Researchers recreated the harsh conditions at the heart of the Earth to prove a theory that the iron core was created over a long period of time in multiple steps and layers.
"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," study leader Wendy Mao said, 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, said.
The Earth's core is composed of different layers of "silicates overlying an iron-rich metallic core." Researchers have wondered for years how the layers developed, a Stanford University press release reported.
The neat layers puzzled scientists because the core was believed to have been formed in the "chaos" shortly after the birth of the Sun about 4.5 million years ago.
Researchers are now working to find an answer to the mystery of the perplexing layers.
One theory is that radioactive decay in isotopes could have generated so much heat that they melted Earth's rocks and metal. This could have created a "magma ocean" that separated and solidified in distinct layers. The iron would have moved down towards Earth's center, leaving the silicates behind.
Other scientists have suggested that even if the Earth did not get hot enough to melt silicates, iron still could have separated and "percolated" through the silicate layer.
"The thought was that pockets of molten iron trapped in the mantle layer could tunnel through the surrounding rock to create channels, or capillaries. This network of tunnels could have helped funnel molten iron towards the planet's center to join the spherical metallic heart that was slowly amassing there," the press release reported.
The "percolation" theory was all but disproven when scientists found that when the upper mantle layer was molten it formed separated "spheres" that did not interact. This phenomenon can be compared to "when water beads up on a waxed surface."
This new experiment found the percolation could still be a possibility.
The research team placed small amounts of iron and silicate rock into metal chambers using a diamond-tipped device. Squeezing the diamonds together recreated the extraordinary pressure that would have been present at the time of formation. A laser beam was used to heat the sample to an appropriate temperature.
After the conditions were created, and then cooled down, the researchers created a 3D image of the result.
The researchers confirmed that the upper mantle would have formed "isolated blobs" before anything else.
"In order for percolation to be efficient, the molten iron needs to be able to form continuous channels through the solid," Mao said.
The team found high pressure and temperatures in the Earth's lower mantle caused the silicates to change in a way that would allow for that continuous channel to form.
"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."
There is still a possibility the Earth formed in the proposed "magma ocean" state.
"We don't know which mechanism happened first, or if the two happened together," study first author Crystal Shi, a graduate student in Mao's lab, said. "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."
