Six years ago, the Cassini spacecraft, which had spent nearly two decades in orbit around Saturn, ended its mission with a grand finale, plummeting into the depths of Saturn’s atmosphere. These final orbits and crash revealed a wealth of information about Saturn’s interior. A team of astronomers has gathered all available data and is now drawing a portrait of the interior of the solar system’s second largest planet.
One of the key questions astronomers have asked about the nature of Saturn is the properties of its deep interior and potential core. With long-distance observations, we can only access the planet’s visible surface, which makes up only the uppermost reaches of its atmosphere. However, several techniques are available to us to explore the deep interior 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 this material will subtly change the orbital motions of the ring particles. By carefully examining how the rings orbit the planet, we can develop models of Saturn’s interior.
Most importantly, we have Cassini itself. As it orbited the world in its final orbits, it collected an enormous amount of data about the gravitational environment it was exposed to. And finally, during its fatal crash, we got our first direct measurements of the upper reaches of Saturn’s atmosphere.
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Recently, astronomers have put all this data together to give us a blurry but compelling picture of the planet’s interior. All available evidence indicates that the planet’s atmosphere undergoes differential rotation down to a depth of 10,000 km, which is about one-sixth of the entire planet’s radius. That means the atmosphere near Saturn’s equator is moving faster than the atmosphere near the pole, which matches what we see on the surface. Deep inside Saturn, however, the differential rotation gives way to a uniform rotation rate of 10 hours and 33 minutes — the true “daylength” on this planet.
It appears that Saturn does indeed have a core made up of a central concentration of heavy elements. This core weighs about 12 to 20 times the mass of Earth. However, this core is not compact like Earth itself, but slowly diffuses and mixes with the surrounding hydrogen and helium atmosphere. This gives a rough limit of the core at about 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 from the upper atmosphere and sinks to deeper depths, transferring energy from the core to the upper atmosphere. However, this has not yet been proven.
The astronomers behind the study are calling for future missions to Saturn, particularly those aimed at penetrating the upper atmosphere and getting as deep into the planet as possible so we can get a clearer picture.