Saturn’s rings are billions of years younger than we thought, say Cornell University researchers analyzing an almost forgotten set of data, collected 10 years ago by NASA’s Cassini mission.

Despite decades observing Saturn’s rings, their age remains unknown.

Following 10 years of Cassini orbiting and observing the planet, scientific thinking suggested an ancient origin, billions of years ago.

However, efforts to pin down a more definite age have stalled as researchers struggled to determine the rings’ exact composition.

Whilst we know they are predominantly composed of water ice particles, up to several meters in size, we also know these particles are continually ‘polluted’ by bombarding micrometeoroids – rocky material from the outer Solar System’s Kuiper belt – and its the proportion of these two ingredient that has proved elusive.

The ice is opaque to many wavelengths of light and therefore able to hide unknown amounts of additional rocky material.

The exact proportion is key because its accumulation should have happened in a predictable way, so combining it with estimates of the average micrometeoroid flux would give a good estimate of age.

This is why a new paper, published in the journal Icarus, from Cornell University’s Zhimeng Zhang could prove a breakthrough.

In it, Zhang navigates a set of complex data collected early in the Cassini mission by instruments intended to analyze the surface of Saturn’s moon Titan.

The Cassini Titan Radar Mapper operates at just the right wavelength in the microwave end of the electromagnetic spectrum to pierce the ice, measuring the entire ring composition, rather just that of the surface layers.

Zhang analyzed data from Saturn’s C ring, which is the most ‘polluted’ due to its relative low mass, making it easier to acquire a higher proportion of non-icy material.

“This makes the C ring ideal for investigating ring contamination and piecing together age and history,” says Zhang.

However, there was a reason the data has been be left unanalyzed for so long.

“This wasn’t low hanging fruit,” says Professor Alexander Hayes, Zhang’s supervisor at Cornell.

“It was reams of difficult to analyze data that sat there for a decade. If ZZ hadn’t come along who knows how long this would have gone untouched.”

After several months of unpicking data, Zhang showed that most regions in the C ring contained 1-2% rocky silicates. Combining this with estimates of the micrometeoroid flux gives a C ring age of somewhere between 15 and 100 million years, billions of years younger than expected.

“None of the current origin scenarios predict the rings are likely younger than 3.8 billion years old,” says Zhang. “This will force a rethinking of ring origin models.”

“It’s a very nice paper,” agrees Professor Sascha Kempf, from the University of Colorado, a proponent of an ancient age for the rings who wasn’t involved in the study.

“I find her findings about the age of the C ring quite persuasive.”

However, the data held another surprise. The analysis revealed a significant ‘hump’ of rock in the middle where silicate composition increased to between 6-11%.

According to Zhang’s models, this suggests the background micrometeoroid flux was boosted by the recent addition of a 20-km diameter rocky body, likely a Centaur from a family of mini-planets located between Jupiter and Neptune.

In her model, the centaur slowly broke up as it passed through the rings a few times, a scenario that Ryuki Hyodo, an expert on Centaurs from Kobe University, who also wasn’t involved in this paper, believes is plausible.

“I have shown recently that Centaurs can be destroyed during these extreme close encounters with Saturn,” says Hyodo. “However, the detailed process of the deposition in the ring is still unclear. We will need direct simulations.”

The centaur model sits less well with Kempf, who suggests the silicates are more likely to be from a core of a disrupted moon that was pushed inward by the spreading of the much larger, more massive ring.

A more definite answer should come from a new period of Cassini radar surveying next year when the craft will be closer to the rings than it ever has before.

To aid the collection and analysis of the new data, Zhang’s paper not only made the case for these new observations but also provides a model for how to take these tricky measurements.

“The paper is like a textbook,” says Hayes. “It describes how to calibrate and analyze the data to test and expand up ZZ’s theories during these closer passes.”

In the meantime, Zhang hopes to soon publish her own support for the new C ring age after applying her methodology and equations to Saturn’s larger A and B rings, and finding a broad agreement in age.

“It is both a nervous and exciting feeling that what we have proposed may be further proved correct in the near future,” says Zhang.

“And the proximal orbit observations next year will provide us with many more insights on the rings.”

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Z. Zhang et al. 2017. Cassini microwave observations provide clues to the origin of Saturn’s C ring. Icarus 281: 297-321; doi: 10.1016/j.icarus.2016.07.020