Researchers have discovered that the Earth's center may contain significant amounts of helium after discovering that the gas can interact with iron at high temperatures and pressures. (Image credit: Yuri_Arcurs via Getty Images)
New research suggests that primordial helium that has existed since the formation of the solar system may have become trapped inside Earth's rocky core, a discovery that could impact long-running debates about how quickly our planet formed.
This rare form of helium is called helium-3 because it has two protons and one neutron in its nucleus. Regular helium, which is 700,000 times more common than helium-3, is known as helium-4 because it contains two protons and two neutrons. While helium-4 is a common decay product of radioactive materials, helium-3 comes almost entirely from the original cloud of dust and gas that formed the solar system.
The existence of this primordial element inside the Earth was known. Each year, about 4.4 pounds (2 kilograms) of helium-3 comes to the surface from mid-ocean ridges where the crust is torn apart and from volcanic hot spots that push magma up from deep in the mantle. But exactly how it remained inside the planet for billions of years remains a mystery.
Helium is a very light gas, and most of the volatiles have long since left the mantle, either ejected by the giant impact that formed the Moon or thrown to the surface by the relentless movements of tectonic plates.
Scientists have suggested that perhaps this primordial helium is locked away in the Earth's core, where it is protected from major disturbances and slowly seeps to the surface. But the core is mostly iron, and helium and iron don't usually mix.
In a new study, scientists in the lab of Kei Hirose, a planetary scientist at the University of Tokyo, and his colleagues found that at core temperatures and pressures, the two elements can indeed mix. In fact, solid iron at high temperatures and pressures can contain up to 3.3 percent helium, the researchers reported Feb. 25 in the journal Physical Review Letters.
The scientists discovered this compatibility by heating iron and helium to temperatures ranging from 1,340 to 4,940 degrees Fahrenheit (727 to 2,727 degrees Celsius, or 1,000 to 3,000 kelvins) while simultaneously compressing them with a diamond-tipped anvil to pressures between 50,000 and 550,000 times greater than atmospheric pressure. They then depressurized the samples at cryogenic temperatures and analyzed their crystal structures. This approach likely prevented helium from escaping during the measurements, Hirose said in a statement.
Sourse: www.livescience.com
