That's really how Saturn got its rings

The destruction of a moon 100 to 200 million years ago led to the formation of Saturn's rings. This also explains the conspicuously large inclination of the planet's axis of rotation. American researchers have come to this conclusion based on a new analysis of data from the Cassini space probe and computer simulations. In order to confirm the model, however, the internal structure of Saturn must be examined more closely, the scientists write in the journal Science.

"The angle between the planet's equatorial plane and the plane of its orbit, at 26.7 degrees, is far too large to have come about during Saturn's formation phase from the rotating disk of gas around the Sun," explains Jack Wisdom of the Massachusetts Institute of Technology (MIT) and his colleagues. Because the conservation of angular momentum ensures that the axis of rotation of a developing planet is almost perpendicular to the orbital plane. So there must be a later cause for the tilt of this axis.

Astronomers have been discussing a resonance between Saturn and Neptune, the outermost planet in the solar system, as a possible cause for almost two decades. Because the axis of rotation of Saturn "wobbles" - the researchers call it precession - in the same rhythm as the orbit of Neptune. As a result, small perturbations from Neptune add up over a long period of time in such a way that what was originally a small tilt of the axis could have increased to the present value.

However, Neptune's impact on Saturn depends both on its moons and on the exact mass distribution of Saturn's interior. This is where Wisdom and his colleagues came in: Using the orbital data transmitted to Earth by the Saturn probe Cassini – especially during the targeted crash of the probe on September 15, 2017 – the researchers constructed an improved model of the planet. And came to an initially disappointing result: Saturn is therefore not in the range of a resonance with Neptune, but just outside.

But the team didn't give up. The researchers' idea: Perhaps Saturn was initially in resonance with Neptune, but was knocked out by an event in that realm. "So we started looking for a way to get Saturn out of resonance with Neptune," says Wisdom.

On the basis of theoretical considerations and a large number of simulations of the planet and its moons, the scientists finally found a solution: Saturn must originally have had another larger moon. Only with this additional moon - which is said to have been about the size of the third largest satellite Japetus - was it possible to keep Saturn in resonance with Neptune for a long time.

But the orbits of Saturn's moons are changing. In particular, the orbit of Saturn's largest moon, Titan, is slowly moving outwards. Sometime between 100 and 200 million years ago, this caused a severe disruption in the orbit of the additional former moon, which Wisdom and his team dubbed "Chrysalis": On a chaotic orbit, it approached Saturn so closely that it was swept away by the planet's tidal forces was torn. With the disappearance of Chrysalis, Saturn moved out of resonance with Neptune.

Much of the debris fell on the planet, eventually forming what are now Saturn's rings. This model can therefore explain both the rings and the axis tilt of Saturn. However, this scenario still depends heavily on the mass distribution inside the planet. More precise data from future space probes are therefore needed to confirm the model.