A mass extinction event that occurred 65 million years ago (the end of Cretaceous) made a large number of reptiles such as dinosaurs extinct, as well as ammonites and arrow stones in marine organisms. Who is the killer of this mass extinction?
The killer is an asteroid?
Many hypotheses have been put forward to explain the extinction of dinosaurs, the most famous of which is the "asteroid impact theory" put forward by American scientist Alvarez and others. This hypothesis is supported by many scientists because the study of hundreds of profiles in many regions such as Italy and Denmark found that there were abnormal contents of iridium and impact quartz particles in the clay layer at the end of Cretaceous. Because the content of iridium on earth is very low, the abnormal iridium content may come from asteroids outside the earth, and the impact quartz particles can be formed only at high temperature. In addition, scientists found a huge crater 180 kilometers in diameter and 900 meters deep on the Yucatan Peninsula in Mexico, indicating that the huge impact did happen. Based on this evidence, scientists have put forward an exciting hypothesis: a small celestial body with a diameter of 10 kilometers fell from the sky and caused the extinction.
Imagine, what is the concept of an extraterrestrial with a diameter of 10 kilometers? The lower troposphere of the earth's atmosphere is only 7 kilometers, while the average depth of the ocean is 3 kilometers. The small celestial body with a diameter of 10 kilometers can be described as "indomitable". Its weight can reach 1 megaton, and the shock wave acting on the atmosphere before hitting the earth can easily pierce the atmosphere into a huge hole in the sky. After hitting the earth, the energy released by it is equivalent to 10000 times the energy contained in nuclear fission minerals all over the world. It is speculated that it will form a huge fireball and produce a huge impact crater. Lava covered the bottom of the pit and ejected a large amount of sediment debris, mixed with water, steam and vaporized meteorites. Soon, it formed a hot gas column with a diameter of tens of kilometers, soared up to an altitude of 30 kilometers, and showed a mushroom cloud, which was scattered on the inner side of the stratosphere, while the fine-grained jet was more likely to be brought into an altitude of 100 kilometers.
The clay layer rich in iridium and quartz particles at the end of Cretaceous may be the fallout accumulation of meteorites after hitting the earth for several years. These huge amounts of dust may block the sunlight for a long time, making the ground plants unable to accept enough sunlight, stopping photosynthesis, so a large number of deaths, resulting in the interruption of the food chain. At the same time, this thick "blanket" reflects the sunlight, which may also make the global climate dry and cold in a short time. Many organisms that lack the ability to resist the cold are quickly frozen to death, and large forests become deserts.
However, as long as people think further, there will be no end to the question. Why did such a thorough extermination kill only some creatures, including dinosaurs? On the other hand, if the destructive effect of asteroid impact is not as severe as assumed, is it really capable of killing all dinosaurs? Some studies have pointed out that the sudden disappearance of dinosaurs at the end of the Cretaceous may not be a real phenomenon in the past. Some data analysis shows that the decline of dinosaurs actually experienced a long time, maybe a million years or even longer, at the end of the Cretaceous; In other words, the dinosaur family had begun to decline before the asteroid impact. Is this the case? More evidence is clearly needed to blame asteroid impacts for the extinction of all dinosaurs.
Are there any other "accomplices"?
Now many people believe that the extinction of dinosaurs is caused by complex reasons, and asteroids may be just one of them. Several other hypotheses are introduced below.
Large scale volcanic eruptions in the Indian subcontinent and other places are also considered to be the cause of the extinction of dinosaurs. Volcanic eruptions will produce a large number of toxic gases, hot lava flows, pyroclastic flows, volcanic ash and other harmful substances, and also cause secondary disasters such as earthquakes and tsunamis. These factors can not only directly kill dinosaurs, but also cause long-term pollution to the surrounding environment. The research of Chinese scholars shows that this trace element pollution can affect the reproductive function of dinosaurs, lead to the pathological changes of dinosaur eggs, significantly reduce the hatching rate, and dinosaurs "lose their children and grandchildren".
Other hypotheses suggest that dinosaurs may have been poisoned because their bodies could not adapt to emerging angiosperms, eliminated because they could not compete with emerging mammals, or frozen to death because they could not adapt to lower temperatures. These hypotheses reveal the possible adverse factors in the living environment of dinosaurs from all aspects. However, it should be emphasized that any hypothesis and the evidence supporting it have applicable scope and limitations, and can not explain the reasons for the extinction of all dinosaurs. So for a complex event like the extinction of dinosaurs, we may never know exactly how it happened. But that's why scientists have the motivation to keep exploring.