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The mystery surrounding the origin of Saturn’s rings finally solved

The origin story behind the creation of the rings surrounding planets such as Saturn and Uranus has finally been understood thanks to the results from a new computer simulation.

The giant planets found in our solar system have very diverse rings. While Saturn’s are made of more than 95 per cent icy particles, the rings of Uranus and Neptune are darker and have higher rock content.

Although the rings of Saturn were first observed in the 17th century, their origin has remained unclear and the mechanisms that lead to the diverse ring systems were unknown.

A team from Kobe University in Japan has launched a new study that focuses on a period in history called the ‘Late Heavy Bombardment’, which is believed to have occurred four billion years ago.

During this period, it is thought that several thousand Pluto-sized (one-fifth of Earth’s size) objects from the Kuiper belt existed in the outer solar system beyond Neptune.

First the researchers calculated the probability that these large objects passed close enough to the giant planets to be destroyed by their tidal force during the Late Heavy Bombardment. Results showed that Saturn, Uranus and Neptune experienced close encounters with these large celestial objects multiple times.

The group then used a computer simulation to investigate disruption of the Kuiper belt objects by tidal force when they passed the vicinity of the giant planets.

The results of the simulations varied depending on the initial conditions, such as the rotation of the passing objects and their minimum approach distance to the planet.

However, they discovered that in many cases fragments comprising 0.1-10 per cent of the initial mass of the passing objects were drawn into orbit around the planet.

The combined mass of these captured fragments was found to be sufficient to explain the mass of the current rings around Saturn and Uranus. In other words, these planetary rings were formed when sufficiently large objects passed very close to giants and were destroyed.

The researchers also simulated the long-term evolution of the captured fragments using supercomputers at the National Astronomical Observatory of Japan.

From these simulations they found that captured fragments with an initial size of several kilometres can be expected to undergo high-speed collisions repeatedly and are gradually shattered into small pieces. Such collisions between fragments are also expected to circularise their orbits and lead to the formation of the rings observed today.

The same model can also explain the compositional difference between the rings of Saturn and Uranus. Compared to Saturn, Uranus (and also Neptune) has a higher density ring which means that in the objects can pass within close vicinity of the planet, where they experience extremely strong tidal forces.

Saturn on the other hand has a lower density and a large diameter-to-mass ratio, so if objects pass very close they will collide with the planet itself.

As a result, if Kuiper belt objects have layered structures such as a rocky core with an icy mantle and pass within close vicinity of Uranus or Neptune, in addition to the icy mantle, even the rocky core will be destroyed and captured, forming rings that include rocky composition.

However if they pass by Saturn, only the icy mantle will be destroyed, forming icy rings. This explains the different ring compositions.

These findings illustrate that the rings of giant planets are natural by-products of the formation process of the planets in our solar system.

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