Approximately 66 million years ago, an asteroid the size of Mount Everest struck Earth, causing a cataclysm that devastated 75% of living beings. But how do we know this happened? If the meteorite was so massive, where is it? Have we found it?
In 1834, English doctor Gideon Mantell classified the first dinosaur fossil in history. From that moment on, discoveries of fossils belonging to creatures humanity had never seen continued to surface worldwide. Dinosaurs, famous for capturing the fascination of people young and old, existed long before the first humans walked the Earth. They displayed a remarkable diversity in size and shape. Despite their dominance, the most mysterious aspect of dinosaurs remains their mass extinction. Why did such colossal creatures vanish from the face of the Earth? This question puzzled scientists for many years.
During the 1990s, paleontologists and geologists made significant efforts to solve the mystery of dinosaur extinction. Numerous expeditions were conducted in major fossil excavation sites worldwide, revealing a pattern. Fossil dating showed that dinosaur remains ceased to appear at a specific point in history, indicating a mass extinction event. Geological evidence pointed to a sudden mass extinction around 66 million years ago, marking the end of the Cretaceous period and the Mesozoic Era while ushering in the Paleogene, the first period of the Cenozoic Era. This extinction event was classified as the Cretaceous-Paleogene (K–Pg) event. It is identified in the geological record by a thin layer of sediment known as the K–Pg boundary, found globally in both marine and terrestrial rocks. Curiously, this boundary layer contains unusually high concentrations of iridium, a metal more common in asteroids than in Earth's crust.
In 1980, a research team led by physicist Luis Álvarez and his son, geologist Walter Álvarez, discovered a global iridium anomaly in sediment layers dated to around 65 million years ago. This led to the "Álvarez hypothesis," which proposed that a massive meteorite impact triggered the extinction of dinosaurs and other life forms. However, the theory faced a major challenge: to cause such a mass extinction, the meteorite would have needed to leave a crater at least 150 kilometers in diameter, and no such crater had been discovered.
To confirm the asteroid theory, scientists needed to locate the impact crater. For years, geologists searched without success. The challenge was significant since the event occurred 66 million years ago, and Earth's continents had shifted considerably since then. However, the crater had already been discovered decades earlier, purely by accident. In the 1970s, geophysicists Antonio Camargo and Glen Penfield, working for Petróleos Mexicanos (PEMEX), identified a gravimetric anomaly in the Gulf of Mexico while searching for oil. The anomaly, extending several kilometers deep, had a circular shape characteristic of a meteorite impact.
Further analysis was conducted by the National Autonomous University of Mexico (UNAM) in collaboration with NASA. Their studies confirmed the presence of a crater over 2.5 kilometers deep and more than 200 kilometers wide, making it the largest and best-preserved impact crater on Earth. The center of this massive structure was located near Puerto Progreso, close to the town of Chicxulub in Yucatán, Mexico. Thus, it became known as the Chicxulub Crater. When the Álvarez research team learned of the discovery, they visited the site and found iridium concentrations matching those in the K–Pg boundary layer, confirming this as the impact site of the extinction-causing meteorite.
But where was the meteorite itself? Despite the discovery of the crater, the meteorite no longer exists. A meteorite is a smaller celestial body that strikes a planet's surface, while asteroids and comets are larger space objects before impact. The Chicxulub impactor was initially believed to be a large metallic asteroid, but recent evidence suggests it may have been a comet. The impactor, estimated to be 9 to 11 kilometers in diameter, likely disintegrated upon impact. If it was a comet, much of its outer layers would have vaporized before reaching the surface, leaving the inner core exposed. The extreme heat generated upon impact caused the entire body to vaporize instantly, while the ground liquefied, triggering massive tsunamis several kilometers high.
The effects of this colossal impact were catastrophic for life on Earth. The collision triggered global wildfires, raised temperatures dramatically, and caused earthquakes that activated volcanic eruptions worldwide. Volcanic ash, dust, and steam filled the atmosphere, blocking sunlight and plunging Earth into darkness for years. This led to a dramatic cooling effect, disrupting photosynthesis and causing plants to die en masse. Herbivores depending on plants soon perished, followed by carnivores that fed on them. Only species capable of surviving on roots, seeds, and underground food sources, such as small burrowing mammals and certain birds, managed to endure.
While non-avian dinosaurs perished, the impact also affected other species, including pterosaurs, plesiosaurs, mosasaurs, and numerous fish, mollusks, and plankton species in the oceans. Despite the devastation, the extinction event paved the way for evolutionary opportunities. With dominant species like dinosaurs gone, small mammals diversified into new ecological niches, eventually evolving into horses, whales, bats, and primates. Birds, the surviving lineage of dinosaurs, adapted and thrived, evolving into the diverse species we see today. So, when asked what happened to dinosaurs, remember: modern birds are their descendants.
Interestingly, the Chicxulub impact might not have been the sole factor in the mass extinction. The "Multiple Impact Theory" suggests Earth was struck by several meteorites around the same time. One such impact site, the Silverpit Crater in the North Sea, discovered in 2002, shares a similar age with Chicxulub, hinting that multiple celestial objects may have contributed to the extinction event.
Though the extinction of the dinosaurs happened 66 million years ago, the possibility of another catastrophic impact still exists. If a comet like the one that struck Chicxulub were to head toward Earth today, could we defend ourselves? Only time will tell.