Meteor craters on Earth and in space
Few people know that the moon is covered with craters. But not everyone knows about the fact that the Earth is also covered by craters from meteorite impacts. In this article I will talk about meteorite craters in general and on Earth - in particular.
On KDPV - Phobos.
Two hypotheses about lunar craters
In 1609, Galileo, who had just invented the telescope, directed him to the moon. The landscapes of the Moon turned out to be unlike the earthly ones: it was covered with cup-shaped depressions of various sizes, surrounded by ring mountain ranges. Galileo could not explain the nature of these formations, but gave them a name, choosing the name of the Greek wine bowl as his. Since then, they are known to us as craters .
At the end of the 18th century, Johann Schröter suggested that the craters on the Moon are the result of powerful explosive eruptions that took place there. Such an explosive eruption would not lead to the formation of a volcanic structure - a regular cone, but, on the contrary, a funnel surrounded by a shaft. Many such volcanoes are known on Earth - they are called calderas and actually somewhat resemble lunar craters.
In contrast to this hypothesis, which quickly gained generally recognized status in science, Franz von Gruytuisen in 1824 made an assumption about the meteorite origin of the craters. The weak point of this theory was that it could not explain the fact that almost all craters have the shape of a regular circle, whereas with an oblique fall the crater should have turned out to be oval and such oval craters would have prevailed. Because of this, for a long time, this theory was not popular.
Only in the first half of the 20th century, in connection with the development of ideas about the phenomena occurring during high-speed impacts (which were extremely important in the military sphere), it became clear that this weak point of the meteorite theory is imaginary. A collision at space speeds leads to an explosion during which the meteoroid and rocks of the planet’s surface at the site of the impact instantly evaporate and the system “forgets” the direction of arrival of the meteoroid. Further expansion of gases and vapors and the propagation of shock waves occur in all directions in the same way, which forms a round crater regardless of the direction of the body trajectory. In 1924, this process was first qualitatively described by the New Zealand astronomer A. Gifford, and then the theory was developed by the Soviet scientist K.P. Stanyukovich, who at the time of the first publication in 1937 was still a student.
(from the book: Khryanina L.P. Meteorite craters on Earth. M .: Nedra, 1987. C. 16.)
And interplanetary space flights hit the last nail in the hypothesis of the volcanic origin of lunar craters - it turned out that they were almost equally densely dotted with craters and Mercury, and ancient sections of the surfaces of the moons of Jupiter and Saturn, and even the tiny Martian moons Phobos and Deimos, in which it would be difficult to even imagine volcanic activity. The intensity and nature of the latter should substantially depend on the structure of the interior of the cosmic body, its mass and size, but they did not affect the density of the craters. It turned out that the reason for their appearance was not inside, but outside the planets. And this reason is a meteorite bombardment.
Meteor Craters on Earth
Moreover, meteorite craters were found not only on other planets. Ring structures similar to lunar ones were also known on Earth, and with the development of aerial and then space photographs, they began to open in dozens of them. To date, they are known to more than 160 pieces.
So, the crater in Arizona has long been known. For the first time, his geological description was made by A.E. Foot in 1891. He discovered an unusual formation, which is a depression with a diameter of 1200 meters with very steep steep slopes, surrounded by a shaft 30-65 m high. At the same time, the depth of the crater is 180 m and its bottom is much lower than the surrounding plain. But the main oddity was that there were no signs of volcanic activity in the crater - neither lava, nor tuff. One limestone, the layers of which were turned out and turned over in the reverse order on the shaft, and inside the crater are twisted, crushed, or even crushed into flour. The Indians called this funnel Devil's Canyon and found native iron in it, which they used for their own purposes, which led to suggest the meteorite origin of the funnel. A.E. Foot during his expedition found three kilometers from the crater a block of meteorite iron weighing 91 kg. In subsequent studies, a large amount of meteorite material was found in the crater - from small particles formed during steam condensation to large pieces of iron. Balloons of a heavily oxidized shell-sized cannonball are characteristic of the Arizona crater. They formed during the melting, evaporation and condensation of the meteoroid at the time of impact. The total mass of metal located in the crater, as a result of geophysical studies, was estimated at tens of thousands of tons. This (with the exception of a number of practically unchanged meteorite fragments) is a deeply molten metal that has lost the original characteristic structure of meteorite iron. In addition to him, swollen and foamed glassy material was found, resembling pumice - this glass was formed as a result of melting of the soil upon impact (a similar glass was subsequently found in places of nuclear explosions). The rocks in the crater, except those that arose after its formation (there was a lake at the bottom of it in the Pleistocene, from which there was a layer of sediment, and the age of the crater was determined from these sediments), were greatly changed as a resultshock metamorphism under the influence of shock waves, ultrahigh temperatures and pressures. All these findings undoubtedly proved the meteorite origin of the crater.
Arizona Crater is not the only and not the largest meteorite crater in size. But it belongs to the most well-preserved impact structures on Earth. Unlike craters on the Moon on Earth, they are ruthlessly destroyed by erosion, so many ancient astroblems do not look like funnels with a shaft for a long time. They are given out only by the presence of characteristic fault systems, characteristic clastic breccia rocks with signs of melting (up to complete melting and subsequent formation of a kind of igneous rock - tagamite), signs of shock metamorphism, such as high-pressure phases - stishovite, coesite, diamond, as well as specifically deformed and cracked crystals of quartz and other minerals. The so-called fracture cones are also signs of an impact event - a system of cracks in the rocks, giving the rock fragments the appearance of cones directed by the apex to the center of the crater.
Of the other well-preserved meteorite craters, I would mention the Sobolevsky crater with a diameter of 50 m in Primorye, in the vicinity of Cape Olympiad in the eastern spurs of Sikhote-Alin. Geologist V.A. discovered this crater. Yarmolyuk in the process of searching for fragments of the Sikhote-Alin meteorite immediately after its fall. The crater was investigated using seismic exploration and it turned out that with its small size, its structure is surprisingly similar to larger craters. The most interesting is that this crater formed less than 1000 years ago (probably no more than 250-300 years ago), and in addition to rocks metamorphosed by the shock wave, numerous organic remains were found in it - blades of grass, slivers of wood converted by a high-temperature pulse and pressure in glassy carbon - fusen (interesting to find a chip of cedar, which partially turned into ordinary soft charcoal, and its other part into fusen). The presence of explosive conditions in the Sobolevsky crater is evidenced by numerous finds of silicate glasses, the drops of which reach a millimeter. Numerous iron-nickel balls were also found - the remnants of a meteorite substance evaporated upon impact.
Currently, the Sobolevsky crater, unfortunately, is undergoing gradual destruction by miners - in contrast to such well-known objects that are considered unique natural monuments and carefully protected from destruction - the Ries craters (Germany), Wolf Creek (Australia), the above-described Arizona and many others.
From craters formed during the explosive braking of high-speed bodies (even such small ones as Sobolevsky), one should distinguish funnels formed during low-speed incidence of large meteorites and their fragments that have lost cosmic velocity in the atmosphere. Explosion, evaporation of the meteorite and target rocks are not observed in such cases, and such craters often acquire an oval or even elongated shape due to an incessant fall. In such craters there are practically no signs of impact metamorphism - only occasionally characteristic fracturing and fracture cones are observed, the formation of allogeneic (formed by debris ejected from their place by the impact) and authigenic (remaining in the place of impact) impact breccias and mountain flour. It is such craters that were found at the site of the fall of large fragments of the Sikhote-Alin meteorite. Their dimensions are always small and do not exceed the first tens of meters. Despite the fact that no explosion occurs during the formation of such craters, microscopic signs of melting of the target rocks can sometimes be detected - in the form of tiny silicate glassy balls, which, in particular, are found in the largest craters of the Sikhote-Alin crater field.
In large impact structures, the dimensions of which are measured in tens and hundreds of kilometers, the characteristic signs of meteorite origin become especially striking. The rocks melted upon impact form lava lakes; after cooling, they form tagamite stratiform bodies ; the fault systems formed upon impact go deep into the lithosphere and generate secondary hydrothermal processes. There are two important differences between impact structures and volcanic ones: the surface character and very high temperatures achieved in impact melts compared to magma of terrestrial origin. This is manifested in the wide distribution of cristobalite, crystallizing from 1700 ° C and tridymite with a crystallization temperature of 1450 ° C, which are rare in igneous rocks.
Large shock structures are characterized by the formation of a central uplift (“central hill”) due to the release of stresses arising from shock deformation, and some structures of hundreds of kilometers are characterized by a multi-ring structure. Such multi-ring structures are well known on the Moon and their existence was considered an argument against the meteorite origin of the craters - it was believed that for this several meteorites would have to fall, which is unlikely. However, a more thorough examination of the processes of propagation of shock waves and the subsequent discharge of deformations showed that the formation of multi-ring structures is associated with this process. The formation of such structures on a small scale was observed in artificial craters after nuclear explosions.
The largest impact structures found on Earth are hundreds of kilometers in size. So, the famous Chiksulub crater on the Yucatan Peninsula, formed just at the boundary of the Cretaceous and Paleogene (when the dinosaurs became extinct), has a diameter of 180 km. There are no visual signs of this crater on the ground - it was discovered by arched geophysical anomalies, and its meteorite origin was proved by the discovery of impactites - shock partially molten breccias ( zuvites) A global geochemical anomaly, the iridium peak, is also associated with this crater. The iridium content in the layer corresponding to the boundary between the Cretaceous and the Paleogene, worldwide is ten times higher than usual, due to the evaporation of a huge amount of meteorite material, in which the iridium content is much higher than its content in the earth's crust. The fall of the asteroid, which caused the formation of this crater, undoubtedly caused a global impact on the entire globe. Explosion power reached
Mt and a gigantic amount of dust was generated into the atmosphere, formed during the condensation of the vaporized asteroid and target rocks, which, together with soot from forests that were set ablaze almost all over the world by a shock wave and debris falling from near space, closed the Earth from sunlight for several years, which probably caused the Cretaceous – Paleogene extinction. 
Unlike Chiksulub, the Wredefort crater, whose diameter reaches 300 km, is clearly visible in space images and is the only well-preserved multi-ring structure on Earth. What is surprising for its safety is the age of this crater - 2 billion years.
With an increase in the diameter of the crater, its morphology changes significantly. In addition to the formation of a central hill and then multi-ring structures, which I mentioned above, the crater flattenes with increasing diameter, and its shaft is formed not from a mound of debris, as in small craters but from large pushed blocks. Craters of a planetary scale on Earth could not survive due to plate tectonics. Nevertheless, there is a marginal hypothesis that the Pacific Ocean is such a gigantic crater (in a less bold version - that the first oceanic crust and moving lithospheric plates formed during the destruction of the primary continental crust by impacts of large planetisimals.
Other planets
Like Earth, craters of clearly meteorite origin were also found during Venus radar, which made it possible to obtain detailed relief maps of its surface. Due to the very dense atmosphere, only very large bodies are able to overcome it, while maintaining cosmic velocity. Therefore, the minimum diameter of the craters of Venus is not less than tens of kilometers. The craters of Venus, like the Earth, are subject to erosion and tectonic processes that destroy them, so there are not many of them.
Many craters are also known on Mars. The atmosphere of Mars is practically not an obstacle to space bombardment, except for micrometeorites. However, most of the small craters of Mars are quickly covered with sand, and for this reason the surface of Mars in large-scale images looks much less cratered than the surface of the Moon. Nevertheless, the density of large craters, not subject to wind erosion and falling asleep by sand, is approximately the same on the Moon and Mars. At the same time, like lunar seas, on Mars, territories almost devoid of craters stand out. The explanation for this is that their surface is much younger; it underwent processes in the relatively recent past that destroyed the former relief, including its elements of impact origin.
Thus, the density of craters is a characteristic that allows you to establish the approximate age of the surface of a particular planet and to distinguish ancient and young sites. This is clearly visible on the Moon, where there are strongly cratered ancient continents, and seas with a lower density of craters, whose age is about a billion years younger than the rest of the surface; on Ganymede, the stripes of the young crust of which are also almost devoid of craters (in comparison with the ancient "continents", the density of craters on which is similar to the lunar one).
If for planets with the atmosphere there is a limit to the size of craters, then for atmosphereless there is no such limit. A single continuous dependence of the frequency of craters on their size extends from the largest craters on a planetary scale to microcraters having microscopic dimensions, which indicates the unity of the mechanisms of their occurrence.
The surfaces of planets devoid of a dense atmosphere are always, to one degree or another, processed through meteorite bombardment. In the absence of atmosphere and appreciable tectonic and volcanic processes, it is the only force that changes the surface. For billions of years of meteorite bombardment, the planet is covered with a layer of regolith. Regolith is not just fragmented and ground rock - it is subjected to shock and metamorphism, melting and hardening, evaporation and condensation in a deep vacuum, fractionation, etc., which has led to the formation of new minerals, including completely unique ones.
Crater wealth
Most of the data on the geological structure of the Arizona meteorite crater was obtained against the background of a kind of “iron gold rush”. The crater was purchased by Daniel Barringer (Barringer), who was planning to extract a meteorite from it, the size of which, according to his ideas, reached 120 meters, and the mass - one and a half tens of millions of tons of pure iron, which did not need to be smelted from ore. It was fabulous wealth and could only take it.
But everything turned out not so rosy. Instead of a giant block of iron, a mass of small fragments and drops of strongly oxidized metal turned out to be in the crater, the number of which did not allow talking about any kind of industrial production. Barringer was not aware that when the impact occurred, it was not just the formation of a funnel, but an explosion with the almost complete evaporation of the fallen cosmic body, and it seemed that it had gone deeper, but his search was doomed to failure. According to modern estimates, it turned out that Barringer was also mistaken in terms of the size of the iron asteroid - its mass was 200 times less than he expected.
So the idea to develop meteorite craters in order to extract iron from there failed. But this does not mean that shock structures are barren. Mineral deposits are often formed in them - but they, as a rule, are in no way associated with meteorite matter. Their formation is associated with two things: residual heat, causing the development of hydrothermal processes, and the formation of faults and the development of mineralization along them.
So, one of the world's largest copper-nickel deposits is confined to the ring faults of the Södbury astrobleme in Canada. Signs of shock metamorphism were found in the rocks of the Aktogay and Kounrad copper deposits and the Almaly gold-silver deposit in Kazakhstan. Sulphide mineralization caused by the mobilization of hydrothermal solutions was noted in the nearby Shunak Crater. Such mineralization is generally characteristic of meteorite craters, including craters of kilometer sizes.
In some cases, individual structures of meteorite craters, due to their geometry, contribute to the formation of mineral deposits. So, the dome-shaped structures of the central uplifts of large astroblems are often the reservoir of oil fields (oil fields of Sierra Nevada, Red Wing, USA). The hollow of the Boltysh crater has become a place of formation of deposits of sapropel oil shales.
Non-meteorite craters
Enthusiasts, eager for discoveries, often “discover” new and new meteorite craters in space images. Often these are already well-known structures whose origin has nothing to do with impact processes.
The "astroblem" Conder in the Khabarovsk Territory is indicative here. The myth of the meteorite origin of this structure is very persistent - and not without reason. It really is very similar in appearance to a meteorite crater - it looks like a mountain chain of a perfectly regular ring-shaped shape. However, the geological structure of the Conder massif is completely unlike the structure of a meteorite crater - it is based on a stock-shaped body formed by ultrabasic igneous rocks (dunites, pyroxenites), which goes deep into the earth's crust. On the contrary, structures of impact origin occur superficially, disappearing with depth.
No signs of meteorite origin were found in another ring structure, which is often cited as an example of astroblems - the Rishat structure in the Sahara. The nature of this “Sahara eye” has not yet been reliably elucidated, but the fact that it is not a crater is fairly well established.
Another example of such a likely pseudo-crater is Smerdyachye Lake in the Shatursky District of Moscow Region. In many publications on the Internet in the meteorite origin it is not even doubted. At the same time, a version of the meteorite origin of Smerdyachy is considered, but so far there is too little data to confirm this. There are isolated finds of material similar to impactite - fragments of red-brown rock, composed of fused grains of various minerals (quartz, feldspar, zircon), cemented by bubble glass. There is still a similarity of the geometrical parameters of the depression with meteorite craters of similar size. And there is nothing more than the very great desire of the author of the article (Engalychev S.Yu. Meteor crater in the east of the Moscow region. // Bulletin of St. Petersburg University. 2009. Ser. 7. Vol. 2. P.3-11 ) to see a meteorite crater in this lake.
But if Lake Smerdyachye nevertheless has certain features hinting at a meteorite origin, then many round lakes and other landscape elements are declared by seekers of the unknown meteor craters completely arbitrarily, on the basis of only their circular shape. Nevertheless, a variety of processes can form a structure similar to a meteorite crater: karst dips, water work, manifestations of explosive volcanism (Maars and calderas), and even the activities of our ancestors. So not everything is round - a meteorite crater.
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The process of impact surface transformation is a single mechanism that transforms the solid surfaces of all the planets that have them, as well as satellites, minor planets and asteroids up to the surface of cosmic dust particles. And the meteoroid that left the crater on the Moon or Earth also had craters! .. There are none only where there is no solid surface. But even there, on Jupiter or Saturn, when an asteroid or comet flies into the dense layers of the atmosphere and there, when they explode, cease to exist, something forms in the cloud layer that is very reminiscent of all the same meteorite craters - though not existing for long. What then to speak about planets and their satellites with a solid surface? The absence of craters on it usually does not mean that they do not form - just active erosion or tectonics erases them from the face of the cosmic body.
Crater formation is not a simple change in surface topography. This is a deep physical and chemical processing of surface material, in which new types of rocks are formed - impactites, new minerals are formed under conditions of superhigh temperatures and pressure.