You wouldn’t expect the impact of a Pluto-sized object with the Earth to have an upside. Yet this event, 4.5 billion year ago, may be the reason why our planet has significant quantities of gold, platinum, and other highly siderophile elements—those which readily bond to iron—according to research published 8 December in Science.

The Earth was a molten ball after it was struck by a Mars-sized object early in its history, which flung out ejecta and created the Moon. The cataclysm would also have allowed heavier material, such as iron and the highly siderophile elements, to sink down to the Earth’s core, leaving the crust and upper mantle bereft of their shine.

Many researchers believe that a subsequent rain of metal-rich asteroids could have replenished reserves in the planet’s mantle, says William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who co-authored the paper.

But this scenario has a problem. If a rain of asteroids hit the Earth, they should have also impacted our nearest neighbor, the Moon, at the same rate.

“Based on their respective sizes, the Earth should have been struck about 20 times more often than the Moon, yet our planet has 1200 times more siderophile elements than its satellite,” says Bottke.

So he and his team created a model where the rain of asteroids is dominated by massive projectiles that impact the planets at random. If most of the impactors are very big objects, and nature plays roulette, the earth is more likely to get hit and it gets the lion’s share of elements, says Bottke.

If the hypothesis is correct, it means the fact that Earth has minable reserves of gold, platinum, and other siderophile elements is a random event, says Richard Walker, a geologist from the University of Maryland in College Park and another co-author.

“It could have just have easily not have happened, and then you wouldn’t be wearing a gold ring after your wedding,” he says.

While the research presents some interesting new ideas, it has a number of problems, says Jay Melosh, a geophysicist from Purdue University in West Lafayette, Indiana. Chiefly, Pluto-sized objects would themselves have a core of heavy elements and few highly siderophile elements on their surface, he says. These masses would have had to strike the Earth in just the right way to break open and spill their contents across the planet’s surface, he adds. The article does not make a compelling argument for how this could have happened, he says.

“It’s a fun, stimulating paper, it just may not very well be right,” says Melosh.

Image: Wikimedia