WASHINGTON (AFP) - Dinosaurs were wiped out by a huge asteroid that smashed into Earth 65 million years ago with the force of a billion atomic bombs, scientists said, hoping to lay an age-old debate to rest once and for all.
The definitive verdict came from an international panel of experts who reviewed 20 years' worth of evidence about what caused the Cretaceous-Tertiary (KT) extinction that wiped out more than half the species on the planet.
They determined it was a massive asteroid, measuring around 15 kilometres (nine miles) wide, which smashed into what is today Chicxulub in Mexico.
The event marked a pivotal point in history because it cleared the way for mammals to become the dominant species on Earth.
"The asteroid is believed to have hit Earth with a force one billion times more powerful than the atomic bomb at Hiroshima," the researchers said in a report published in the journal Science. "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."
The 41 scientists in the panel hope their findings will lay to rest once and for all the debate about what caused the KT extinction.
Some scientists have argued that dinosaurs and species including bird-like pterosaurs and large sea reptiles were wiped out by a series of volcanic eruptions in what is now India that lasted some 1.5 million years.
The eruptions spewed enough basalt lava across the Deccan Traps in west-central India to fill the Black Sea twice and were thought to have caused a cooling of the atmosphere and acid rain on a global scale.
But the evidence gathered for the study published in Science showed that marine and land ecosystems were destroyed rapidly in the KT extinction, leading the scientists to rule out volcanic activity as the culprit, because its effects would have whittled away at dinosaurs and other species over time.
"Despite evidence for relatively active volcanism in the Deccan Traps at the time, marine and land ecosystems showed only minor changes within the 500,000 years before the time of the KT extinction," the scientists said.
"Computer models and observational data suggest that the release of gases such as sulphur into the atmosphere after each volcanic eruption... would have had a short-lived effect on the planet and would not cause enough damage to create a rapid mass extinction of land and marine species."
The Chicxulub asteroid, on the other hand, could very well have made short shrift of dinosaurs, pterosaurs and other species, the scientists said.
The impact of the large asteroid would have "triggered large-scale fires, earthquakes measuring more than 10 on the Richter scale, and continental landslides which created tsunamis," said Joanna Morgan, a lecturer in geophysics at Imperial College, London and co-author of the study.
The asteroid hit Earth 20 times faster than a speeding bullet and exploded into a deadly mix of hot rock and gas which would have "grilled any living creature in the immediate vicinity that couldn't find shelter," said Gareth Collins, a research fellow at Imperial College.
"The final nail in the coffin for the dinosaurs happened when blasted material was ejected at high velocity into the atmosphere," shrouding the planet in darkness and causing a global winter that killed off species that "couldn't adapt to this hellish environment," added Morgan.
Another clue that the KT extinction was caused by a huge asteroid and not volcanic activity was evidence in geological records of "shocked" quartz in rock layers at KT boundary levels around the world.
Quartz is "shocked" when it is hit very quickly by a massive force -- such as a 15-kilometre-wide asteroid traveling 20 times faster than a bullet.
The KT extinction marked the end of the 160-million-year reign of the dinosaurs and allowed mammals, and eventually humans, to become the dominant species on earth.