October 8, 2013
Earth’s Core Formed As Iron Trickled Down Like Dripping Coffee
Lee Rannals for redOrbit.com – Your Universe Online
Scientists writing in the journal Nature Geoscience say that a similar process to that which allows water to yank oils from ground coffee in order to make a cup of joe in the morning, could be how the Earth's core formed.Stanford University scientists recreated the intense pressures and temperatures inside Earth and found that an iron melt network may have helped grow the Earth's core. The finding revisits a theory first proposed about 50 years ago that suggested the Earth's iron-rich core and layered internal structure might have formed in a series of steps that took place over millions of years.
"We know that Earth today has a core and a mantle that are differentiated. With improving technology, we can look at different mechanisms of how this came to be in a new light," said study leader Wendy Mao, an assistant professor of geological and environmental sciences at Stanford, and of photon sciences at the SLAC National Accelerator Laboratory, which is operated by the university.
The interior of Earth is divided into layers, with the rocky mantle composed mostly of silicates overlying an iron-rich metallic core. Scientists believe the beginnings of this core were the result of small bodies made up of rock and metals crashing and clumping together shortly after the formation of the sun and the birth of the solar system about 4.5 billion years ago.
One theory says that the heat generated by the collisions and by the radioactive decay of certain isotopes helped warm the Earth. This means the planet would have gotten so hot that its rocks and metals melted, creating a "magma ocean" that would have separated into distinct layers as a result of the metals' different densities. Iron would have drifted downward towards the planet's center, while silicates remained on top.
Pockets of molten iron trapped in the mantle layer could tunnel through the surrounding rock to create channels. This network of tunnels could have helped funnel molten iron towards the planet's center.
Scientists debunked this "percolation" theory when they discovered that the molten ironin the mantle tended to form into isolated spheres that didn't interact with one another. However, a recent experiment has brought new evidence that percolation could still be a viable mechanism for explaining the formation of Earth's core.
The experiment confirmed the findings from previous studies that molten iron in the upper mantle tended to form isolated blobs, which would have prevented percolation from happening.
"In order for percolation to be efficient, the molten iron needs to be able to form continuous channels through the solid," Mao said in a press release.
The scientists found that at the higher pressures and temperatures that would have been present in the early Earth's lower mantle, the structure of the silicates changed in a way that permitted connections to form between pockets of molten iron, making percolation possible.
“Scientists had said this theory wasn’t possible, but now we’re saying, under certain conditions that we know exist in the planet, it could happen,” Mao said. “So this brings back another possibility for how the core might have formed.”
Although the team's findings renews the idea of percolation, it doesn't necessarily mean that other theories are out. The study's first author, Crystal Shi, a graduate student in Mao’s lab, said that they do not know which mechanism happened first, or if the two happened together.
“At the very beginning, Earth would have still been very hot, and the magma ocean mechanism could have been important. But later as the planet cooled, percolation may have become the dominant mechanism," Shi said in a press release.