Gigantic collisions on Earth, the moon and Mars 4.5 billion years ago injected precious elements such as gold and platinum into the developing worlds, a new study suggests.
In the last days of planet formation, a body as big as Pluto likely slammed into Earth after the planet had been clobbered by a Mars-size object, researchers said. Mars and the moon absorbed smaller but still devastating blows, they added.
These traumatic cosmic crashes may have knocked Earth off its axis by 10 degrees. But they also delivered gold and other elements into the bodies' upper reaches, and possibly brought huge amounts of water to the moon, researchers said.
"We can use this information to tell us about the last great growth spurt of the Earth, moon and Mars," said study leader Bill Bottke of the Southwest Research Institute in Boulder, Colo. [Photos: Impact Craters of the Solar System]
The mystery of the plentiful platinum
Gold, platinum, palladium and other so-called "siderophile" elements have a strong affinity for iron. So they should have followed iron down into the cores of Earth, the moon and Mars as the bodies were forming, leaving a near void in their mantles and crusts, researchers said.
But siderophiles are found in these bodies' upper reaches in perplexing abundances.
"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."
One possibility is that the siderophiles were replenished shortly after core formation by collisions with planetesimals, the smaller building blocks of full-grown planets.
Bottke and his colleagues favor this explanation, and they marshal a variety of evidence in the new study to support it. They also put some numbers on how big the impacts likely were.
Modeling planet trauma
To account for present abundances of gold, platinum and the other siderophiles, researchers said, the impacts would need to deliver about 0.5 percent of Earth's mass to our planet's mantle, 10 times less mass than that to Mars and about 1,200 times less to our moon.
Using numerical models, the research team determined that this could happen if the impactors were dominated by a small number of huge space rocks.
"The populations that were hitting Earth, the moon and Mars were pretty top-heavy," Bottke said. "Most of the mass was in the big guys."
The results show that the largest Earth impactor was likely 1,500 to 2,000 miles (2,414 to 3,219 kilometers) wide. The biggest rocks to hit Mars and our moon were probably about 1,000 miles (1,609 km) and 200 miles (322 km) across, respectively, researchers said.
These big planetesimals may have delivered much of the moon's ample water along with the siderophiles, according to the study.
This bombardment came toward the end of our solar system's planet-formation period, around 4.5 billion years ago, researchers said. It likely took place within a few tens of millions of years of the most traumatic event in Earth's history: the collision with a Mars-size body that blasted a giant chunk off our planet, creating the moon.
Bottke and his colleagues published their findings online Dec. 9 in the journal Science.
Other evidence mounts
The results from the numerical model are buttressed by other evidence, researchers said.
For example, other models describing how planetesimals form and evolve suggest the biggest ones gobble up smaller ones very efficiently, leaving behind a top-heavy population. This same top-heaviness is seen in the asteroid belt, the last surviving population of planetesimals in the inner solar system, researchers said.
"The distribution of their sizes is actually remarkably consistent with what we'd expect based on our models," Bottke said.
A 1,000-mile-wide impactor is also about the right size to blast out Mars' enormous Borealis Basin if this ancient landform is indeed an impact crater, as some scientists think.
Taken together, the signs all point toward giant impacts pummeling the Earth, moon and Mars toward the end of planet formation, according to the researchers.
"We have a number of different lines of evidence to suggest that this is probably a real thing," Bottke said.
Is this violence typical?
The study paints a violent picture of the solar system's early days, with the inner planets being walloped heartily shortly after their birth. Such pummelings might be common in other newly forming solar systems, according to Bottke.
"Because we can reproduce some of these things in our models, I think that probably means we're fairly typical," he said. "But that's just a guess."
If our solar system is atypical if young planets in other solar systems were spared pummelings by massive impactors late in the formation process then alien planets might be different in significant ways, Bottke said. Their mantles, for example, might be lacking in siderophiles like gold and platinum, which we value for a variety of reasons.
In any event, Bottke said he hopes the study serves as a conversation starter, getting scientists thinking about the planet-formation process in a more general sense.
"If you can figure out how it worked here, you may be able to figure out how it worked elsewhere," Bottke said. "That's really exciting."
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You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall.