“It’s a shame we don’t have it naturally here on earth, but on Jupiter, there are oceans of metallic hydrogen. We want to find out how these oceans give rise to Jupiter’s enormous magnetic field,” observed Mohamed Zaghoo with the University of Rochester’s Laboratory of Laser Energetics (LLE) and colleague Gilbert ‘Rip’ Collins, director of the high-energy-density physics program. Astrophysicists have long thought that planets with magnetic fields are better able to sustain gaseous atmospheres and are more likely to harbor life.
One of the biggest mysteries surrounding Jupiter is how it generates its powerful magnetic field, the strongest in the solar system. One theory is that about halfway to Jupiter’s core, the pressures and temperatures become so intense that the hydrogen that makes up 90 percent of the planet looses hold of its electrons and begins behaving like a liquid metal. Oceans of liquid metallic hydrogen surrounding Jupiter’s core would explain its powerful magnetic field.
“Dynamo theory and magnetic fields are key conditions of habitability,” said Zagoo who has held research appointments at Harvard’s Lyman Laboratory of Physics, Max Plank Institute for Quantum Optics, Stanford University, and MIT’s Kavli Institute for Astrophysics and Space Research. “There are hundreds of exoplanets discovered outside our solar system every year and we think many of these planets are like Jupiter and Saturn. We cannot go to these planets yet, but we can apply our knowledge about the super giants in our own solar system to make models of what these planets might be like.”
The question of how hydrogen transitions into a metallic state — whether that is an abrupt transition or not — has huge implications for planetary science. How hydrogen transitions inside Jupiter, for example, says a lot about the evolution, the temperature and the structure of these gas giants interiors.”
Metallic hydrogen is one of the rarest materials on Earth, yet more than 80 percent of planets–including Jupiter, Saturn, and hundreds of exoplanets –are composed of this exotic form of matter. “Metallic hydrogen is the most abundant form of matter in our planetary system,” says Zaghoo in July 2018 about is abundance in our solar system–despite its rarity on Earth–makes metallic hydrogen an intriguing focus for researchers who study planet formation and evolution, including how planets both inside and outside our solar system form magnetic shields.
Every element acts differently under intense pressure and temperature. Heating water, for example, generates a gas in the form of water vapor; freezing it creates solid ice. Hydrogen is normally a gas, but at high temperatures and pressures–the conditions that exist within planets like Jupiter–hydrogen takes on the properties of a liquid metal and behaves like an electrical conductor.
Although scientists theorized for decades about the existence of metallic hydrogen, it was nearly impossible to create on Earth. “The conditions to create metallic hydrogen are so extreme that, although metallic hydrogen is abundant in our solar system, it has only been created a few places on earth,” Zaghoo said. “The LLE is one of those places.”
At the LLE, researchers use the powerful OMEGA laser to fire pulses at a hydrogen capsule. The laser impinges on the sample, developing a high-pressure, high-temperature condition that allows the tightly bound hydrogen atoms to break. When this happens, hydrogen is transformed from its gaseous state to a shiny liquid state, much like the element mercury.
By studying the conductivity of metallic hydrogen, Zaghoo and Collins are able to build a more accurate model of the dynamo effect–a process where the kinetic energy of conducting moving fluids converts to magnetic energy. Gas giants like Jupiter have a very powerful dynamo, but the mechanism is also present deep within Earth, in the outer core. This dynamo creates our own magnetic field, making our planet habitable by shielding us from harmful solar particles.
Researchers can map the earth’s magnetic field, but, because the earth has a magnetic crust, satellites cannot see far enough into our planet to observe the dynamo in action. Jupiter, on the other hand, does not have a crust barrier. This makes it relatively easier for satellites–like NASA’s Juno space probe, currently in orbit around Jupiter–to observe the planet’s deep structures, Collins says.
“It is very humbling to be able to characterize one of the most interesting states of matter, liquid metallic hydrogen, here in the laboratory, use this knowledge to interpret satellite data from a space probe, and then apply this all to extrasolar planets.”
Zaghoo and Collins focused their research on the relationship between metallic hydrogen and the onset of the dynamo action, including the depth where the dynamo of Jupiter forms. They found that the dynamo of gas giants like Jupiter is likely to originate closer to the surface–where the metallic hydrogen is most conductive–than the dynamo of Earth. This data, combined with revelations from the Juno Mission can be incorporated into simulated models that will allow for a more complete picture of the dynamo effect.
“Part of the mandate for the Juno mission was to try to understand Jupiter’s magnetic field,” Zaghoo says. “A key complementary piece to the Juno data is just how conductive hydrogen is at varying depths inside the planet. We need to build this into our models in order to make better predictions about current planet composition and evolution.”
Better understanding the planets in our own solar system also provides more insight into the magnetic shielding of exoplanets outside of our solar system–and may help determine the possibility of life on other planets.
The Daily Galaxy, Sam Cabot, via University of Rochester’s Laboratory of Laser Energetics
Image at the top of the page: the giant planets are known for spectacular auroras at their polar regions. Scientists figured that heat generated by the auroras was blown toward the equator by some unknown process. An artist’s impression of Saturn’s auroras as viewed from the nightside of the planet. © John Clarke, Denis Grodent, ESA, and NASA)