Saturn’s Rings Might Be Really Old After All

Saturn’s rings are among the most glorious, stunning, and well-studied features in the Solar System. However, their age has been difficult to ascertain. Did they form billions of years ago when the planet and the Solar System were young? Or did they form in the last few hundred millions of years?

The latest new research shows that the iconic rings are, in fact, very old.

We first became aware of Saturn’s opulent rings hundreds of years ago. Galileo was the first to see them, though he couldn’t tell they were rings in his early telescope. Nobody had ever seen anything like them before, obviously, and he thought they were moons. When he observed the planet two years later, the ‘moons’ had disappeared, leaving him confused. Another two years passed, and when he observed Saturn again, they had returned. However, the viewing angle had changed, and what he once thought were moons he concluded were ‘arms’ of some sort.

Top: Galileo’s sketch of Saturn from 1610. Bottom: Galileo’s sketch of Saturn from 1616. Image Credit: Galileo Galilei. ;<)

Decades later, Christian Huygens had a much better telescope and deduced that the features were actually rings. He described them as a “thin, flat ring, nowhere touching the planet, inclined to the ecliptic plane, and surrounding the planet without touching it.”

Fast forward to our modern age of space exploration, and scientists have gotten much better looks at Saturn and its rings. Voyager 1 and Voyager 2 opened our eyes to Saturn’s unique rings when they flew past the planet in 1980 and 1981. Those images began to reveal some of the rings’ complexity, including unusual ‘spoke’ shapes. The mystery deepened.

This Voyager 2 image from August 1981 shows the unusual dark, spoke shapes in the rings. Image Credit: NASA/JPL-Caltech

When the Hubble Space Telescope launched, it brought Saturn’s rings to life with its stunning images. It confirmed that the rings aren’t uniform and contain many fainter inner rings and ringlets. It also found that icy particles from the rings rain down on Saturn and help heat its atmosphere.

However, the Cassini spacecraft has revealed the most about Saturn’s rings. It spent 13 years investigating Saturn, its moons, and its rings.

Cassini’s data has transformed our understanding of the gas giant. No longer were scientists restricted to telescope images or fleeting flybys from the Voyager spacecraft. Cassini captured unprecedented close-up views of Saturn and its rings and gathered detailed measurements.

This is the highest-resolution image ever captured of Saturn’s rings. It shows part of the B ring. The different ringlets are part of the B-ring’s irregular structure. Cassini captured this image in July 2017. Image Credit: NASA/JPL-Caltech/Space Science Institute

Cassini revealed the complex dynamics at play in the rings and intricate details, including kinks and clumps. It showed us how the rings change over time due to Saturn’s gravity and all of its moons and moonlets. One of its biggest discoveries is that the rings are largely composed of water ice.

However, scientists are still uncertain exactly how old the rings are. Different researchers come up with different results. Some say they’re billions of years old, while others say they’re as young as 100 million years old.

New research in Nature Geoscience suggests that the rings cannot be only a few hundred million years old. It’s titled “Pollution resistance of Saturn’s ring particles during micrometeoroid impact.” The lead author is Ryuki Hyodo, a planetary scientist associated with JAXA and several universities and space agencies.

The young estimates for Saturn’s rings’ ages stem from their colouration. They appear to be clean despite their expected bombardment by micrometeoroids. The models that arrived at youthful estimates were based on high accretion rates for micrometeoroids. The logic says that if micrometeoroids bombard the ring particles and accrete efficiently, the rings should be much darker than they appear to be. Hence, they must be young. Estimates based on this arrive at an age of between 100 and 400 million years for Saturn rings.

However, those models are based on highly efficient accretion rates for micrometeoroids onto icy particles in the rings.

In the new research, Hyodo and his fellow researchers simulated the hypervelocity impacts of micrometeoroids striking icy particles. They found that the accretion may not be as efficient as previous research suggested. Instead, the non-icy micrometeorites can be vaporized, expand, and then form charged particles and ions.

These particles then leave the ring system via three main processes. They either collide with Saturn, leave the planet’s gravitational field, or are dragged into Saturn’s atmosphere electromagnetically.

This figure from the research summarizes the simulation results. a) Micrometeoroid impacts on Saturn’s rings occur at impact velocities of ~30 km?s–1. b) The impactor materials are highly shocked (>100?GPa) and form hot expanding vapour (>10,000?K). Only a small fraction of the ring particles (mass comparable to the impactor) is vaporized. c) The impact-generated vapour expands with a high velocity (on average >14?km?s–1), producing atoms/molecules and forming nanoparticles as condensates. The silicate vapour is more prone to condensation than water vapour. d) Atoms or molecules are ionized, nanoparticles are charged in Saturn’s magnetosphere, and impactor materials are removed from the ring plane by direct collision with Saturn, by escape from Saturn’s gravitational field, or by being dragged into Saturn by interaction with the electromagnetic field. Image Credit: Hyodo et al. 2024. Credit: d, NASA Goddard Space Flight Center.

The critical part of the study and how it differs from previous efforts is in the accretion efficiency of micrometeorites. Previous models used an accretion efficiency of greater than or equal to 10%. However, this study shows that the actual accretion efficiency might be much lower, greater than or equal to only 1%. That means that the rings could be much older and only appear to be clean because micrometeoroids don’t accrete as efficiently as thought and don’t ‘dirty’ the appearance of the rings.

“Thus, we suggest that the apparent youth of Saturn’s rings could be due to pollution resistance rather than indicative of young formation age,” the authors write.

This won’t be the last word on Saturn’s rings and their ages. All models have limitations, and Hyodo and his co-researchers acknowledge some limitations in theirs. Their model doesn’t account for porosity, strength, or the granularity of the ring particles.

Still, the study emphasizes that dynamic forces are at play that need to be considered in the evolution of planetary bodies and that some of our long-held assumptions need to be questioned.

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