Last Days Of The Dinosaurs

[Amelie Robertos]

Some66 million years ago, give or take several millennia, a 10-kilometer-wide asteroid slammed into our planet. The impact blasted out an enormous crater and heaved large amounts of material into the atmosphere. Some of the sulfur-rich debris poisoned the sky, unleashing downpours of acid rain. Heat generated by ejecta falling back to Earth ignited wildfires worldwide that blazed for months, if not years. In the wake of the event, as many as 75 percent of all species were wiped out.

Yet in devastation lay opportunities: Ecological roles that had been occupied by dinosaurs for at least 100 million years were suddenly available, setting the stage for the slow but steady rise of mammals and the world we inhabit today (SN: 2/4/17, p. 22).

A capsule carrying rocks and dust from the asteroid Bennu safely parachuted down to Hill Air Force Base in Utah on September 24. Scientists prepared for the event by dropping a mock version of the capsule (shown) in August.

When beginning to open the capsule that collected the asteroid sample, NASA scientists found extra material outside the collection container. That has delayed opening the main capsule but led to a few science discoveries.

Metallic nodules (such as the 10-centimeter one seen above being collected from the North Atlantic Ocean seafloor in 2021) can act like weak batteries and produce enough voltage to split seawater into hydrogen and oxygen, new research suggests.

A satellite captured a potential milky sea (middle) in the Indian Ocean, south of the Indonesian island of Java (the bright area outlined at the top of the image) in 2019. A yacht happened to sail by in August, its track denoted by the white line that turns turquoise where it crosses the milky sea, confirming this was indeed what the satellites observed.

Brain imaging data show small everyday changes in brain activity in a single person (green and yellow colors). After a dose of psilocybin, activity changes dramatically as neural collectives fall out of sync (yellow, orange and red). After the drug wears off, activity returns to normal.

A National Fisheries and Aquaculture service employee wearing biosafety gear approaches a sea lion at a beach in Chile that was closed in May 2023 due to the spread of bird flu in the region. That year, at least 24,000 South American sea lions died from an H5N1 infection.

Skin from a mammoth that died 52,000 years ago was so well-preserved that even the 3-D structure of its DNA was intact. Here, researchers Valerii Plotnikov and Dan Fisher examine the skin after it was excavated from Siberian permafrost.

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The aftermath is history: a monstrous crater, major extinctions and the end of dinosaurs. In fact, the impact forever changed the course of life on Earth. With dinosaurs gone, mammals rose to dominate the land. New ecosystems formed. From the ashes, a new world arose.

The fossil record clearly shows a major extinction at the end of the Cretaceous. Dinosaurs that had walked the Earth for tens of millions of years vanished suddenly. Why remained a mystery for many years.

Then in the 1980s, geologists noticed a distinct layer of rock in many places around the world. The layer was very thin, generally no more than a few centimeters (several inches) thick. It always occurred at exactly the same place in the geologic record: where the Cretaceous ended and the Paleogene Period began. And everywhere it was found, the layer was packed with the element iridium.

The iridium-rich layer was all over the Earth. And it appeared at the same moment in geologic time. That suggested that a single, very big asteroid had struck the planet. Bits of that asteroid had flown into the air and traveled around the globe. But if the asteroid was so big, where was the crater?

In 2016, a new scientific expedition set out to study the 66-million-year-old crater. The team brought a drill rig to the site. They mounted it on a platform that stood on the seafloor. Then they drilled deeply into the seabed.

The expedition drilled more than 850 meters (2,780 feet) into Chicxulub. As the drill spun deeper, it cut a continuous core through the rock layers. (Imagine pushing a drinking straw down through a layer cake. The core collects inside the straw.) When the core emerged, it showed all the rock layers the drill had gone through.

The scientists arranged the core in long boxes. Then they studied every inch of it. For some analyses, they just looked at it very closely, including with microscopes. For others, they used laboratory tools such as chemical and computer analyses. They turned up many interesting details. For example, the scientists found granite that had splashed to the surface from 10 kilometers (6.2 miles) below the Gulf floor.

Along with studying the core directly, the team also combined data from the drill core with simulations that it made using a computer model. With these, they reconstructed what had happened on the day the asteroid struck.

By studying the site, DePalma and other scientists have determined that Tanis was a riverbank near the shore of the shallow sea. They believe that the remains at Tanis were dumped within minutes of the impact by a powerful wave called a seiche (SAYSH).

Mixed into the debris at Tanis are little beads of glass called tektites. These form when rock melts, gets blasted into the atmosphere, then falls like hail from the sky. Some of the fossilized fish even had tektites in their gills. While taking their last breaths, they would have choked on those beads.

The age of the Tanis deposit and the chemistry of its tektites are an exact match for the Chicxulub impact, DePalma says. If the creatures at Tanis really were killed by the effects of the Chicxulub impact, they are the first of its direct victims ever found. DePalma and 11 co-authors published their findings April 1, 2019, in the Proceedings of the National Academy of Sciences.

When the asteroid hit, a plume of sulfur, dust, soot and other fine particles shot well over 25 kilometers (15 miles) into the air. The plume quickly spread around the globe. If you could have seen Earth from space then, Gulick says, overnight it would have transformed from a clear blue marble to a hazy brown ball.

Even within the Chicxulub crater, life came back surprisingly quickly. The intense heat of the impact would have sterilized much of the area. But Christopher Lowery found signs that some life returned within just 10 years. He studies ancient marine life at the University of Texas in Austin.

Over millions of years, the crater disappeared beneath new layers of rock. Today, the only above-ground sign is a semicircle of sinkholes that curves across the Yucatn peninsula like a gigantic thumbprint.

Those sinkholes, called cenotes (Seh-NO-tayss), trace the rim of the ancient Chicxulub crater hundreds of meters below. The buried crater rim shaped the flow of underground water. That flow eroded the limestone above, making it crack and collapse. The sinkholes are now popular swimming and diving spots. Few people who splash in them might guess that they owe their cool, blue waters to the fiery end of the Cretaceous Period.

The vast Chicxulub crater has all but disappeared from view. But the impact of that single day continues 66 million years later. It changed the course of life on Earth forever, creating a new world where we and other mammals now flourish.

Alamosaurus A genus of enormous, long-necked sauropod dinosaurs that lived in the late Cretaceous. The name comes from the sandstone deposits in New Mexico where their fossils were first found. The size of their vertebrae suggest these could have been one of the biggest dinosaurs in North America.

asteroid A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

Caribbean The name of a sea that runs from the Atlantic Ocean in the East to Mexico and Central American nations in the West, and from the southern coasts of Cuba, the Dominican Republic and Puerto Rico down to the northern coasts of Venezuela and Brazil. The term is also used to refer to the culture of nations that border on or are islands in the sea.

chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

crater A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body. Such an impact is sometimes referred to as a cratering event.

dinosaur A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs.

element A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

glass A hard, brittle substance made from silica, a mineral found in sand. Glass usually is transparent and fairly inert (chemically nonreactive). Aquatic organisms called diatoms build their shells of it.

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