Iron, carbon and oxygen subjected to intense temperatures and pressures form a pocket of iron oxide (bottom, center) and a darker pocket of diamond (bottom, right). Electron micrograph courtesy of Ohio State University.
SAN FRANCISCO, US: A new study suggests that some stars in the Milky Way could harbor “carbon super-Earths” – giant terrestrial planets that contain up to 50 percent diamond. But if they exist, those planets are likely to be devoid of life.
The finding comes from a laboratory experiment at Ohio State University, where researchers recreated the temperatures and pressures of Earth’s lower mantle to study how diamonds form there.
The larger goal was to understand what happens to carbon inside planets in other solar systems.
Wendy Panero, associate professor, Ohio State University and doctoral student Cayman Unterborn used what they learned from the experiments to construct computer models of the minerals that form in planets composed with more carbon than Earth.
“It’s possible for planets that are as big as fifteen times the mass of the Earth to be half made of diamond,” Unterborn said.
Earth’s core is mostly iron and the mantle mostly silica-based minerals, a result of the elements that were present in the dust cloud that formed into our solar system. Planets that form in carbon-rich solar systems would have to follow a different chemical recipe with direct consequences for the potential for life.
Diamonds transfer heat so readily, however, that a carbon super-Earth’s interior would quickly freeze. That means no geothermal energy, no plate tectonics, and – ultimately – no magnetic field or atmosphere.
“Our results are striking, in that they suggest carbon-rich planets can form with a core and a mantle, just as Earth did. However, the cores would likely be very carbon-rich – much like steel – and the mantle would also be dominated by carbon, much in the form of diamond.”
“We think a diamond planet must be a very cold, dark place,” said Panero.
She and former graduate student Jason Kabbes subjected a tiny sample of iron, carbon, and oxygen to pressures of 65 gigapascals and temperatures of 2,400 Kelvin.
As they watched under the microscope, the oxygen bonded with the iron, creating iron oxide – a type of rust – and left behind pockets of pure carbon, which became diamond.
Based on the data from that test, the researchers made computer models of Earth’s interior, and verified what geologists have long suspected – that a diamond-rich layer likely exists in Earth’s lower mantle, just above the core.
When they modeled what would happen when these results were applied to the composition of a carbon super-Earth, they found that the planet could become very large, with iron and carbon merged to form a kind of carbon steel in the core, and vast quantities of pure carbon in the mantle in the form of diamond.
“We’re looking at how volatile elements like hydrogen and carbon interact inside the Earth, because when they bond with oxygen, you get atmospheres, you get oceans – you get life,” said Panero. “The ultimate goal is to compile a suite of conditions that are necessary for an ocean to form on a planet.”
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