An asteroid collided with the still-forming Mercury (http://www.nationa-lgeographic.com/solarsystem/planets/mercury.html) 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.
A second simulation followed Mercury's jettisoned particles through space. The bits, Horner says, would have traveled for millions of years.
"Mercury particles would've ended up on everything in the solar system," he said.
Some particles were swept up by Jupiter's gravity and flung from our solar system. Some landed on Earth.
Most would have gone to Venus. "It's the nearest stop,'' Horner said.
Exactly where they ended up was heavily influenced by where Mercury may have been at the time of the collision and exactly where the planet was hit, the simulations found.
The simulations determined that the particles wouldn't have fallen back to Mercury.
Because of the distance the particles would have been flung during the collision, it would have taken four million years for 50 percent of the particles to fall back to Mercury. By then, they would have already been carried away by solar radiation.
John Chambers, a research astronomer with the Carnegie Institution in Washington, D.C., agreed that Mercury bits may have landed on Earth.
"If Mercury was hit, especially by something big, pieces could have escaped and hit other planets,'' he said.