Saturn

Saturn is light enough to float in a giant bathtub and wears the most spectacular rings in the solar system. How it formed, what its rings are made of, and why its moons Titan and Enceladus matter.

Diameter: 120,536 km (9.4 × Earth)
Mass: 95 × Earth
Gravity at cloud tops: about 1.07 × Earth
Distance from Sun: 9.5 AU (about 1.43 billion km)
Year: 29.4 Earth years
Day: about 10 hours 40 minutes
Moons: more than 270 known
Cloud-top temperature: about −140 °C

The build: a smaller, lighter giant

Saturn formed the same way as Jupiter — a heavy core beyond the frost line pulling in hydrogen and helium — but it ended up with less than a third of Jupiter's mass. Farther from the Sun, material was more spread out and growth was slower, so Saturn had less time to collect gas before the disk faded.

Saturn's average density is lower than water's. If you could find a big enough bathtub, it would float. Its fast spin and low density also make it the most flattened planet: it is about 10% wider at the equator than from pole to pole.

The rings

Saturn's rings stretch hundreds of thousands of kilometres across but are typically only around ten metres thick. They are made of countless pieces of water ice, from dust grains to chunks the size of houses.

The rings sit inside Saturn's Roche limit, the distance within which tidal forces tear apart anything held together only by its own gravity. That is why the material stays as rings instead of gathering into a moon.

How old are they? Measurements from NASA's Cassini spacecraft suggested the rings are surprisingly clean and might be only a few hundred million years old — younger than the dinosaurs. Other scientists argue they could be as old as Saturn itself and have stayed clean in other ways. The question is still open.

Remarkable moons

Titan — larger than Mercury and the only moon with a thick atmosphere, mostly nitrogen. It has rain, rivers and lakes, but of liquid methane and ethane. NASA's Dragonfly rotorcraft is being built to fly there.
Enceladus — a small icy moon that sprays jets of water from a hidden ocean out of cracks near its south pole. Cassini flew through the plumes and detected salts and organic molecules.

A hexagon at the pole

Around Saturn's north pole, a jet stream forms a six-sided pattern wider than Earth. Similar shapes have been made in laboratory tanks of spinning fluid, but why Saturn's is so stable is still being studied.

Lessons for planet builders

Timing and location control how big a giant planet gets.
Rings form when material is trapped inside the Roche limit.
Small icy moons can hold liquid oceans if something heats them from inside.