Hydrogen has a dark side.
For the first time, scientists from the Carnegie Institution for Science and the University of Edinburgh have transformed hydrogen into a state that exists between gas and liquid metal. It is known as dark hydrogen, and it likely occurs naturally on gas giants like Jupiter and Saturn.
“This dark hydrogen layer was unexpected and inconsistent with what modeling research had led us to believe about the change from hydrogen gas to metallic hydrogen inside of celestial objects,” said Carnegie’s Alexander Goncharov, co-author of a paper published in the journal Physical Review Letters, in a news release.
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Although hydrogen is the most-abundant and simplest element in the universe with just one electron in each atom, a big unknown is what happens to hydrogen under the extreme pressures and temperatures found in the interiors of gas giants. In these extreme conditions, hydrogen molecules separate into single atoms, and the electrons of these atoms start behaving as if they were in a metal. What results is metallic hydrogen that is capable of conducting electricity.
On the surface of gas giants like Jupiter, hydrogen is a gas. According to the recent findings of the paper, between this gas surface and the liquid hydrogen found in the planet’s core is a layer of dark hydrogen.
Illustration of the layer of dark hydrogen found beneath the surface of gas giant planets. Photo credit: Stewart McWilliams/University of Edinburgh, NASA
This surprising new state was found using a laser-heated diamond anvil cell. Goncharov, as well as Edinburgh’s Stewart McWilliams, the first author of the paper, compressed hydrogen between two diamond tips, exerting a force way beyond what we experience here on Earth. The hydrogen was subjected to pressures ranging from 10,000 to 1.5 million times normal atmospheric pressure, and up to 5,538 degrees Celsius (10,000 degrees Fahrenheit).
When hydrogen is in its dark phase, it does not reflect or transmit visible light — hence the name — but it does transmit infrared radiation, or heat. “This observation would explain how heat can easily escape from gas giant planets like Saturn,” said Goncharov.
The researchers also noted that this intermediate dark hydrogen is slightly metallic and is capable of conducting a very weak electric current. Although the electric current is not overly strong, it could still play a role in the process that produces a magnetic field around a gaseous planet.
This is the second recent discovery about the interesting properties of hydrogen. You may remember that, earlier this year, researchers at the University of Edinburgh produced a metallic form of hydrogen by subjecting it to 3.25 million times the pressure of Earth's atmosphere.
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