Rings

Saturn’s rings look solid, like a hat brim. They are not.

They are made of billions of separate pieces of ice. Some are as small as grains of dust. Some are as big as a house. Every piece goes round Saturn on its own, like a tiny moon.

The rings are enormously wide. If you laid them between the Earth and the Moon they would fill most of the gap.

But they are also amazingly thin: in most places only about ten metres from top to bottom. For their width, they are as thin as a sheet of paper three kilometres across.

Saturn is not the only planet with rings. Jupiter, Uranus and Neptune have them too. Theirs are dark and faint, and were only found in the last fifty years.

Why they are not solid. In 1859 James Clerk Maxwell proved on paper that a solid ring round a planet would be torn apart: its inner edge would need to go round faster than its outer edge. A ring has to be a crowd of separate bodies, each on its own orbit, the inner ones lapping the outer ones. Spacecraft later showed he was right.

Why they are flat. A piece on a tilted orbit crosses the ring twice each lap and collides with others as it does. Every collision takes away some of the up and down motion. After enough of them everything has settled into one thin sheet over the planet’s equator. Saturn’s main rings are about 280,000 km across and mostly about 10 m thick.

Why they do not gather into a moon. Close to a planet, tides are strong: the planet pulls harder on the near side of anything than on its far side. Inside a certain distance, called the Roche limit, that stretching beats the gravity that would hold a loose pile of rubble together. For Saturn the limit lies around two and a half times the planet’s radius from its centre. Rings live inside it, and large moons outside.

Gaps and edges. The rings are full of structure, and moons cause most of it.

• A moon outside the rings can clear a gap where ring pieces would orbit in step with it. The Cassini Division, the wide dark band you can see in a small telescope, is where a piece would go round exactly twice for each orbit of the moon Mimas.
• Small moons orbiting beside a narrow ring keep its edges sharp. They are called shepherd moons. Saturn’s thin, twisted F ring runs between two of them, Prometheus and Pandora.
• A few tiny moons orbit inside gaps in the rings and sweep them clear.

The four ring systems.

Planet	Found	What they are like
Saturn	1610 by Galileo, understood as a ring by Huygens in 1655	Broad and bright, almost pure water ice
Uranus	1977, when a star flickered before and after passing behind the planet	Thirteen narrow, dark rings
Jupiter	1979, by Voyager 1	Faint dust knocked off small inner moons
Neptune	Confirmed in 1989 by Voyager 2	Dark and dusty, with the outer ring gathered into clumps

How old are Saturn’s rings? This is still argued over. In 2017, at the end of its mission, the Cassini spacecraft flew between the rings and the planet and weighed them. They came out light, with less than half the mass of the small moon Mimas, and remarkably clean. Dust from space should have darkened them if they were old, so many researchers concluded they formed no more than a few hundred million years ago, perhaps from a moon or comet that strayed too close. Others reply that the rings may have ways of staying clean and could be as old as Saturn. Cassini also found ring material steadily falling into the planet, so the rings may not be permanent.

Seen edge on. Saturn is tilted, so we see the rings from different angles as it goes round the Sun. About every 14 years the Earth passes through their plane and they all but vanish, because there is nothing to see of a sheet ten metres thick from more than a billion kilometres away. It last happened in March 2025.