Meteorites and impact craters bear witness to the fact that large impacts occasionally occur on Earth. Meteor Crater in the northern Arizona desert of the United States formed about 50,000 years ago when a metallic impactor roughly 50 meters across crashed to Earth with the explosive power of a 20-megaton hydrogen bomb. Although the crater is only slightly more than one kilometer across, an area covering hundreds of square kilometers was probably battered by the blast and ejecta—the debris ejected or displaced during the formation of an impact crater. Far bigger impacts have occurred, sometimes with catastrophic consequences for life on Earth.

While collecting geological samples in Italy in 1978, the father-son team of Luis and Walter Alvarez discovered a thin layer of dark sediment that had apparently been deposited 65 million years ago—at about the same time that the dinosaurs and many other organisms suddenly became extinct. Subsequent studies found similar sediment deposited at the same time at many sites around the world. Careful analysis showed this worldwide sediment layer to be rich in iridium, and element that is rare on Earth’s surface. But iridium is common in primitive meteorites, which led the Alvarezes to a stunning conclusion: the extinction of the dinosaurs was caused by the impact of an asteroid or comet. This conclusion was not immediately accepted and still generates some controversy, but it now seems clear that a major impact coincided with the death of the dinosaurs. While the dinosaurs were the most famous victims of this mass extinction, it seems that up to 99 percent of all living things were killed and that 75 percent of all species living on Earth were wiped out at that time.

How could an impact lead to mass extinction? The amount of iridium deposited worldwide suggests that the impactor must have been about 10 kilometers across. After a decade-long search, scientists identified what appears to be the impact crater from the event. Located off the coast of Mexico’s Yucatan peninsula, it is 200 kilometers across, which is close to what one would expect for a 10-kilometer impactor, and dates to 65 million years ago. Further evidence that the Yucatan crater is the right one comes from the distribution of small glassy spheres that formed when the molten impact ejecta solidified as it rained back to Earth. More of these glassy spheres are found in regions near the crater, and careful study of their distribution suggests that the impactor crashed to Earth at a slight angle. These pieces of once molten rock are evidence of an explosion powerful enough to instantly melt bedrock and propel it far from its origin. The impact almost immediately sent a shower of debris raining across much of North and South America and generated huge waves that may have sloshed more than 1,000 kilometers inland. Many North American species thus may have been wiped out shortly after impact. For the rest of the world, death may have come more slowly. Heat from the impact and returning ejecta probably ignited wildfires in forests around the world. Evidence of wildfires is found in the large amount of soot (a black powdery form of carbon produced when coal, wood, or oil is burned) that is also present in the indium-rich sediment from 65 million years ago. The impact also sent huge quantities of dust high into the stratosphere, where it remained for several years, blocking out sunlight, cooling the surface, and affecting atmospheric chemistry. Plants died for lack of sunlight, and effects propagated throughout the food chain.

Perhaps the most astonishing fact is not that 75 percent of all species died, but that 25 percent survived. Among the survivors were a few small, rodent-like mammals. These mammals may have survived because they lived in underground burrows and managed to store enough food to outlast the long spell of cold, dark days. Small mammals had first arisen at about the same time as the dinosaurs, more than 100 million years earlier. But the sudden disappearance of the dominant dinosaurs made these mammals dominant.

陨石和撞击坑证明了地球上曾不定期地发生过巨大的撞击。美国北部亚利桑那州沙漠中的陨石坑是在大约5万年前形成的，当时一个大约50米的金属撞击物，以20兆吨的氢弹的爆炸力撞击地球。尽管火山口仅有大约一公里，但数百平方公里的地区可能受到爆炸物和喷射物的冲击 - 在形成撞击坑期间会喷出或排出的碎片。有时，会产生更大的影响，对地球上的生命造成灾难性的打击。 1978年，在意大利采集地质样品时，路易斯和瓦尔特•阿尔瓦雷斯的父子团队发现了一层薄薄的黑色沉积物，这些沉积物显然是在6500万年前沉积而成的 – 这与恐龙和许多其他生物的突然性灭绝大约是处于同一时期。随后的研究发现，在同一时间，世界各地的许多地方沉积了类似的沉积物。经过仔细的分析，可以发现这个世界范围内的沉积层富含铱元素，而这是一种地球表面上罕见的元素。但是铱在原始陨石中很常见，这使得阿尔瓦雷斯得出了一个惊人的结论：恐龙的灭绝是由小行星或彗星的撞击造成的。这个结论并没有马上得到人们的认可，并且还引发了一些争议，但是现在我们可以明确的是，一次重大的撞击与恐龙的灭绝时间相吻合。虽然恐龙是这场大灭绝中最为人们熟知的受害者，但是事实上似乎有高达99％的生物都遭到了灭绝，并且当时生活在地球上的所有物种中都有75％的动物遭到了灭绝。 撞击为什么会导致大规模的灭绝呢？全球的铱储存量表明，冲击器必须距离地球约10公里远。在经过长达十年的搜索后，科学家们似乎确定了这次事件所造成的撞击坑。撞击坑位于墨西哥尤卡坦半岛的海岸线上，横跨200公里，因而人们预计这应该是一个接近于10公里大的撞击物，可追溯到6500万年前。进一步的证据表明，进一步证据表明，尤卡坦半岛火山口是当熔融撞击喷射物后，固化降落到地球时所形成分布的小型球状颗粒物。更多的这些玻璃球是在火山口附近的地区被发现的，对其分布进行仔细研究，表明冲击物以微小的角度撞向地球。这些熔化的岩石的碎片可以证明，这是一场威力十足的爆炸，足以立即融化基岩，并将其推离其原点。这次撞击几乎立即给北美洲和南美洲造成了大量的碎片，并产生了可能给内陆造成了超过1000公里的大浪。因此，很多北美物种在撞击后可能已经消失了。对于世界其他地方而言，灭绝的速度可能会慢一点。撞击和返回喷射物所带来的热量可能在全球的森林中引发大火灾。大火灾发生的证据就是大量的烟灰（一种煤炭，木材或石油被燃烧时产生的黑色粉末状碳），在6500万年前，也可以在富含铟的沉积物中找到该物质。这种撞击还将大量灰尘带入平流层，并在平流层中滞留了数年，阻挡了阳光的照射，降低了表面温度，并对大气化学造成一定的影响。植物因缺乏阳光而死亡，进行影响了整个食物链。 也许，最令人震惊的事实并不是所有物种的75％的动物都遭到灭绝，而是有25％的动物存活了下来。在所有的幸存动物中，有一些是小型的啮齿动物哺乳动物。可能这些哺乳动物幸存下来的原因，是因为他们生活在地下洞穴中，并设法储存足够的食物，来度过漫长，寒冷且黑暗的日子。小型哺乳动物与恐龙出现的时间大致相同，都是在距今1亿多年前出现的。但是，占据统治地位的恐龙突然消失，这使得这些哺乳动物可以成为统治者。