Saturn’s rings
The hoops around the second-largest planet are iconic, but what do we know about them?
mission began its 13-year visit to the planet, discovering many new moons and ringlets.
The particles in the rings consist of almost pure water ice; this makes them extremely bright. Saturn’s albedo, the amount of light it reflects, varies from –0.5 when the rings are edge-on to +0.9 when they are fully open. The particles range from the size of a sand grain to 10m or more in diameter. The gaps are not well-defined empty spaces, but are areas where particle density drops. The largest of these, the Cassini Division, is 4,700km wide.
Building blocks
From inner to outermost, the main structures are: the D, C and B rings; Cassini Division; A ring; Roche Division; F ring; Janus/Epimetheus ring; G ring; Methone and Anthe ring arcs; Pallene ring; E ring and finally the Phoebe ring. Many of these have their own subdivisions, gaps, structures and moonlets within.
The rings orbit in the same direction as Saturn but at different rates, the inner rings faster than the outer ones in accordance with Kepler’s third law. The main rings
is a freelance science writer, astronomer and broadcaster
Rich textures: an image taken by the Cassini spacecraft during a close flyby in 2017 reveals the exquisite detail of Saturn’s A ring extend from 7,000km to 80,000km from the planet’s equator, yet they are only an average of 10m thick. The disappearance of the rings that so perplexed Galileo is due to the 27° tilt of Saturn’s axis to the ecliptic. From Earth we see the rings from different angles as both planets orbit: first from below, then they almost disappear as we see them edge on, then they widen again as we see them from above and then they appear edge on again, so the rings seem to vanish roughly every 15 years. Their total mass is equal to that of Saturn’s moon Mimas.
All rings considered
The origin of Saturn’s rings has been hotly debated. One theory is that they are all that remains of one of Saturn’s moons that was ripped apart by gravitational forces when its orbit decayed, bringing it too close to its parent planet. That would mean the rings are younger than the planet. Another theory is that the rings are the same age as the planet, being remnants of the nebula from which Saturn formed.
In April 2017 at the end of Cassini’s tour of the Saturn system, the spacecraft performed the Grand Finale phase of its mission, passing between Saturn and its rings 22 times before diving into and burning up in the planet’s atmosphere. Data from these acrobatics on the rings’ brightness and purity revealed they may have formed just 100 million years ago, when dinosaurs still ruled the Earth. If they were older, the theory goes, they would have been darkened over time by cosmic dust, although other scientists suggest that recycling of material within the rings, as particles collide, break apart and reform, could dilute any pollutants.
However they were formed, the rings are continually losing material in the form of an icy rain falling onto Saturn, at a rate of a swimming pool full of material every half hour. There’s still plenty of time to view them: it’s estimated they won’t be gone for good for 100 million years.