Long 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.