In the final period of planet formation, a body possibly as big as Pluto probably collided with the Earth after the planet had been hit by an even large object, scientists at the Southwest Research Institute in Boulder, Colo., said.

Mars and the moon absorbed smaller but still devastating blows, they said.

Gold, platinum, palladium and other so-called "siderophile" elements have a strong affinity for iron and should have followed iron down into the cores of Earth, the moon and Mars as the bodies were forming, leaving a scarcity of those metals in their mantles and crusts, study leader Bill Bottke said.

But precious metals are found in these bodies' upper reaches in puzzling amounts, he said.

"The abundance of these elements is actually surprisingly high," Bottke told SPACE.com. "People have wondered, 'How can this be?' They've been arguing about it for decades."

Bottke and his colleagues favor the theory that siderophiles were replenished shortly after core formation by collisions with planetesimals, the smaller building blocks of full-grown planets.

This bombardment came around 4.5 billion years ago, near the end of our solar system's planet-formation period, researchers said.

It probably took place within a few tens of millions of years of a collision with a Mars-size body that blasted a giant chunk off our planet, creating the moon, they said.

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