Saturn’s rings are very chemically complex, new research shows, and actively change the makeup of the planet’s atmosphere.


Titan in front of Saturn and its rings.
Image credits NASA / JPL-Caltech / Space Science Institute.

Data beamed back from the Cassini spacecraft during its final descent into the depths of Saturn shows that the giant’s rings are more chemically complex than we’ve believed.

If you like it, study the rings on it

“This is a new element of how our solar system works,” said Thomas Cravens, professor of physics & astronomy at the University of Kansas and a co-author of the new paper.

Cravens is a member of Cassini’s Ion and Neutral Mass Spectrometer (INMS) team. Back in 2017, as Cassini plunged into Saturn’s upper atmosphere, it sampled the chemical makeup of points at various altitudes between Saturn’s rings and atmosphere using its onboard mass spectrometer.

The paper reports finding a surprising chemical complexity in the planet’s rings. This challenges the current view, based on past observations, that the rings “would be almost entirely water”, Cravens explains.

“Two things surprised me. One is the chemical complexity of what was coming off the rings — we thought it would be almost entirely water based on what we saw in the past. The second thing is the sheer quantity of it — a lot more than we originally expected.”

“the mass spectrometer saw methane — no one expected that. Also, it saw some carbon dioxide, which was unexpected,” Cravens explains. “The rings were thought to be entirely water. But the innermost rings are fairly contaminated, as it turns out, with organic material caught up in ice.”

The INMS-readings were performed in the gap between the inner ring and upper atmosphere. They uncovered the presence of water, methane, ammonia, carbon monoxide, molecular nitrogen, and carbon dioxide in the rings.

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Dust grains from Saturn’s D (innermost) ring constantly rain down into the planet’s upper atmosphere, carrying a coating of this ‘chemical cocktail’. This process takes place at an extraordinary rate, the team adds — 10 times faster than previously estimated. This process is powered by the different spin rates of the planet and its rings (the rings spin faster than the planet’s atmosphere). Over time, this process likely changed the carbon and oxygen content of Saturn’s atmosphere.

“We saw it was happening even though it’s not fully understood,” Cravens adds. “What we saw is this material, including some benzine, was altering the uppermost atmosphere of Saturn in the equatorial region. There were both grains and dust that were contaminated.”

The findings not only shed light on the chemical complexity of planetary rings, but also raise important questions pertaining to their formation, lifespan, and interaction with the host planet.

For example, given the very high rate of material transfer to the atmosphere, it may be safe to assume that planetary rings are much more short-lived than previously estimated. In the absence of a source of fresh material to make up for this particle flow, rings may simply drain away into nothingness. One possibility that derives from these findings is that Jupiter likely also had its own set of fully-fleshed out rings, which gradually drained into the wispy trail that surrounds the gas giant today.

The origin of these complex materials is also of interest to astronomers; “[is material in the rings] left over from the formation of our solar system? Does it date back to proto pre-solar nebula, the nebula that collapsed out of interstellar media that formed the sun and planets?”

Finally, the team reports that this influx of matter also impacts the planet’s ionosphere by converting hydrogen ions and triatomic hydrogen ions into heavier molecular ions — thereby depleting the ionosphere of charged particles.

But Cravens’ main contribution involved interpreting that data with a focus on how materials from the rings are altering Saturn’s ionosphere.

“My interest was in the ionosphere, the charged-particle environment, and that’s what I focused on,” Cravens said. “This gunk coming in chews up a lot of the ionosphere, affects its composition and causes observable effects — that’s what we’re trying to understand now. The data are clear, but explanations are still being modeled and that will take a while.”

The paper has been published in the journal Science.