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Planets’ magnetic fields come from more complex inner flows than thought

An illustration of the experimental tank with rotating, convecting liquid gallium. Credit: Eric King.

An illustration of the experimental tank with rotating, convecting liquid gallium. Credit: Eric King.

The magnetic fields of Earth and other planets result from their dynamos—vast oceans of electrically conductive fluids such as liquid metals in their innards that roil turbulently due to convection of heat left over from the birth of those worlds. Now scientists find liquid metal may behave very differently in real life than in computer models developed so far, potentially explaining the variety of magnetic fields seen on gas and ice giant planets.

Earth’s magnetic field was likely vital to the evolution of life, preventing hydrogen from leaking away, which helped keep its water. However, much remains uncertain about how Earth’s magnetic field came to be and how, for instance, that of Mars vanished. Computer models exist to simulate dynamos, but physical experiments are needed to see how realistic those models really are.

Models of planetary dynamos generally assume fluids diffuse momentum faster than they do heat. As such, these fluids have what are called Prandtl numbers that are greater than one. However, liquid metals actually often have Prandtl numbers that are less than one, meaning they diffuse heat more readily than they do momentum.

“In metals, electrons are free to roam, and these electrons carry both thermal energy and electrical charge, making metals good conductors of both heat and electricity,” says researcher Eric King, a planetary scientist at the University of California at Berkeley. “This is why a room temperature metal often feels cold to the touch—it is very effective at pulling heat from your skin, which is warmer than room temperature, by a process known as thermal diffusion. Thermal diffusion is essentially the natural response of any material to smooth out temperature differences. Because of their free electrons, metals are better than most common materials at diffusing heat.”

Now experiments with liquid metal gallium show the convective behavior of liquid metal differs substantially from that seen in conventional models.

“All else being equal, convection in liquid metal is more turbulent than convection in a moderate Prandtl number fluid such as water,” King says.

To see how liquid metals actually behave, researchers experimented with a cylinder filled with liquid gallium, one 20 centimeters wide and 20 centimeters tall. The metal was heated up to 110°C, and the cylinder was spun at up to 40 times per minute to see what effects rotation had on convection, just as Earth’s spin might have on its core. Sensors sticking into the tank measured the flow of the metal.

“The experimental apparatus took several years to design, build, and implement,” King says. “Many of the challenges we faced centered around the use of the liquid metal gallium. Gallium freezes just above room temperature, at 85°F (29°C), and, like water, expands upon freezing. So similar to plumbing in a perpetually frozen environment, gallium storage and transfer systems had to be actively heated to avoid cracking pipes leading to potentially catastrophic spills.”

“Gallium is also corrosive, and reacts with commonly used lab materials such as aluminum and copper,” King adds. “The tank walls, which were constructed from copper for its favorable thermal properties, had to be coated with a thin layer of tungsten to prevent corrosion. Fortunately, our lab being in Los Angeles, I was able to take advantage of the local aerospace industry’s expertise in such high performance material treatments.”

The researchers discovered that convecting liquid metals interact with Coriolis forces—the same forces that help drive major winds and ocean currents on Earth—differently than more conventional fluids do.

“Coriolis forces are felt by fluids flowing in rotating containers such as Earth’s core, and we mimic this effect in the lab by rotating an experimental convection tank,” King says. “Coriolis forces tend to organize turbulent flow, and this organization is thought to be responsible for the near-alignment of Earth’s geomagnetic and geographic poles.”

More turbulent flows required stronger Coriolis forces—faster rotation rates—to be well-organized, as expected. “What is surprising is that the liquid metal flows are more easily organized by Coriolis forces than are moderate Prandtl number fluids such as water,” King says. “Our research suggests that this difference is due to how heat and momentum are transferred from the [container] to the fluid, and so depends strongly on viscosity and thermal diffusion.”

“There are two basic implications for this work,” King says. “The first is that models of planetary systems where flowing liquid metals are important, such as Earth’s core, ought to pay attention to the special material properties of the fluid conductors. This may be important as well for astrophysics, where flowing plasmas are responsible for much of the dynamical behavior of stars like our sun, and these plasmas are thought to have very small Prandtl numbers.”

“The second implication that we offer is a possible explanation for the strange dichotomy between the observed magnetic fields of the gas giants, Jupiter and Saturn, whose magnetic fields are generated by flowing metallic hydrogen, and the ice giants, Uranus and Neptune, whose magnetic fields are believed to be generated by flowing mixtures of water, methane, and ammonia. By treating the former as a low Prandtl number fluid, and the latter as a moderate Prandtl number fluid, we predict that these planetary systems may be in different dynamical states—convection within Jupiter and Saturn being well organized by Coriolis forces and giving rise to well-organized magnetic fields, and Uranus and Neptune being relatively weakly affected by Coriolis forces, giving rise to disorganized magnetic fields.”

The researchers also investigated how liquid metal convects in the presence of strong magnetic fields.

“For this purpose, we designed a custom electromagnet that encapsulates the experimental convection tank,” King says. “The entire apparatus then had to be built entirely from non-magnetic materials. Experiments on rotating convection in liquid metals with strong magnetic fields show even more surprising results, and a publication of these results is currently in preparation.”

King and his colleague Jonathan Aurnou detailed their findings online April 8 in the Proceedings of the National Academy of Sciences.

[youtube http://youtu.be/5uW0DPNYNwM]

A movie showing the experimental apparatus rotating when filled with liquid gallium. Credit: Eric King.

Categories: Applied Physical Sciences | Earth, Atmospheric, and Planetary Sciences
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