This diffuse core extends to about 60% of Saturn’s radius — a huge jump from the 10 to 20% of a planet’s radius that a traditional core would occupy.
One of the wildest aspects of the study is that the findings didn’t come from measuring the nucleus directly — something we’ve never been able to do. Instead, Mankovich and Fuller turned to seismographic data about Saturn’s rings first collected by NASA’s Cassini mission, which explored the Saturn system from 2004 to 2017.
“Saturn is essentially always ringing like a bell,” Mankovich says. As the core wobbles, it creates gravitational disturbances that affect the surrounding rings, creating subtle “waves” that can be measured. As the planet’s core oscillated, Cassini was able to study Saturn’s C ring (the planet’s second block of rings) and measure the small but consistent gravitational “ring” created by the core.
Mankovich and Fuller looked at the data and created a model for Saturn’s structure that would explain these seismographic waves — and the result is a hazy interior. “This study is the only direct evidence so far for a diffuse core structure in a liquid planet,” Mankovich says.
Mankovich and Fuller think the reason the structure works is that the rocks and ice near Saturn’s center are hydrogen soluble, causing the core to behave like a liquid rather than a solid. Their model suggests that Saturn’s diffuse core contains rocks and ice that make up more than 17 times the mass of the entire Earth, so that’s a lot of material floating around.
A diffuse core can have some big implications for how Saturn works. Most importantly, it would stabilize some of the interior from convective heat, which would otherwise swirl Saturn’s interior with turbulence. In fact, this stabilizing influence gives rise to the internal gravitational waves that affect Saturn’s rings. In addition, the diffuse core would explain why Saturn’s surface temperatures are higher than what traditional convective models suggest.
Still, Mankovich acknowledges that the model is limited in a number of important ways. It cannot explain what scientists have observed about Saturn’s magnetic field, which is bizarre in many ways (for example, it exhibits near-perfect symmetry on its axis, which is quite unusual). He and Fuller hope that future research can narrow down the interior more closely and give scientists an idea of how the planet’s core might affect the magnetic field.
They also hope that NASA’s Juno mission could reveal a similar diffuse core in Jupiter. That would go a long way to confirm suspicions that when giant planets form, the process naturally creates gradients of material as opposed to clean and solid cores. Some research using gravity data collected by Juno seems to support this idea too.