June 24, 2016 – Hydrogen is the most-abundant element in the universe. It’s also the simplest — sporting only a single electron in each atom. But that simplicity is deceptive, because there is still so much we have to learn about hydrogen.
One of the biggest unknowns is its transformation under the extreme pressures and temperatures found in the interiors of giant planets, where it is squeezed until it becomes liquid metal, capable of conducting electricity.
New work published in Physical Review Letters by Carnegie’s Alexander Goncharov and University of Edinburgh’s Stewart McWilliams measures the conditions under which hydrogen undergoes this transition in the lab and finds an intermediate state between gas and metal, which they’re calling “dark hydrogen.”
On the surface of giant planets like Jupiter, hydrogen is a gas. But between this gaseous surface and the liquid metal hydrogen in the planet’s core lies a layer of dark hydrogen, according to findings gleaned from the team’s lab mimicry.
Using a laser-heated diamond anvil cell to create the conditions likely to be found in gas giant planetary interiors, the team probed the physics of hydrogen under a range of pressures from 10,000 to 1.5 million times normal atmospheric pressure and up to 10,000 degrees Fahrenheit.
They discovered this unexpected intermediate phase, which does not reflect or transmit visible light, but does transmit infrared radiation, or heat.
“This observation would explain how heat can easily escape from gas giant planets like Saturn,” explained Goncharov.
They also found that this intermediate dark hydrogen is somewhat metallic, meaning it can conduct an electric current, albeit poorly. This means that it could play a role in the process by which churning metallic hydrogen in gas giant planetary cores produces a magnetic field around these bodies, in the same way that the motion of liquid iron in Earth’s core created and sustains our own magnetic field.
“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,” Goncharov added.
Studying hydrogen conditions on Jupiter is one of the objectives of the Juno mission.
“On Jupiter the pressure is so high from Jupiter’s enormous gravity, that not only has that hydrogen gas been squeezed down into a liquid, it’s squeezed so much that the electrons are coming right off the atoms and conduct electricity,” said Dr. Steven Levin, Juno Project Scientist. “It’s the swirling motion of that liquid metallic hydrogen that we think generates Jupiter’s enormous electric magnetic field.”
Pressure at Jupiter’s core is millions of times that of Earth. Scientists expect Jupiter’s core to be more liquid than solid because of the pressure, but also expect it to be much denser than Earth’s core. At these enormous pressures, the hydrogen acts like an electrically conducting metal, which is believed to be the source of the planet’s intense magnetic field.
This powerful magnetic environment creates the brightest auroras in our solar system, as charged particles precipitate down into the planet’s atmosphere. Juno will directly sample the charged particles and magnetic fields near Jupiter’s poles for the first time, while simultaneously observing the auroras in ultraviolet light produced by the extraordinary amounts of energy crashing into the polar regions. These investigations will greatly improve our understanding of this remarkable phenomenon, and also of similar magnetic objects, like young stars with their own planetary systems.
Understanding the formation of Jupiter is essential to understanding the processes that led to the development of the rest of our solar system and what the conditions were that led to Earth and humankind. Like the sun, Jupiter is mostly hydrogen and helium, so it must have formed early, capturing most of the material left after our star came to be. How this happened, however, is unclear. Did a massive planetary core form first and capture all that gas gravitationally, or did an unstable region collapse inside the nebula, triggering the planet’s formation? Differences between these scenarios are profound.
Juno gives us a fantastic opportunity to learn about the structure of Jupiter in a way never before possible and will allow us to take a giant step forward in our understanding on how giant planets form and the role that plays in putting the rest of the solar system together.