Planetary History Written in Saturn’s Rings


Saturn may appear calm and motionless from afar, but the immense planet is subtly pulsing and oscillating — and those oscillations impose a pattern on the planet’s rings that could tell us about Saturn’s history.

A Planet in Motion

Close-up of Saturn's rings

This extreme close-up of Saturn’s rings from Cassini shows the alternating dark and light bands of spiral density waves. [NASA/JPL-Caltech/Space Science Institute]

As the Cassini spacecraft orbited Saturn, it watched light trickle through the planet’s icy rings as they passed in front of distant stars. The flickering starlight revealed density waves — alternating stripes of compacted and loose material. Those density waves tell us much more than just what’s going on in the rings — they also tell us about the motions of Saturn’s surface.

Yanqin Wu (University of Toronto, Canada) and Yoram Lithwick (Northwestern University) combined observations and theory to study Saturn’s surface oscillations. They found that impacts from small objects were the most likely cause of the oscillations, with convection and atmospheric storms playing a minor role. Each of those impacts caused Saturn to “ring” like a bell, and the volume of the “sound” that we hear now depends on how hard it was struck, how many times, how long ago, and how quickly it fades.

Wu & Lithwick 2019 Fig. 4

Energies associated with different oscillation modes as derived from Cassini observations (black squares) and theory (colored circles and grey dashed line). While the impact theory matches the observations well for high l-values, it’s several orders of magnitude too low at low l-values. Alternative explanations, shown in the right-hand plot, match the data more closely at those low l-values. Click to enlarge. [Wu & Lithwick 2019]

Ringing Like a Bell

Saturn’s oscillations diminish as energy is carried away by the density waves in its rings, a process that can take up to 20 million years. By considering the expected frequency and size of impacts over that time period, the authors find that collisions in the distant past could have imparted enough energy to set Saturn ringing in the way we see today — with the exception of a few oscillation modes.

The authors explored several possibilities to explain the mismatch. Saturn could have experienced a once-in-a-million-year impact within the past 40,000 years — a so-called “lucky” strike. It’s also possible that some oscillation modes fade away more quickly than others or that energy is transferred between modes.

Another intriguing possibility is that those missing modes are excited not by impacts but by something more exotic: rock storms. These massive storms might begin deep within Saturn, where the atmospheric pressure is roughly ten thousand times higher than the pressure at Earth’s surface. Since it’s still not clear whether these massive storms actually exist, the authors acknowledge that the theory can’t yet be proved or disproved.

Wu & Lithwick 2019 Fig. 6

Simulations of two potentially observable signatures of the impact of a 150-km object: gravitational moments (left) and radial velocity (right). [Wu & Lithwick 2019]

From One Gas Giant to Another

Could oscillations be used to learn about the impact history of other planets? Since Jupiter lacks an extensive ring system to act as a dampener, any impact-induced oscillations would last far longer — potentially as long as billions of years — and we may be able to spot them.

To show this, Wu and Lithwick estimated how Jupiter would respond to a collision with a 150-km body a billion years ago. They found that the resulting changes in Jupiter’s gravitational field and surface velocity should be detectable by Juno and ground-based spectroscopy, respectively. With further study, we may be able to read the oscillations of Saturn and Jupiter to look back in time.


“Memoirs of a Giant Planet,” Yanqin Wu and Yoram Lithwick 2019 ApJ 881 142. doi:10.3847/1538-4357/ab2892