Saturn’s core is a large, diffuse, rocky snowball

The formation of a gas giant involves a race against time. Planets are born when newly formed stars heat up, a process that quickly drives all parasitic gas out of nearby planet-creating regions. To create a gas giant, a large rocky planet must form before this process and generate enough gravitational pull to suck the gas out before everything is pushed back.

The process should leave planets like Jupiter and Saturn with a solid, rocky core buried deep within the gas envelope. But confirming this basic composition was difficult. Now, researchers have used features in Saturn’s rings to detect subtle gravitational influences in the nucleus. Although not definitive, the results suggest that the core is large and that the solid and rocky part is widely distributed in this area.

A layered look?

Planets like Earth and Mars were hot enough during their formation to establish a layered structure, with the heaviest elements at the core and lighter material above. The same should happen in a planetary body large enough to attract a massive envelope of gas. As a result, early designs of gas giant interiors suggested a series of layers: a metallic inner core surrounded by a rock layer, then metallic gases compressed by the layers of gas atmosphere above them.

By following the movement of the Cassini probe around the system, we obtained data on the gravitational field of Saturn. Additional data has come from the recognition that the movement of materials within the planet also creates regions of altered density in the rings, creating patterns that can be imaged when the rings are backlit by the Sun.

The new work is based on the characteristics of the waves we detected in Saturn’s rings. In essence, the researchers built several models of what Saturn’s nucleus might look like and checked whether the models would create the models we actually see. Real-world data is then used to impose constraints on possible elements of Saturn’s core.

The mere existence of certain characteristics in the rings, for example, means that there must be internal divisions within Saturn. The characteristics are formed by the influence of internal gravitational waves (note: no gravitational waves) in the inner core. The presence of gravity waves implies that there is a boundary between two layers, separated by something like density or chemical composition, which maintains their distinction against any internal convection in the nucleus.

Set limits

All in all, the characteristics of the ring can eliminate many elements. For example, if there was a sharp boundary between the core and the gas envelope, the waves seen in the ring would have a high frequency. Since this is not the case, the line between the two must be somewhat blurred. At the same time, the boundary cannot be so blurry that there are no clear boundaries between the layers inside Saturn. If this were true, there would be no way to produce any of the features seen in the ring.

Overall, the models that match the data place Saturn’s core-envelope boundary a significant distance from the center of the planet, about 60 percent of the distance to the surface. It is a radius of nearly 60,000 kilometers, or more than nine times the radius of the Earth.

The exact composition of the core is much more difficult to determine, since the constraints are quite broad. The total mass of the heaviest elements in the core is about 19 times the mass of Earth, which matches the formation patterns of gas giants that place rock and iron at the center, although much of this material could also be water ice. Still, the total mass of the nucleus could reach 55 times the mass of the Earth, indicating that there are a lot of other materials there, probably metallic hydrogen and helium.

If the hydrogen reaches the inner core, it should form a metallic fluid that can easily mix with iron and silicate rocks.

In any case, it is clear that the carefully structured layers that we might expect based on models of planetary formation do not seem to actually exist. Combined with clues that Jupiter may also have a diffuse core, this idea would appear to favor alternative models in which the planetary cores of gas giants do not undergo the same evolutionary processes that are observed in rocky bodies.

The alternative is that the core became diffuse, as the conditions of the inner core would turn hydrogen into a metallic fluid that can easily mix with iron and molten silicate rocks. It is therefore possible that an early layered structure slowly eroded and dissolved over time.

Yet this article should not be taken as the last word on what is going on inside Saturn. Even after trying several ways to match the data, the researchers conclude that “none of the models are entirely satisfactory,” which means that there are many opportunities for researchers to change the parameters or add additional data. features to get a better fit.

Nature astronomy, 2021. DOI: 10.1038 / s41550-021-01448-3 (About DOIs).