The gold ring around your finger may symbolize “till death do us part” for you, but for scientists, it poses a problem.
That shiny band probably cost a small fortune at the jewelry store, but gold is actually abundant on the Earth’s surface (which helps explain why it’s the ideal form of money). The difficulty is, when scientists apply what they know about how the solar system formed, it’s hard to explain how all that gold (and other precious metals that bond easily to iron, like palladium and platinum) got into the Earth’s crust, where bling-loving humans could get at it. A new study in Science sets forth an explanation: In the Earth’s younger days, impacts by huge objects—perhaps even one as big as Pluto—may have brought it here.
To explain this theory, let’s start with the most dramatic impact in our planet’s history: the one that formed the moon and re-melted the solidifying Earth in the process.
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. [National Geographic]
The fact that gold and platinum settled on the surface and did not get sucked down below by molten iron appears to suggest the metals arrived via asteroids and other objects that hit the Earth after it started cooling down from moon-creating smash-up. It’s a nice and logical idea, but carries one problem: Where’s the gold on the moon?
The moon should have siderophiles [those iron-loving metals] too, and it doesn’t. Rock samples show that it has 1200 times fewer than Earth. So how to explain this discrepancy? Earth is bigger than the moon, so it attracted 20 times as much material, but that still left them with a factor of 60 to deal with. [New Scientist]
So William Bottke and his team modeled the early solar system, when “leftover planetesimal populations” were floating around after the planets we see today had formed. Bottke argues that larger objects would have dominated the scene the way that the largest asteroids, like Ceres and Vesta, are much bigger than the main population of the Asteroid Belt. If one of those Pluto-sized rocks whacked the Earth but missed the moon, he says, that could explain our siderophile discrepancy. It would also mean that gold’s place in human history in just a happy accident.
However, this argument doesn’t tie everything off with a neat little bow. There’s still one problem.
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. [Nature]
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Image: Wikimedia Commons