the meteorite they will give a beautiful, but without any legal force certificate of what is it reallymeteorite. And the price requested by them for fragments of the recently fallen meteorite Chelyabinsk is already going over the roof for tens of thousands of rubles, and the total number of these fragments that are being fired by gullible seekers of rarities clearly exceeds a reasonable amount.
And here they come on the scene - pseudo-meteorites.
For a meteorite take (or try to give out) a variety of things. Slag of various kinds, splashes of metal from electric welding, metallurgical waste, aluminum re-melted in a fire, molten pieces of brick and even pieces of anthracite that were not completely burnt in the furnace, pretending to be rare carbonaceous chondrite.
Sometimes there are really difficult cases. Achondrite is often distinguished from a piece of terrestrial rock only by melting and thincompositional features (including isotopic composition). But most meteorites and pseudo-meteorites are recognized by specialists literally at first sight.
Here are signs that it’s probably 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 (palassites are such an exception);
- it has signs of deep penetration, consists wholly or partially of a glassy substance (with the exception of tektites, which, while not being meteorites in the full sense, are still meteorites);
- inside it there are pores and shells. Meteorites, as a rule, do not contain pores;
- meteorites never give a red or brown line to unglazed porcelain (with the exception of the weathered melting crust);
- unequivocally rejects the meteorite origin of the presence of signs of sedimentary rock - stratification, carbonate minerals (calcite, etc.) in the composition, fossils.
And here are the signs that indicate that you have a real meteorite in your hands.
The un-weathering melting crust of real stone meteorites is usually thin and black, even if the rock under it is light. Melted industrial waste is often the opposite, covered with a light melting crust, the thickness of which sometimes reaches several millimeters.
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 spangles. It is easy to distinguish these spangles from sulfides (pyrite, chalcopyrite, galena, etc.) that are often found in terrestrial rocks by their magnetism and ductility.
Iron meteorites and iron in stone and iron-stone meteorites always contain a lot of nickel, and industrial metallurgical waste (except stainless steel) and shell fragments often mistaken for iron meteorites practically do not contain it. The simplest way to detect it, which does not require any equipment at all, is just a few drops of two reagents - a solution of dimethylglyoxime in alcohol and a solution of ammonia (pharmacy ammonia). Apply a drop of ammonia solution to the surface to be examined and after a few minutes transfer it to a piece of filter paper, and drop it with a solution of dimethylglyoxime. Nickel will find itself pink or red.
But for the exact establishment of meteorite origin, it is advisable to seek help from a specialized scientific institution. In Moscow, it’s GEOCHI RAS. Expertise meteorites engaged Anna Yakovlevna Skrypnik from the laboratory of meteorics. As far as I am aware of the current situation, the examination is carried out free of charge, cutting off 20% of each received sample of this meteorite to replenish the collection. You can send a sample for examination by mail (without declared value) or bring it personally after an appointment. By the way, “meteorite hunters” simply hate our institute: here they (as well as collectors deceived by them) immediately explain why this stone or piece of iron is not a meteorite.
* * *
The earthly atmosphere is not only the air we breathe, it is also a protection. Meteorites are the few that made their way through this shield. Flying through the Earth’s atmosphere for a meteorite is a serious test, and it is possible to pass it only after losing almost everything. But the little that remains, surprisingly, remains unchanged in its internal structure.
If meteorite subjects interest Habr, the following article will be about meteorite craters.