How old are Saturn's rings?

Nobody knows for certain, and that is the point: astronomers are split between rings as old as Saturn itself and rings that are surprisingly young. Cassini data now suggest the rings most likely formed within the last 100 million years, and that they may be only between 10 million and 100 million years old. There is no consensus on when they formed, because some theoretical models suggest they formed early in the Solar System's existence.

What are they? Saturn's rings are particles in orbit around the planet, from micrometres to metres in size, made almost entirely of water ice. The dense main rings are only 10 metres to 1 kilometre thick. Their ice is 99.9 percent pure, and that purity is one of the clues for youth: meteoric dust should have darkened old ice, though new research suggests one ring, the B Ring, may be massive enough to dilute the dust.

A second clue is how fast the rings are disappearing. Ring material is continually spiralling down into Saturn, and the faster this infall, the shorter the lifetime of the ring system. Research on infall rates favours an age of hundreds of millions of years.

A third question is where the rings came from. A hypothesis proposed in the 19th century by Édouard Roche says they were once a moon whose orbit decayed until tidal forces ripped it apart. Simulations in 2022 support it and named the lost moon Chrysalis.

The rings are also a story of slow understanding. Galileo was first to see them in 1610 but could not make out their nature. Huygens described them as a disc in 1655, and in 1859 James Clerk Maxwell proved they must be countless small particles.