The city-size asteroid that slammed into Earth with the force of a million nuclear bombs is what snuffed out the dinosaurs and half of all other species on Earth 65 million years ago, an international group of scientists who reviewed all available evidence said today.

The asteroid that ended the 160 million-year reign of the dinosaurs was about 10,000 times more massive than the total mass of the human world population, according to the University of Texas at Austin. “At impact, the asteroid is estimated to have been traveling at 20 kilometers per second (44,640 miles per hour), roughly 20 times the speed of a rifle bullet,” the university said in a news statement. Impacts of this size on Earth are thought to happen on average about once every hundred million years.

Illustration courtesy of NASA

The cataclysmic extinction of that time was not caused by massive volcanic activity, as another theory has suggested, according to the new analysis, published today in the journal Science.

A panel of 41 experts from Europe, the U.S., Mexico, Canada and Japan analyzed new data from ocean drilling and continental sites and reviewed the research of palaeontologists, geochemists, climate modelers, geophysicists and sedimentologists who have been collecting evidence over the last 20 years to determine the cause of the Cretaceous-Tertiary (KT) extinction, which happened around 65 million years ago.

“The impact hypothesis has grown stronger than ever.”

“They find that alternative hypotheses are inadequate to explain the abrupt mass extinction and that the impact hypothesis has grown stronger than ever,” the University of Texas at Austin said in a news statement.

“Today’s review of the evidence shows that the extinction was caused by a massive asteroid slamming into Earth at Chicxulub (pronounced chick- shoo-loob) in Mexico,” said Imperial College London, in a separate statement.

Scientists from both institutions participated in the study.

The KT extinction wiped out more than half of all species on the planet, including the dinosaurs, birdlike pterosaurs and large marine reptiles, clearing the way for mammals to become the dominant species on Earth, Imperial College added in its release.

The impact released about a billion times more energy than the atomic bombs dropped on Hiroshima and a million times larger than the largest nuclear bomb ever tested, said the University of Texas. “The initial impact crater was about 100 kilometers (60 miles) wide and 30 kilometers (18 miles) deep.”

Illustration courtesy of NASA

“The asteroid, which was around 15 kilometers [9 miles] wide, is believed to have hit Earth with a force one billion times more powerful than the atomic bomb at Hiroshima. It would have blasted material at high velocity into the atmosphere, triggering a chain of events that caused a global winter, wiping out much of life on Earth in a matter of days,” Imperial College said .

The asteroid smacked Earth on what is today’s Mexico’s Yucatan peninsula.

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Illustration courtesy of NASA

Some scientists have suggested that the Chicxulub impact happened 300,000 years before the KT boundary, and therefore came too early to have been the major cause of extinctions, the University of Texas said.

The KT boundary, also known as the Cretaceous-Paleogene (K-Pg) boundary, is how geologists refer to the time of the great extinction. Life on Earth after the event that caused the extinction was dramatically different to what was on the planet before .

The Cretaceous Creekbed diorama in the Prehistoric Journey exhibit at the Denver Museum of Nature & Science shows two Stygimoloch spinifer dinosaurs fighting in the woodlands of North Dakota. The scene, based on an actual fossil site, represented the final ecosystem of the Cretaceous. Paleontology from this site documents the extinction of the dinosaurs as well as more than 50 percent of plant and insect species.

Image courtesy of the Denver Museum of Nature and Science

Scientists who suggest that the Chicxulub impact came earlier point to deposits at sites around the Gulf of Mexico with a layer of tiny glass-like blobs of melted impact material that, according to their interpretation, was deposited at about 300,000 years before the mass extinction, the University of Texas said.

“As an alternative, they suggest the Deccan Traps–unusually active volcanoes in what is now India–led to global cooling and acid rain, and were the major cause of mass extinction, not the Chicxulub impact in Mexico.”

However, the University of Texas added, the reviewers of the study published today find that what appears to be a series of layers neatly laid down over 300,000 years near the impact site were actually violently churned and then dumped in a thick pile in a very short time.

The Earth Impact Effects Program website created by Gareth Collins, from the Department of Earth Science and Engineering at Imperial College London, allows the user to select different effects such as air blasts, seismic shaking and thermal radiation to determine their effect on Earth after the Chicxulub asteroid impact. The illustration at the top shows intensity of seismic shaking around the point of impact. At bottom is the area covered by ejecta, or material blasted into the air.

Illustrations courtesy of Imperial College London

“Models suggest the impact at Chicxulub was a million times more energetic than the largest nuclear bomb ever tested. An impact of this size would eject material at high velocity around the world, cause earthquakes of magnitude >10, continental shelf collapse, landslides, gravity flows, mass wasting and tsunamis and produce a relatively thick and complex sequence of deposits close to Chicxulub.”

“If we are to unravel the sequence of events across the K-Pg boundary, perhaps the last place in the world we should look is close to the Chicxulub impact site, where the sedimentary deposits will be most disturbed,” write the reviewers.

In addition, the reviewers note, as you go farther from the impact site, these layers become thinner and the amount of ejected material decreases until it becomes one layer that can be found globally exactly at the KT boundary coincident with the mass extinction. Moreover, the ejecta (blasted material) within the global KT layer is compositionally linked to the specific sediments and crystalline rocks at Chicxulub .

The K-Pg boundary as exposed along the side of Interstate 25 near Raton Pass in southern Colorado. The obvious white layer is the K-Pg ejecta layer, says the University of Texas at Austin. It contains elevated levels of iridium and shocked mineral grains. Pollen and spores from Cretaceous plants are found immediately below this layer but not above it, a pattern that is seen from the southern United States all the way north to the Arctic Ocean. “This direct link between impact ejecta and plant extinction suggests a very strong cause and effect relationship between impact and extinction.”

Image courtesy of Kirk Johnson, Denver Museum of Nature and Science

The reviewers find that despite evidence for relatively active volcanism in India, marine and terrestrial ecosystems showed only minor changes within the 500,000 years before the KT boundary. “Then, precisely at the boundary, there was an abrupt and major decrease in productivity (a measure of the sheer mass of living things) and species diversity,” the University of Texas explained.

“ The Deccan hypothesis is further weakened by a review of models of atmospheric chemistry, according to the latest study. “Although significant volumes of sulfur may be emitted during each volcanic eruption and form aerosols in the stratosphere, these sulfur aerosols fall out rapidly and any adverse environmental effects are apparently only short-lasting. In comparison, during the Chicxulub impact, much larger volumes of sulfur, dust and soot were released in a much shorter time, leading to extreme environmental perturbations (such as darkening or cooling).”

“Combining all available data from different science disciplines led us to conclude that a large asteroid impact 65 million years ago in modern-day Mexico was the major cause of the mass extinctions,” says Peter Schulte, assistant professor at the University of Erlangen in Germany and lead author of the review paper.

The Earth Impact Effects Program website created by Gareth Collins shows the extent of the fireball that engulfed the impact zone (top) and the area covered by an air blast.

Illustrations courtesy of Imperial College London

“Far from Chicxulub, the geologic record clearly shows a single large meteorite hit the Earth exactly at the K-Pg boundary. Thickening of the K-Pg boundary layer towards Chicxulub shows Chicxulub was the impact site. The significant changes in Earth’s ecosystems all occur precisely at this boundary and thus, say the reviewers, a large asteroid impact into the sulfate-rich sediments at Chicxulub remains the most plausible cause for the K-Pg boundary mass extinction,” the University of Texas said.

“We now have great confidence that an asteroid was the cause of the KT extinction,” said Joanna Morgan, co-author of the review from the Department of Earth Science and Engineering at Imperial College London. “This triggered large-scale fires, earthquakes measuring more than 10 on the Richter scale, and continental landslides, which created tsunamis.

“However, the final nail in the coffin for the dinosaurs happened when blasted material was ejected at high velocity into the atmosphere. This shrouded the planet in darkness and caused a global winter, killing off many species that couldn’t adapt to this hellish environment.”

Illustration courtesy of NASA

“The asteroid was about the size of the Isle of Wight and hit Earth 20 times faster than a speeding bullet,” said Gareth Collins, Natural Environment Research Council Fellow and another co-author from the Department of Earth Science and Engineering at Imperial College London. “The explosion of hot rock and gas would have looked like a huge ball of fire on the horizon, grilling any living creature in the immediate vicinity that couldn’t find shelter.

“While this hellish day signalled the end of the 160 million-year reign of the dinosaurs, it turned out to be a great day for mammals.”

“Ironically, while this hellish day signalled the end of the 160 million-year reign of the dinosaurs, it turned out to be a great day for mammals, who had lived in the shadow of the dinosaurs prior to this event.

Paved the way for humans to become dominant

“The KT extinction was a pivotal moment in Earth’s history, which ultimately paved the way for humans to become the dominant species on Earth.”

The Earth Impact Effects Program website shows the estimated tsunami zone created by the Chicxulub impact.

Illustration courtesy of Imperial College London

In the review, the panel sifted past studies to analyze the evidence that linked the asteroid impact and volcanic activity with the KT extinction, Imperial said.

“One key piece of evidence was the abundance of iridium in geological samples around the world from the time of the extinction. Iridium is very rare in Earth’s crust and very common in asteroids. Immediately after the iridium layer, there is a dramatic decline in fossil abundance and species, indicating that the KT extinction followed very soon after the asteroid hit.

“Another direct link between the asteroid impact and the extinction is evidence of ‘shocked’ quartz in geological records. Quartz is shocked when hit very quickly by a massive force and these minerals are only found at nuclear explosion sites and at meteorite impacts sites. The team say that an abundance of shocked quartz in rock layers all around the world at the KT boundary lends further weight to their conclusions that a massive meteorite impact happened at the time of the mass extinction,” Imperial College said.