What is the significance of Saturn's rings

Computer simulations bring old theory back to life - Saturn's rings could be the remains of a decayed moon

Saturn

Boulder (USA) - The formation of the extraordinary ring system around the planet Saturn is still a mystery to astronomers. Computer simulations by an American researcher are now breathing new life into an old theory: the rings could have been created by the decay of a large moon. The simulations presented in the online edition of the journal "Nature" show that during this process the outer, water-rich layers of the moon are detached, while the rocky core falls onto the planet.

"So far there is no adequate explanation for the origin of Saturn's rings," emphasizes Robin Canup from the Southwest Research Institute in Boulder, Colorado. "The current rings consist of 90 to 95 percent water ice. Originally, they must have consisted of almost pure water ice, as they have been contaminated by meteoroids over time." And this is exactly the weak point of the previous models: They cannot explain the high water content.

With one exception: As early as the 19th century, the French astronomer and mathematician Édouard Albert Roche suggested that a moon could have come so close to Saturn that it was torn apart by the tidal forces of the planet. However, since it was not clear why a moon should approach the planet, this theory became less popular than alternative ideas such as the collision of a moon with an asteroid or comet. In recent years, however, it has been shown that migration plays an important role in the formation phase of planets and moons.

This is where Canup's simulation comes in. The scientist assumes a large moon similar to Titan and the large moons of Jupiter, which has a core of rock and metal and a mantle of frozen water. In the last phase of planet formation, this moon moved too close to the planet due to the interaction with the still existing gas cloud from which Saturn and its moons were formed. The strong tidal forces led to the dissolution of the moon: the outer layers formed a ring that initially had a thousand times the mass of today's rings of Saturn, while the core of the moon fell on Saturn.

Over the billions of years, the ring system has lost a large part of its mass, but has retained its high proportion of water. Part of the ring material flowing outwards has formed new, water-containing moons. Canup suggests using the Cassini space probe to determine the exact proportion of impurities in Saturn's rings - and thus the age of the ring system. If she is correct in her theory, the rings would have to be around 4.5 billion years old, roughly the same as the planet itself.