for National Geographic News
An asteroid collided with the still-forming Mercury some 4.5 billion years ago, sending chunks of the planet hurtling through space, scientists say.
What's more, the collision was big enough to send up to 16 million billion tons (16 quadrillion tons) of Mercury's rocky material tens of millions of miles to Earth, new computer simulations suggest.
Some scientists believe that Mercury was much larger as it was forming than it is today. It had a lighter, rockier outer layer, similar to that of Earth's, which was blasted away in the great crash, they say.
This would explain why Mercury is so different from its neighbors Venus and Earth. Mercury is very heavy for its size, due to an unusually large amount of iron, scientists believe.
All the planets are thought to have formed in much the same way, so how Mercury ended up being so different is not completely understood.
A team of scientists in Bern, Switzerland, decided to tackle the mystery. They ran a pair of extensive computer simulations to test the collision theory.
"You always try to prove an idea wrong. This work shows it could have happened in this way,'' said astronomer Jonti Horner, who will present his results tomorrow at a Royal Astronomical Society meeting in Leicester, England.
First, the scientists simulated the catastrophic collision of the young Mercury with a giant asteroid traveling at 16 miles (25 kilometers) a second.
The collision would have been so violent and energy-packed that it would have caused Mercury's outer layer to melt. Shock waves would have flung the material from the planet.
The rapidly escaping debris would have eventually cooled into particles 1 centimeter (0.4 inch) in diameter or smaller, Horner said.
At the end of this hours-long process, Mercury would have been left at 35 percent of its original size.
"You've lost more [of Mercury] than what is left,'' Horner said.
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