Huge Asteroids Brought Gold to Infant Earth, Study Says

Just as wise men are said to have brought gold to baby Jesus, huge asteroids may have brought gold and other precious metals to infant Earth, new research suggests.
Scientists have long known that there's a mysterious amount of siderophile ("iron-loving") metals in Earth's mantle. Such metals, including gold, tend to affiliate with iron in their liquid forms.
The best explanation has been that some sort of space object brought the elements to the planet just after it formed its core, but the exact nature of the impactor has been a matter of debate.
Based on computer simulations, the new study says that a small number of enormous, random impacts roughly 4.5 billion years ago are the sources of Earth's iron-loving materials.
These impactors were rocky objects left over from our solar system's planet-formation phase. The largest one that hit Earth was roughly the size of Pluto—up to 2,000 miles (3,220 kilometers) wide, the study suggests.
And young Earth wasn't the only recipient: Cataclysmic collisions delivered iron-loving metals to the moon and Mars around the same time, the study authors say. What's more, the impacts may have been the source of water on the moon.
"These elements are telling us about what was hitting these worlds in sort of the 'last gasp' growth spurt that they had," said study leader William Bottke, of the Southwest Research Institute in Boulder, Colorado.
Impact Odds Like Rolling the Dice
Moon rocks brought back during the Apollo missions led to the now widely accepted theory that the moon formed when a Mars-size object crashed into early Earth.
Energy from the impact would have spurred the still forming Earth to develop its mostly iron core. When this happened, iron-loving metals should have followed molten iron down from the planet's mantle and into the core.
But we know that gold and other iron-lovers are found in modest abundances in Earth's mantle. (Explore Earth's insides.)
Using a mathematical approach called Monte Carlo analysis, Bottke's team calculates that iron-loving metals were delivered in a limited number of massive impacts that just happened to miss the moon.
In cross section, the moon is about one-twentieth Earth's size, so one might expect the moon to have one-twentieth as many precious metals in its mantle, if the materials were delivered by impacts.
That's because, if millions of impacts had occurred, the odds of objects hitting Earth and the moon would likely have evened out to sustain the 1-to-20 ratio.
Instead, the moon has one-thousandth as many iron-loving metals as Earth. According to the study, published this week in the journal Science, those odds can be explained if a limited number of massive impactors were involved.
A lucky roll of the dice could easily have meant that a huge object missed the smaller moon but smashed into Earth—creating the metal discrepancies we see today.
Asteroid Belt Puts Proof in the Pudding?
"It's a cute result, but how do you prove such a thing?" Bottke said. The key, he thinks, is to look at the existing remnants of planet formation in our solar system, aka asteroids.
In the inner asteroid belt, the three largest space rocks—Ceres, Pallas, and Vesta—range from 300 to 600 miles (483 to 966 kilometers) across.
These bodies are much larger than the biggest of the rest, which measure only 150 miles (241 kilometers) across, and no "in between" sizes seem to exist.
"Most of the mass is in the biggest objects," Bottke said. "It's a top-heavy size distribution that is consistent with the kinds of populations needed to make what we see on the Earth and moon."
Martian craters tell a similar tale. The sizes of the oldest impact basins on the red planet appear consistent with a theory that Mars was hit by a population of space objects dominated by a few large asteroids, Bottke said