Why are there no meteorites in Earth's craters? Could someone have taken them away?

Humans are always very complicated. They believe in science but cannot help but be superstitious. For example, when we see a shooting star in the sky, we know it is a "meteorite" and has no magical power, but we still silently make a wish, hoping that it can be fulfilled.

Legend has it that if you make a wish towards a shooting star, it will come true.

However, due to environmental and time constraints, many people never have the chance to see a meteor in their lifetime, so why not take a look at the craters left behind by the falling meteors!

Meteor craters are the final destination of meteors, so they should contain their "remains", but the fragments of meteorites seem to be outside the craters. So why are there no meteorites in the craters? Who took these meteorites away?

Barringer Crater in the American desert

Mechanism of crater formation

In fact, when it comes to the number of meteorite craters, the Earth really cannot compare with other celestial bodies. Therefore, after scientists examined the meteorite craters on other celestial bodies in the solar system, they determined that they can be divided into three types: simple craters, complex craters, and multi-ring basins.

The appearance of a simple crater is consistent with everyone's understanding of a meteorite crater, which is shaped like a bowl with a relatively small diameter. In addition to a large diameter, a complex crater also has a raised part in the center.

Multi-ring basins can also be called giant craters, whose diameters often exceed 100 kilometers. They are flat in the middle and surrounded by craters.

Three types of craters on the moon

The reason why different types of meteorite craters show these differences is mainly because the size of meteorites is obviously different. However, regardless of the difference in size, the formation mechanism of meteorite craters is similar. It can be divided into three stages: compression and extrusion, excavation and adjustment after crater formation.

First, let's talk about the first stage, compression and extrusion. Although the meteorite will become smaller and slower due to friction after entering the atmosphere, it will still form a high-pressure transient compression wave when it hits the ground at such an ultra-high speed, which is what we often call a shock wave.

Schematic diagram of the compression stage of the crater

If the speed of a meteorite hitting the ground is 10 km/s, the pressure of the shock wave can reach several hundred GPa. In this case, granite rocks can be evaporated and eventually turned into gas.

When the shock wave hits the ground first, the rarefaction wave behind it will decompress the system, causing meteorite fragments and other materials to be ejected at high speed at the interface between the two, eventually forming a pit.

The final adjustment after the crater is the accumulation of material debris and meteorite fragments along the edge of the crater, eventually forming a sedimentary layer, the sequence of which is opposite to the bottom layer of the meteorite impact target area. The crater will also be affected by gravity and rock mechanics, resulting in subtle changes.

Schematic diagram of the last two stages of the crater

From the perspective of the formation mechanism of meteorite craters, after a meteorite hits the ground, it will undergo a series of "dissolution" processes or gasification. In this process, smaller meteorites are directly and completely decomposed, larger meteorites will break into pieces, and a large number of meteorite fragments will splash out of the crater.

Therefore, the meteorites in the crater were not picked up by anyone, but turned into slag under the impact, and some of them even cannot be found.

"Clean" craters on Mars

At this point, you may be wondering, are meteorites really that powerful? Although some have very large diameters, they are not necessarily that lethal. In fact, this is similar to "high-altitude throwing". If you drop an egg from the 30th floor, it may hurt someone, let alone a meteorite that enters the earth at a higher speed?

Data shows that if an iron meteorite with a diameter of 1km hits the surface at a speed of 25km per second, its kinetic energy is E=1/2mV2=1.31×10^21 joules. This kinetic energy is equivalent to the explosive energy of 3.12x10^11 tons of TNT; while the energy of the 8.9 magnitude earthquake in Indonesia in 2004 was only 184×10^16 joules.

Conceptual image of a falling meteorite

It can be seen that meteorites are sometimes like a cannonball from the universe. After the cannonball hits the ground, the "cannonball itself" will definitely not maintain its original shape, and its shrapnel will be scattered around. In this way, everyone may understand it better. After all, meteorites are not "duds", and their "impact" is still very strong.

As for where the meteorite fragments scattered around eventually went, if they were meteorites that fell recently, they might be taken away by scientists for research. If they were meteorite craters formed many years ago, then these meteorites would have long been slowly worn away by geological processes.

The Manicourt crater has a reservoir at its rim.

In fact, due to the frequent geological activities on Earth, there are not many craters left, which is why scientists specifically investigate other celestial bodies in the solar system. So, what kind of later "transformation" will the craters undergo?

Later "transformation" of the crater

The "transformation" of meteorite craters after their formation includes landslides, erosion, filling, volcanic action, etc., so if a meteorite crater is not large enough and has a long history, it is basically impossible to preserve it.

According to scientists' statistics, meteorite craters on Earth with a diameter of more than 20 kilometers can still be seen after 600 million years, while meteorite craters of 1 kilometer will be impossible to find after a hundred years.

Beaverhea crater is 600 million years old

Let's take erosion as an example. Whether it is erosion by wind or water, as long as it is on the surface, it will be destroyed by erosion. If the location of the crater happens to be in a place with heavy rains, it will gradually lose its original appearance under the erosion of rain.

Precisely because the "transformation" that a meteorite crater has undergone over a long period of time is still very obvious, people often need to start from three aspects when determining whether it is a meteorite crater, namely, landform, mineral rocks, and geochemistry.

Triplet crater on Mars

You know, when a meteorite hits the ground, it can be said to be "hurting the enemy by a thousand and hurting yourself by eight hundred". Not only will it shatter itself into pieces, but it will also be affected by other unknown rocks it hits. The high temperature and high pressure generated will leave special deformation and metamorphic characteristics in these rocks.

Therefore, scientists often determine whether a rock has the characteristics of a meteorite crater by identifying its metamorphism. Of course, if you want to be 100% sure, you still have to combine the three aspects.

Scientists take samples from the crater

So why do we go to so much trouble to study craters and search for meteorites? What role do these extraterrestrial visitors play in human history?

Why study meteorites?

When it comes to studying meteorites, we have to mention a subject that many people have never heard of, that is astrochemistry. It is mainly a subject that studies the chemical composition and evolution of matter in space, and it involves the cross-penetration of multiple disciplines such as earth science and astronomy. Meteorites are important for solving problems related to astrochemistry, or they are "physical evidence."

It is worth mentioning that different types of meteorite research have different goals and uses. Scientists will conduct research based on their types.

Chondrite slices from different celestial bodies

For example, the study of chondrites is to solve the origin and evolution of the early solar nebula. You know, humans have always been very curious about the origin of the sun. Although we have made predictions based on some things, it would be great if we could find more evidence from the products of condensation and accretion of the solar nebula.

There is also research on primitive achondrites, which have similar chemical compositions to chondrites, but they hold the secrets of the origin of planets. This type of meteorite is commonly found in Antarctica and deserts. For example, two achondrites were found in the Gobi Desert in Xinjiang, my country.

IAB-MG in the category of primitive achondrites

As for Martian meteorites and lunar meteorites, since these two are the primary exploration targets of mankind at present, the number of meteorites obtained is still very large. Especially with the development of science and technology in various countries, the frequency of exploration has increased, and the number of meteorites has also increased wildly.

According to statistics from the International Meteorite Database, from January 2011 to December 2020, the number of lunar meteorites increased from 157 to 435, and the number of Martian meteorites increased from 105 to 291.

A lunar meteorite named NWA 11220

Take Martian meteorites as an example. As a planet that is considered as a future immigration site, humans must have a clear understanding of Mars, especially the magma process on Mars and how the water disappeared. Of course, some people believe that traces of extraterrestrial biological activities can be found through organic carbon in meteorites.

In addition to the meteorites mentioned above, there are also studies on rare gases in meteorites, non-traditional isotopes of meteorites, etc. In short, meteorites have become another "magic weapon" for humans to explore the secrets of the extraterrestrial world.

A complete rock slice from Mars

Under such circumstances, every year many scientists travel around to collect precious meteorites from various regions, and large meteorite craters are often protected.

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