The internal structure of Saturn has been mapped by using data from the Cassini spacecraft to observe seismic oscillations in the planet’s rings. The study reveals that the core is both larger and more diffuse than previously thought.
The research is described in a paper in Nature Astronomy and could improve our understanding of the Saturn’s formation and evolution.
“The conventional picture of Saturn’s interior is of a compact core of rocks and ices that is surrounded by an envelope of hydrogen and helium,” explains the paper’s co-author Christopher Mankovich who is at the California Institute of Technology (Caltech). “Based on the unique information now available from Cassini ring seismology, we found that this distinction between core and envelope is not so tidy. The transition must be gradual, hence the ‘diffuse’ or ‘dilute’ core.”
Rock and ice
Along with co-author Jim Fuller at Caltech, Mankovich has found a rock and ice-dominated fluid at the centre of the planet. The hydrogen and helium content of the fluid gradually increases, moving outwards from the core as does the fraction of heavier elements.
In addition to discovering the lack of a clear boundary separating the core from the planet’s outer layers, the duo also found that the core is considerably larger than previous models had suggested.
“The diffuse core region to occupies the inner 60% of Saturn by radius, a dramatically larger number than the 10% or 20% expected from conventional models with a neatly separated core and envelope,” explains Mankovich. “At 60% of Saturn’s total radius this is a dramatic departure from previous models for Saturn’s structure, which came as a surprise to both of us. But after a long and careful investigation it does simply seem to be what the data require.”
What separates this latest work from previous studies of Saturn is the unique use of seismology data collected from the rings of Saturn — arguably the gas giant’s most famous feature.
Shaken from the core
Saturn’s rings were first observed by Galileo in 1610. They comprise a multitude of objects made of ice and traces of silicates, ranging in size from microns to metres. The closest ring to the surface of the planet is about 7000 km away, so it might come as a surprise that monitoring this stunning feature can reveal details of the interior of Saturn.
Mankovich explains that this is possible thanks to spiral patterns stirred up in Saturn’s rings by the planet’s gravitational influence and natural oscillations. “The planet itself is constantly ringing at a variety of frequencies, just like a musical instrument has its own rich spectrum of sounds at any given time,” he explains, adding “These oscillations in the planet cause small amounts of mass in the planet to essentially wobble back and forth slightly as a function of time, and this carries over into a wobbling gravitational field that can stir up waves in the rings.”
Saturn’s ring system is sub-divided into separate bands and data from Cassini has revealed dozens of waves in the C ring of Saturn driven by the gas giant’s oscillations. The frequencies of these waves allowed Fuller and Mankovich to better constrain the planet’s interior than previous methods have allowed.
Internal gravity waves
“Typically the interior structures of the outer solar system planets are constrained using their gravity fields, but this information only goes so far, since the gravity measurements are not very sensitive to the deepest parts of the interior,” Mankovich says. “Seismology is a handy and independent way to study the interior, especially at Saturn where the ring waves include those produced by Saturn’s internal gravity waves which inherently probe the deepest parts of the interior. It’s the frequencies of these internal gravity waves that turned out to eliminate many otherwise plausible interior models and let us arrive at our surprising result.”
Cassini’s grand finale
Mankovich says that when it comes to their approach to mapping Saturn, he and Fuller took considerable inspiration from helioseismology — the use of the Sun’s regular oscillations to model its interior. Despite decades of development in this field and the growth of the related field of asteroseismology, a powerful method of charting the interiors of stars, understanding gas giants with seismology is still no mean feat.
“It’s difficult! This success story never would have happened were it not for the Cassini mission, which orbited Saturn for longer than a decade and collected a wealth of data,” concludes Mankovich. “The crucial next step will be to search for an interior model in which both of these stable regions might coexist, with the aim of explaining the ring seismology, gravity field, and magnetic field simultaneously. The best picture ever for Saturn’s structure is really starting to come into focus.”
