Graduate student Hao Cao decodes planetary magnetic fields

HaoCaoLong ago, scientists discovered that when a compass points north on Earth, it is not actually pointing to the North Pole.  The axis of Earth’s magnetic field is tilted away from the axis that the planet spins about. Every planet in our solar system with a magnetic field follows this rule, except one: Saturn.

When fourth-year graduate student Hao Cao came to UCLA, his advisor, Professor Chris Russell, presented him the opportunity to study data from NASA’s Cassini mission.  Arriving at Saturn in 2004, Cassini has been orbiting the planet ever since, measuring Saturn’s magnetic field, among many other things.  Cao wondered, “What does the magnetic field tell us about Saturn?”

To answer that question, Cao needed to determine how Saturn’s magnetic field is generated.  The sixth planet from the Sun, Saturn is a gas giant composed primarily of hydrogen, the simplest and most abundant of elements in the universe.  Inside Saturn, where pressure is a million times greater than at Earth’s surface, hydrogen is thought to exist as liquid metal. The turbulent motions of this electrically conductive hydrogen are what give rise to the magnetic field of Saturn.  But metallic hydrogen is also responsible for producing the tilted magnetic field on Jupiter.  Cao knew he had to look deeper.

He began contemplating the rocky core that may exist deep in the heart of Saturn, which could shape the metallic hydrogen layer that lies above it. “Zonal winds that move across the planet could reach deep inside the planet and influence the shape of the magnetic field being generated by the metallic hydrogen layer,” said Cao.  As a result of these interactions, Cao has produced the best size estimate of Saturn’s core to date.  Twice the size and ten times the mass of Earth, but only 1/5th the size of Saturn, this core is the first to be assessed using magnetic field data.

Recently, Cao has begun trying to explain Mercury’s puzzling asymmetric magnetic field using information from NASA’s MESSENGER spacecraft.  “When you study a place like Saturn or Mercury, there are many things you learn for the first time,” said Cao.  Beyond magnetic field research, Cao is interested in many aspects of planetary science.  “Dynamo studies are only part of understanding planets. Formation, internal structure, and dynamics are all related – it’s not an isolated problem,” he said.

Watch a video profile of Hao Cao here.  Learn more about his research here.

UCLA laboratory puts a spin on fluid dynamics

Simulated “Red Spot” on Jupiter created by rotating forced vortices

The Simulated Planetary Interiors Laboratory, known more fondly as the SPINLab, is a state-of-the-art fluid dynamics research facility among only a handful of such unique labs in the world.  Funded by the National Science Foundation, the group is led by Associate Professor Jonathan Aurnou, who has dedicated over ten years of his life to building functional models of planetary cores and atmospheres.

The daily routine for Jon, his graduate students, post-doctoral scholars and researchers involves spinning large, heat-driven containers of water or liquid metal in order to understand the fundamental physics of rotating bodies.  “We are interested in explaining how strongly turbulent systems, like planetary cores and planetary atmospheres, organize into planetary-scale magnetic fields, jet systems, and vortices,” said Aurnou.

The primary device used in the lab, a rotating magnetic convection device (RoMag), is a fluid-filled cylinder that to Aurnou represents “a parcel of fluid inside a planetary core.” “The idea is to study all the ingredients that are involved in planetary core convection and dynamo generation in their simplified state,” said Jonathan Cheng, a fourth-year graduate student with Aurnou.  Dynamos, large-scale magnetic fields generated from the motions of an electrically conducting fluid, are known to exist within planets, stars and even galaxies.  Yet the detailed physics of these natural dynamos remain largely mysterious.

The Earth has a very organized magnetic field, created by convective motions in its rapidly rotating molten metal core, but other bodies such as Uranus and Neptune, the ice giants, and Jupiter and Neptune, the gas giants, have much “messier” dynamos, Aurnou said.  In his lab, however, Aurnou is more concerned with studying the underlying dynamics of fluid systems than reproducing these dynamos.  “I know there are dynamos. There are dynamos all over the solar system and on just about every star,” Aurnou said.  “I’m interested not so much in building a dynamo in my laboratory, but instead in building experiments that allow me to better understand the fundamental physics that underlie dynamo processes.”
And fundamentals have proved successful so far for the SPINLab.  Using RoMag, the team has been able to show drastic differences in rotating convection systems that are metal versus those that are water.  In water experiments, rapidly rotating systems become turbulent much faster than numerical models had predicted.  The interpretation is that planets with deep-water layers can easily break down into turbulent systems that create disordered dynamos, like those we see on the ice giants.

Marie-Curie fellow Michael Le Bars, has been working in the SPINLab for a year, taking part in a long-standing relationship between the lab and French researchers.  Le Bars investigates mixed systems that are partially convecting and partially layered, like those in our atmosphere, oceans, and stars.  These systems were thought to be well understood, but when Le Bars decided to ship his experiments all the way from France to Los Angeles and try rotating them in the SPINLab, the results were surprising.  “Rotation changes everything,” said Le Bars.  One interesting result was the production of “inverse cascades” that create columns of spinning fluid that cut across stratified layers similar to the Great Red Spot on Jupiter.

The fluid dynamics of most turbulent systems studied in the SPINLab are simply too complex for even the most advanced supercomputers to model or predict, but Aurnou and his team realize the importance of combining the two approaches.  They hope to build bridges between experimental and computational methods in order to determine “how to make models that better describe the examples we see in nature.”

Learn more about SPINLab on their website.  Watch the SPINLab educational film project here.