What did Cassini’s “grand finale” teach us about Saturn’s interior?

Six years ago, the Cassini spacecraft, which had spent nearly two decades in orbit around Saturn, ended its mission with a grand finale, plunging itself deep into Saturn’s atmosphere. Those last few orbits and final dives revealed a wealth of information about Saturn’s interior. A team of astronomers has collected all the available data and is now mapping out the interior of the second largest planet in the solar system.

One of the main questions astronomers have about the nature of Saturn is the properties of its deep interior and possible core. With distant observations, we can only reach the visible surface of the planet, which is only its upper reaches atmospheres. However, we do have many techniques available to probe the depths of the planet. First we have the rings themselves. All of the trillions of particles that make up the rings respond to Saturn’s gravity. Different compositions of the interior and different mixtures of that material will subtly alter the orbital motions of the ring’s molecules. By examining how the loops carefully orbit Planet, we can develop models of the interior of Saturn.

Most importantly, we have Cassini itself. As it circled the globe on its last few orbits, it collected a huge amount of data about the gravitational environment it experienced. And finally, during its fatal descent, we got our first direct measurements of the upper reaches of Saturn’s atmosphere.

Astronomers recently pieced together all of this data to give us a fuzzy but convincing picture of the planet’s interior, published as a preprint on arXiv. All available evidence indicates that the planet’s atmosphere undergoes differential downward circulation to a depth of 10,000 km, which is approximately 1/6 of the entire radius of the planet. This means that the atmosphere near Saturn’s equator is moving faster than the atmosphere near the pole, matching what we see on the surface. But deep within Saturn, the differential rotation gives way to a uniform rotation rate of 10 hours and 33 minutes—that’s the length of a true “day” on that planet.

Saturn appears to have a core made up of a central concentration of heavy elements. This core weighs somewhere between 12 and 20 times the mass of Earth. However, this core is not as compact as Earth itself, but instead slowly spreads out and mixes with the atmosphere of hydrogen and helium that surrounds it. This gives an approximate boundary to the core somewhere around half the radius of Saturn.

Astronomers still don’t fully understand how Saturn generates all of its heat. One possible mechanism is that helium condenses outside the upper atmosphere and falls to lower depths, serving as a vehicle for energy transfer from the core to the upper atmosphere, but this has yet to be proven.

The astronomers behind the study are calling for future missions to Saturn, especially those designed to break through upper atmosphere And it goes as deep into the planet as possible so we can get a clearer picture.