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Cassini data from last decade reveals insights into 'diffuse' nature of Saturn's core

Internal structure extends to 60% of gas giant's radius, study shows

New analysis of data from the Cassini space probe has revealed Saturn possesses a fuzzy or diffuse core without clearly defined boundaries.

Scientists had until now figured that the sixth planet from the Sun has a metallic core surrounded by an envelope composed mostly of hydrogen and helium.

A research team from California Institute of Technology interpreted data based on Cassini stellar occultation data – an approach to measuring the density of the Saturn's rings by tracking how they filter the starlight through – published in papers between 2014 and 2019, taken between 2007 right up to the end of mission in 2017.

The idea that Saturn's – and indeed Jupiter's – structure is neatly divided into a compact core of heavy elements such as rocks and ices and an envelope of light ones such as hydrogen and helium goes back decades, postdoctoral research associate Christopher Mankovich told The Register.

"It's a convenient way of matching the best structure constraints that we typically have for our local gas giants, namely their size and their gravity fields," said Mankovich, who worked with assistant professor Jim Fuller. "However, since the size and gravity field constraints are not sensitive to the deepest regions of the interior, this cleanly separated core-envelope structure is not a unique solution, only a convenient one: many other more complex interior structures are possible."

To confirm this idea, the researchers analysed data on Saturn's rings because the usual way of finding out about gas planet cores – using perturbations induced on a planet's gravitational field – are quite weak in Saturn's case and limit the precision to which the internal structure can be defined.

"It is here where the totally unique method of ring seismology can step in and give us precious new information about the deepest parts of the interior," Mankovich said.

"By using frequencies of Saturn's own oscillations observed in the rings, we were able to constrain the internal structure of Saturn better than ever before. The result is that we can rule out any neat division between core and envelope: Saturn's oscillation frequencies instead reveal a large, blurry core devoid of any clean interfaces.

"The rocks and ices still dominate over hydrogen and helium at the centre of the planet, but these are gradually diluted by more and more hydrogen and helium as you move outward in the planet. The 'core' region appears to extend all the way out to a whopping 60 per cent of Saturn's radius."

The result – published in Nature Astronomy – was a striking departure from the 10 per cent to or 20 per cent of a conventional compact-core model and was a "huge surprise" which led to "many more months of checking work and seeking alternatives, just trying to find any way out of this surprising result," Mankovich added. "In the end, none of these alternatives could do a good job for the gravity and seismology simultaneously, and so we concluded that the data really are telling us that Saturn's core is diffuse."

The researchers used computer simulations of the rotating gas giant's internal structure to show how any given interior model should be "ringing" and compared the results to the observed frequencies. Saturn's C-ring in particular turns out to have dozens of waves forced by oscillations inside Saturn itself.

"Just like a wave on a lake, these waves have frequencies – the time for a high point to go down and back up again – and these frequencies immediately give away the frequencies of Saturn's internal oscillations, which tell us a lot about Saturn's hidden depths," Mankovich said. "This is familiar to helioseismologists, who for decades have used the Sun's ringing to probe its internal structure and rotation."

Although Cassini's mission ended when it finally plunged into the gas giant in 2017, ring observers still hope to get more from the data. "There is some hope that some new detections will include waves caused by the Saturn modes with very deep roots that made these results possible, and detections like those could be tremendous for confirming or modifying the picture we're advocating," Mankovich said.

In addition, a network of earthbound observatories is fitted with custom instruments designed to measure the gentle oscillations on the surface of Jupiter or Saturn. The hope is that in the coming years they can provide more detail on the internal structures of these gas giants. ®

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