What are the contents of the theory of dinosaur extinction and meteorite impact?

meteorite impact theory: In 198, American scientists put forward this theory based on the discovery of high content of iridium in the clay layer at the top of Cretaceous in Italy and other places. Because the iridium content on the earth is very small, but the iridium content in celestial bodies can be very high, the iridium in the clay layer should be brought by the impact of celestial meteorites on the earth, and the impact caused a lot of dust, covered the sky, the temperature plummeted, plants withered, and finally dinosaurs were difficult to survive and finally became extinct. This theory has caused a great sensation, but it has received mixed praise. The proponents put forward many evidences one after another. For example, NASA found a circular structure with a diameter of 6 kilometers in the Yucatan Peninsula in the Gulf of Mexico, which contained marine strata in the late Cretaceous period, and a ring structure with a diameter of 18 kilometers in the periphery. There were 2 craters with a diameter of more than 1 kilometers (some people named them Alvaro-Obregon crater, Chicxulu crater, Belize crater, etc.), so it is speculated that these craters were formed by a meteorite with a diameter of 1 kilometers at that time, which struck at a speed of 4 kilometers per second. Some people think that the huge energy generated by the impact is equivalent to 1,-5, times the total energy released by the impact of 21 fragments of comet Shoemaker Levi-9 on Jupiter's surface in July 1994, while others think that it is equivalent to 5 billion times the energy of Hiroshima atomic bomb. In a word, this great energy is enough to throw hundreds of millions of tons of dust into the sky, make the earth dark, stop photosynthesis for 6 months, and keep the temperature low for 1 years; The high temperature at the time of impact can also melt the rock stratum, forming many glass balls around it, or forming a secondary impact Shi Ying around the pit. To make matters worse, the rock layer of the impact crater contains a lot of sulfur, some say that there is a lot of methane on the seabed, or meteorites contain toxic substances, which poisoned the extremely harsh environment at that time. According to another report, in 1994, an impact crater named Sva was also discovered in India, which was 6 kilometers long and 45 kilometers wide. It is estimated that the meteorite is 4 kilometers long, and it is the same meteorite as that found in Mexico. When it reaches the earth's atmosphere, it is divided into two large pieces and many small meteorites, which are distributed in and around the Pacific Ocean. In recent years, the United States has also found meteorites from this period in the middle of the Pacific Ocean. This article was published in the authoritative journal Nature. It is also worth mentioning that scientists found the rare chromium (Cr) isotope-53 in the solar system in the rocks at the top of Cretaceous in Denmark and Spain, which is the same as the chromium isotope in carbon-bearing meteorites. In Denmark, the content of iridium is 16 times higher than that of earth rocks. In addition, there are high anomalies such as hunger, platinum and rhenium, which also prove that there was a meteorite impact.