Friday, September 08, 2017

Getting to the Core of Molten Planetary Cores

Aside from being hot, what does the liquid, metallic core of a newly formed planet have in common with a bowl of chicken noodle soup?

It turns out that in both cases, the liquid can mix and mingle by convection—the process in which cooler, denser regions sink to the bottom and hotter, less dense regions travel upward.

In new work published last week in Physical Review Letters, scientists Elliot Kaplan, Nathanaël Schaffer, Jérémie Vidal, and Philippe Cardin from Grenoble Alps University in France use powerful supercomputers to simulate the convection that takes place in liquid metal planetary cores. Their results suggest that convection could impact planetary magnetic fields in important ways.

“Planets form as hot spheres of molten material and slowly cool over the ensuing eons,” according to Kaplan. Today, the Earth has a solid inner core surrounded by a liquid outer core. The moon has a solid inner core as well, thought to be surrounded by a softer, molten layer. Gas giants like Jupiter and Saturn probably have rocky cores, but we know even less about them.

If a newly formed rocky planet cools quickly enough, the cooling process leads to convection currents along which cooler areas sink and hotter areas rise. This churning has important implications for life—just consider our situation on Earth. As the Earth’s hot, liquid outer core cools, convection currents churn up the liquid. Because the liquid is metallic, the convection currents, aligned by the spin of the planet, are capable of producing strong electric currents—and the associated magnetic fields—via the dynamo effect.

Simulated convection in a liquid metal planetary core. The colors represent temperatures, ranging from cooler (purple) to hotter (orange). The pink lines indicate flow direction.
Image Credit: E. J. Kaplan, N. Schaffer, J. Vidal, and P. Cardin, Phys. Rev. Lett. 119, 094501 – Published 31 August 2017, https://doi.org/10.1103/PhysRevLett.119.094501.
The laws of physics prevent planets with a purely solid core or a liquid core at rest from having magnetic fields, which means they might not be great places to live. After all, the Earth’s magnetic field forms the shield-like magnetosphere that protects the atmosphere from erosion by harmful solar wind.

In order to study the convection in planetary cores, scientists have traditionally used simplified computer models. However, the Grenoble Alps University team was able to attack the problem in more detail than ever before thanks to advances in computing. They used a thorough two-dimensional model and then a fully three-dimensional computer system to explore the conditions for planetary core convection, as well as what happens when variables like the cooling rate and rotation rate change.

Their results verify something hinted at in previous studies but never before confirmed: The vigorous convective motion in liquid metal cores can last longer than expected. In order for convection to occur in its core, a planet must be cooling at or above a minimum rate. However, this research showed that in some cases, the core continues experiencing vigorous convection even when the cooling rate sinks below the minimum rate. This rare property is called subcriticality. Furthermore, their work shows that at some point, convection abruptly stops.

This is a still image from a 3D simulation of thermal convection in the liquid metal of a rapidly rotating planetary core. Check out the animated version and detailed description.
Image Credit: Nathanaël Schaffer..
“This finding was expected from the 70's... but no one could really show that thermal convection in a rapidly rotating sphere could be subcritical. It is really the increasing power of computers which gave us the opportunity to discover this effect,” according to the project supervisor Philippe Cardin. In fact, some of the team members and many people outside of the team weren’t convinced that the 3D computation was possible, but together the team pulled it off.

The subcriticality seems related to the combination of planetary spin and the core’s excellent ability to conduct heat, but it’s still a mystery. “We do not understand fully the physical mechanism responsible for the subcriticality,” says Cardin. The results show that the mechanism for subcriticality is different than everything that has been previously proposed, continues Schaffer. “To our knowledge, no theoretical framework can capture this effect.” The team hopes that this research will inspire theoretical studies that could explain their findings.

The subcriticality could shed light on some questions about magnetic fields in our solar system. For example, as the moon’s liquid core chilled, evidence suggests that its once strong magnetic field suddenly disappeared. This sudden disappearance could be explained by a rapid halt in convection in the moon’s core. There is similar geological evidence for the sudden loss of a magnetic field on mars as well.

Not only can this work help us understand the early history of the rocky planets in our solar system, it may also help us better identify potentially habitable planets. The more we understand about the planetary conditions that support life and how they evolve (and potentially disappear), the better equipped we are to gauge their ability to support life. That’s something to think about the next time you sit down to a bowl of chicken noodle soup.

Kendra Redmond

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