A collision 160 million years ago of two asteroids orbiting between Mars and Jupiter sent many big rock chunks hurtling toward Earth, including the one that zapped the dinosaurs, a trio of scientists said on Wednesday.
Their research offered an explanation for the cause of one of the most momentous events in the history of life on Earth -- a 10km-wide meteorite striking Mexico's Yucatan Peninsula 65 million years ago.
The smash drove a giant sliver of rock into Earth's path, eventually causing the climate-changing impact that ended the reign of the dinosaurs and enabled the rise of mammals.
The impact is thought to have triggered a worldwide environmental cataclysm, expelling vast quantities of rock and dust into the sky, unleashing giant tsunamis, sparking global wildfires and leaving Earth shrouded in darkness for years.
Mixing skills in carbon chemistry, time travel and jigsaw-making, US and Czech researchers -- William Bottke, David Vokrouhlicky and David Nesvorny of Southwest Research Institute in Colorado -- carried out a computer simulation of the jostling among orbital rubble left from the building of the Solar System.
The results of their research was reported in this week's issue of the journal Nature.
They were guided by an intriguing clue -- a large asteroid called (298) Baptistina, which shares the same orbital track as a group of smaller rocks.
Turning the clock back, the simulation found that the Baptistina bits not only fitted together, they were also remnants of a giant parent asteroid, around 170km across, that once cruised the innermost region of the asteroid belt.
Around 160 million years ago -- the best bet in a range of 140-190 million years -- this behemoth was whacked by another giant some 60km across.
From this soundless collision was born a huge cluster of rocks, including 300 bodies larger than 10km and 140,000 bodies larger than 1km.
Over aeons, the fragments found new orbits with the help of something called the Yarkovsky effect, in which thermal photons from the Sun give a tiny yet inexorable push to orbiting rocks.
As the family gradually split up, a large number of chunks -- perhaps one in five of the bigger ones -- crept their way out of the asteroid belt and became ensnared by the gravitional pull of the inner planets.
Around 65 million years ago, a 10km piece crunched into Earth blasting out the Chicxulub Crater -- which measures about 180km -- and unleashing a firestorm and kicking up clouds of dust that filtered out sunlight.
The event is called the Cretaceous/Tertiary Mass Extinction. In this enduring winter, much vegetation was wiped out and the species that depended on them also became extinct. Only those animals that could cope with the new challenge, or could exploit an environmental niche, survived.
"Dinosaurs were around for a very long time. So the likelihood is they would still be around if that event had never taken place," Bottke said.
"Was humanity inevitable? Or is humanity just something that happened to arise because of this sequence of events that took place at just the right time. It's hard to say," he said.
The researchers estimated that there also was about a 70 percent probability that the prominent Tycho crater on the moon, formed 108 million years ago and measuring about 85km across, was also carved out by a remnant of the earlier asteroid collision.
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