殺死恐龍的小行星並不孤單

Scientists have made an exciting discovery about the asteroid that struck Earth and led to the extinction of the dinosaurs 66 million years ago. It turns out that this massive asteroid was not alone; there was a second, smaller asteroid that also impacted Earth during the same time period. This smaller asteroid crashed into the ocean off the coast of West Africa, creating a significant crater in the seabed. Researchers believe that this event would have been catastrophic, potentially generating a tsunami that reached heights of at least 800 meters. Dr. Uisdean Nicholson, a scientist from Heriot-Watt University, first identified the Nadir crater in 2022. Initially, there was uncertainty surrounding its formation, but Dr. Nicholson and his team have now confirmed that the crater, which measures 9 kilometers in diameter, was indeed caused by an asteroid colliding with the ocean floor. While they cannot pinpoint the exact timing of this event or determine whether it occurred before or after the impact that created the Chicxulub crater in Mexico, they do know that both events happened at the end of the Cretaceous period, which marked the extinction of the dinosaurs. As the smaller asteroid entered the Earth's atmosphere, it would have produced a massive fireball. Dr. Nicholson illustrated this by saying, 'Imagine if the asteroid hit Glasgow and you were in Edinburgh, about 50 kilometers away. The fireball would appear 24 times larger than the Sun in the sky, enough to ignite trees and plants in Edinburgh. ' Although there are no photographs of the Nadir crater, scientists have noted that it is similar to the Gosses Bluff crater located in Australia. Following the impact, a loud air blast would have occurred, followed by seismic activity comparable to a magnitude 7 earthquake. This would have caused large amounts of water to be displaced from the seabed, which later cascaded back down, leaving unique imprints on the ocean floor. It is quite unusual for two large asteroids to collide with Earth in such close succession, and researchers are still trying to understand why this happened. The asteroid responsible for the Nadir crater is estimated to be around 450 to 500 meters wide and struck the Earth at an incredible speed of approximately 72,000 kilometers per hour. The closest event of this magnitude in modern history was the Tunguska event in 1908, when a 50-meter asteroid exploded in the atmosphere above Siberia. The Nadir asteroid is comparable in size to Bennu, which is currently regarded as the most hazardous object orbiting near Earth. According to NASA, the most likely date for Bennu to impact Earth is September 24, 2182, but the probability of this occurring is only 1 in 2,700. There has never been an asteroid impact of this scale in human history, and scientists typically study eroded craters on Earth or analyze images of craters found on other planets. To gain a deeper understanding of the Nadir crater, Dr. Nicholson and his team utilized high-resolution 3D data provided by a geophysical company called TGS. Unlike most craters that have been eroded over time, the Nadir crater is well-preserved, allowing scientists to examine the rock layers more closely. Dr. Nicholson expressed his excitement, stating, 'This is the first time that we've ever been able to see inside an impact crater like this - it’s really exciting. ' There are only 20 marine craters in the world, but none have been studied in such detail as the Nadir crater.