Asteroid 1, Dinosaurs 0

FAS Astronomers Blog, Volume 31, Number 15.

The dinosaurs ruled the Earth for millions of years. Despite their dominance, the dinosaurs disappeared. The commonly accepted theory is that a large asteroid hit the Earth, resulting in the extinction of over two thirds of the species on the planet. For more on dinosaurs, see three previous articles: An Introduction to Dinosaurs, The Classification of Dinosaurs, and Meet the Dinosaurs.

Asteroids

Asteroids are chunks of rubble left over from the formation of the Solar System. They generally come in three types.

• C-types are made of clay and rock and are the most common.
• M-types are made of nickel-iron.
• S-types are composed of a combination of silicate and nickel-iron.

Most asteroids are found in the asteroid belt between the orbit of Mars and Jupiter. Some, however, cross the orbit of the Earth and those that travel close to the Earth are called “Near Earth Asteroids.” It is possible that one of these hit the Earth over sixty million years ago.

The Time of the Dinosaurs

The Earth is estimated to be around 4 ½ billion years old. Dinosaurs flourished during the Mesozoic era, which began some 250 million years ago with the Permian-Triassic (P-T) extinction, when a large portion of the species on the Earth died off. The subsequent era of the dinosaurs is further broken into three major periods – The Triassic, Jurassic, and Cretaceous.

At first, dinosaurs were rather small, but they soon took over and became the dominant species on the planet for some 150 million years. Dinosaurs were alive and thriving during the Cretaceous period at the end of the Mesozoic era. Then, suddenly, they were gone at the beginning of the Paleogene (formally Tertiary) period of the Cenozoic era. At the same time mammals who were once small and insignificant became larger and much more dominant.

During the last hundred years or so, scientists have determined that something happened at a point in geologic time 66 million years ago called the Cretaceous-Paleogene (K-Pg) boundary. Geologists found a small stratification band separating the Cretaceous period from the Paleogene period. They also found a significant difference in the fossil remains below and above this boundary. The evidence points to another mass extinction event, the Cretaceous-Paleogene (K-Pg) extinction, when many of the Earth’s species, including the dinosaurs, disappeared.

The Theories

After the discovery of the dinosaurs in the early 19^th century, it was clear that somehow someway the dinosaurs disappeared. For a long time, no one knew why. Beginning around 1830, geology was driven by the concept of Uniformitarianism – the idea that geological changes occur slowly over a very long period of time. This replaced the concept of Catastrophism, which said that changes occur periodically as the result of some type of catastrophe. Because of this, it took a long time for geologists to recognize that both theories had some truth, and catastrophic events resulted in the extinction of species and opened up the opportunity for new species.

There have been several theories as to why the dinosaurs (and other species) vanished, including an asteroid impact, volcanoes, competition from mammals, changes in the Earth’s climate, and continental drift. Today, everything points to a large impact, when an asteroid smashed into the Earth near the town of Chicxulub (Chics-ah-lube), Mexico on the Yucatan Peninsula some 66 million years ago. However, it wasn’t until the 1980s and 1990s that the evidence of an impact, including the crater, was discovered.

The Evidence

In 1980, Luis Alverez, his son Walter Alverez, Frank Asaro, and Helen Michel (Alverez, et al. 1980) found that the K-Pg clay boundary contained a significantly high amount of iridium, much more than expected to be found on the Earth, but at a level consistent with that found in meteorites. This led to the belief that the K-T extinction was caused by the impact of a large asteroid.

Iridium wasn’t the only clue. Around the same time, Jan Smit (Smit 1981) found evidence of microscopic glass spheres (microspherules) at the K-Pg boundary. A few years later Bruce Bohor (Bohor et al. 1984) discovered “shocked quartz”. Both could be attributed to a giant impact.

During the 1980s, Wendy Wolbach and others (Wolbach et al. 1985) discovered soot at the K-Pg boundary, which they attributed to global fires resulting from a huge impact.

The Chicxulub Crater

Despite the evidence that some type of an impact occurred, no one had yet to discover the “smoking gun” – a crater, which would provide definitive proof of an impact.

Mexicanos (aka Pemex), the Mexican state oil company was searching for oil near the Yucatan Peninsula in the mid to late 20^th century. In doing so, they created geologic maps of the area, and those maps identified something interesting.

In 1966, Robert Baltosser may have been the first to discover evidence of something interesting off the coast of the Yucatan Peninsula. However, Pemex, because their focus was on the search for oil and they wanted to keep their findings private, prevented him from publicizing his results.

A few years later, in 1981, Glen Penfield and Antonio Camargo, who also worked for Pemex, made a similar discovery. They identified an arc of some 70 kilometers in the Gulf of Mexico. Again, Pemex did not want company information to be released. However, possibly because of the findings by Alverez, Penfield and Camargo were allowed to present some of their results at a 1981 Society of Exploration Geophysicist conference (Penfield and Camargo 1981). Unfortunately, core samples of the area were not available. So, Penfield and Camargo, lacking additional evidence, turned their attention elsewhere, and no one paid too much attention to their findings.

A decade later, Alan Hildebrand was searching for the evidence of a giant impact in the Caribbean. Florentine Morás had previously published a paper identifying “tektites” in Haiti, which he attributed to volcanic activity. Hildebrand reviewed Morás’ results and found they included tektites as well as shocked quartz, which he presumed could have been produced by a large impact event. Hildebrand, along with his advisor William Boynton, presented their results in 1990 (Hildebrand and Boynton 1990). Around the same time, a Houston reporter named Carlos Byars told Hildebrand about the work Penfield and Camargo had done and that they might have located the crater he was looking for. Hildebrand contacted Penfield and things started to move quickly.

Hildebrand and Boynton, along with David Kring, took a closer look at the evidence provided by Penfield and Camargo. They found a crater over 100 miles in diameter near the Yucatan. Both groups (Kring, et al. 1991) and (Penfield and Camargo 1991) presented papers at a 1991 Lunar and Planetary Science conference. Later that year, they joined with Mark Pilkington and Stein Jacobsen to publish a more comprehensive paper (Hildebrand, et al. 1991). The next year, in 1992, Kring and Boynton (Kring and Boynton 1992) determined that the crater formed at the same time as the K-Pg extinction event.

Studies by Kevin Pope and others (Pope et al., 1991 and Pope et al. 1996) further delineated the outline of a giant crater, lending more support for a giant impact in the Yucatan Peninsula.

The Impact of the Asteroid

The asteroid would have sent an enormous amount of dust and rock into the atmosphere, blocking the sunlight, and cooling the Earth for months. Fires would have erupted from falling debris. There would also have been earthquakes and tsunamis. Soon the food chain would have been disrupted, killing off much of the plant and animal life on the planet.

Deccan Traps and Volcanic Eruptions

Although most scientists agree that the asteroid was the final blow that did the dinosaurs in, some think that the asteroid might not have been the only cause. The Deccan Traps in India released a huge amount of magma around 65 million years ago, which was about the same time as the asteroid impact and the extinction of the dinosaurs.

• There have been some arguments that the lava released from the Deccan Traps may have caused a global climate change that initiated the decline of the dinosaurs before the asteroid caused the final blow.
• Others have found that the bulk of the lava flow occurred after the asteroid impact and the asteroid itself could have triggered the additional eruptions. In this case, the eruptions could have hastened the demise of the dinosaurs, but only after the asteroid hit.

This debate over whether the dinosaurs were already in decline (Pincelli, 2020) or were still thriving  when the asteroid hit (Garcia-Giron, 2022) appears to be continuing even today.

Was it a comet?

Although most scientists support the theory that a large rock from space killed off the dinosaurs, there is still some debate over where the rock came from. Most think it was an asteroid and a recent study (Nesvorny, et al. 2021) suggests that it came from the outer part of the asteroid belt. However, another study (Siraj and Leob, 2021) think it could have been the piece of a comet that broke up near the Sun. In any event, it was probably a very big object of rock or ice.

Conclusion

The impact of a 6- to 9-mile (10 to 15 km) wide asteroid created the Chicxulub crater and killed off the dinosaurs. This is the leading theory for the dinosaurs’ disappearance. In 2010, forty-one scientists published an article (Schulte et al. 2010) saying that the asteroid did it. Although, there is still some discussion that volcanism may have contributed in some way. In any event, the dinosaurs are gone, and we (mammals) are here. So, it was a bad day for the dinosaurs 66 million years ago, but it was a good day for us.