Primitive forest after Tunguska incident
This is a very small comet, and its nucleus is only about 40 meters in diameter. When it collided head-on with our earth, its speed reached 60 kilometers per second, mainly due to the explosion caused by the release of kinetic energy, which was estimated to be equivalent to 6.5438+million tons of TNT explosives and 700 atomic bombs exploding in Hiroshima, Japan.
This "whisper" exploded 8.5 kilometers above the ground. The falling area is sparsely populated, but the explosion can be heard from 1000 km away, and the shock wave of hot air surrounds the world twice. The erupting splash blazed over northern Europe for two consecutive nights. About two weeks later, a large number of burning debris arrived in California, which greatly reduced the transparency of the global atmosphere. The hunter who first entered the disaster area reported that the whole forest was razed to the ground and caused a large-scale forest fire. After 20 years of exploration, it was found that the trees within a radius of 30 ~ 40 kilometers were uprooted and the fire burned radially from the center of the explosion. The explosion area covering an area of 2,000 square kilometers was destroyed by the fire, and Fiona Fang18km became a scorched earth.
meteorite crater
The Tunguska incident is certainly not unique.
1949, an iron meteorite entering eastern Siberia exploded at a height of 6 kilometers from the surface, leaving about 380 fragments with a total weight of more than 23 tons. The explosive force is about 1000 tons of TNT explosive, which is about110000 of the explosion energy of Tunguska. 1965 In March, another fireball blazed over British Columbia, Canada, with an impact energy of about 20,000 tons of TNT, and many meteorite fragments were found.
Broken land
In the course of billions of years on earth, the craters on the mainland have been rapidly destroyed by erosion, and even the largest craters will be erased from the geological records. It is difficult to leave the debris of meteorites in the ocean, which accounts for three-quarters of the earth's surface.
1979, the famous astronomer David Lins published a paper in the international authoritative publication Nature, and summarized some calculation formulas as follows: every 1400 years, an impact crater with a diameter of 1 km can be produced on the earth, and every 1400 years, a crater with a diameter of/kloc-0 can be produced. If the meteorite impact records on the earth can be preserved as on the moon, we can find that there are not a few traces left by the huge destructive impact, but thousands.
In recent years, many scholars have suggested that at the end of Cretaceous, the killer that led to the extinction of dinosaur family may be an asteroid. In this regard, astronomer Weaser Riel believes that asteroids are condensed comet nuclei, which replenish energy from a part called Oort Cloud located in the solar system. Every time a comet approaches the sun along its orbit, its frozen comet head will become an "Apollo star", that is, an asteroid. Apollo asteroids are rocky stars whose orbits intersect the Earth's orbit, and they are often very close to us. They may be the main culprits of impact craters on the earth and the moon in recent 3 billion years.
A few years ago, the spectacular sight of two comets hitting Mars attracted the interest of a large number of scientists. Perhaps it was this blow 65 million years ago that led to the extinction of dinosaurs, a large number of plants and marine plankton on the earth. Because when the comet becomes columnar at the impact point, rises to a fire more than ten kilometers high and spreads out like a mushroom cloud, the temperature will rise to hundreds of thousands of degrees in a very short time, which is high enough to ionize cyanide or other toxic gases. The abnormal enrichment of iridium and osmium in the thin clay layer at the Cretaceous-Paleogene boundary found in many parts of the world also proves this point, because these substances called "trace elements" are relatively high only in celestial bodies or deep in the earth's crust.
A carnivorous dinosaur.
Five mass extinctions in the history of the earth.
The geological period dating back to the present has influenced the late Cretaceous, which coincides with the Deccan igneous rock 65 million years ago, and may also coincide with the impact of planets or comets. 16% of marine life is extinct; 38% ~ 46% of marine life species are extinct; The extinction of dinosaurs, ammonites (shellfish) and many animal species; 24% of marine life became extinct 65.438+0.98 billion years ago in Late Triassic. 43% ~ 58% of marine life species are extinct; Cephalopods are almost extinct; Many reptiles, gastropods, bivalves and brachiopods have become extinct; Conodonts (probably the fish ancestors of vertebrates) ended at the end of Permian 250 million years ago, which is consistent with volcanic eruption; More than 50% of marine life is extinct; 76% ~ 80% of marine life species are extinct; 80% of marine species and 96% of all species are extinct; Trilobites and many terrestrial species ended the late Devonian 357 million years ago, and sea level and climate change may have lasted for 654.38+0 billion years. 22% of marine life is extinct; 47% ~ 57% of marine life species are extinct; Corals, brachiopods, shield fish and trilobites have a high mortality rate, and 24% of marine life was extinct at the end of Ordovician 435 million years ago. 45% ~ 57% of marine life species are extinct; The rapid change of sea level has affected tropical shallow waters. The mass extinction of marine invertebrates such as trilobites, cephalopods and sea lilies is still an important subject in paleontology research. Although there are still different opinions, it is certain that the extinction of dinosaurs could not be caused by a single factor, but was probably the result of a combination of many factors. There are external reasons, such as the impact of asteroids and comets on the earth, and there are factors of strong climate change, which cannot be completely denied; There may also be dinosaurs' own reasons, such as small teeth, single food, unstable body temperature, and even genetic defects caused by environmental changes. At that time, the dinosaur's genes had been largely stable. If the external environment causes gene mutation, then it is difficult for dinosaurs with high degree of evolution to adjust and adapt, so they are extinct.