This diffuse nucleus spans about 60% of Saturn’s radius, a huge leap from the 10-20% of a planet’s radius that a traditional nucleus would occupy.
One of the craziest aspects of the study is that the results did not come from the direct measurement of the nucleus, which we were never able to do. Instead, Mankovich and Fuller turned to seismographic data on Saturn’s rings first collected by NASA’s Cassini mission, which explored the Saturn system from 2004 to 2017.
“Saturn basically rings like a bell at all times,” explains Mankovich. When the nucleus wobbles, it creates gravitational disturbances that affect the surrounding rings, creating subtle “waves” that can be measured. As the planet’s core swayed, Cassini was able to study Saturn’s C-ring (the planet’s second block of rings) and measure the small but constant gravitational “tinkle” caused 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 blurry interior. “This study is the only direct evidence of a diffuse central structure in a fluid planet to date,” explains Mankovich.
Mankovich and Fuller believe the reason the structure works is that rocks and ice near Saturn’s center are soluble in hydrogen, allowing the nucleus to behave like a fluid rather than a solid. Their model suggests that Saturn’s diffuse core contains rocks and ice that are more than 17 times the mass of the entire Earth, causing a lot of matter to wobble.
A diffuse nucleus could have some important implications for the functioning of Saturn. Most importantly, it would stabilize part of the interior against convective heat, which would otherwise disturb Saturn’s interior with turbulence. In fact, this stabilizing influence gives rise to the internal gravity waves that influence the rings of Saturn. 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 some important respects. This cannot explain what scientists observed on 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 are hopeful that future investigations can restrict the interior more narrowly and tell scientists how the planet’s core might affect its magnetic field.
They are also hoping that NASA’s Juno mission could reveal a similar diffuse core within Jupiter. This would go a long way to assert suspicions that when giant planets form, the process naturally creates gradients of matter as opposed to clean, solid nuclei. Certain research using gravity data collected by Juno seems to support this idea too.