Saturn

When Galileo turned his telescope on Saturn in 1610 he was baffled. The planet seemed to have handles, or perhaps ears, that came and went over the years. It took another half century and better optics for Christiaan Huygens to realise that Saturn was circled by a thin, flat ring. Four centuries on, Saturn remains the sight that makes people gasp at the eyepiece, and the planet turned out to be only part of the story. Its moons are among the most remarkable places we know.

A lighter giant

Saturn is the second largest planet, about nine times Earth's diameter, and like Jupiter it is a ball of hydrogen and helium with no solid surface. It is noticeably less dense: its average density is lower than water's, so in a hypothetical ocean large enough, Saturn would float. Its clouds are a muted butterscotch, calmer in appearance than Jupiter's, because a high haze veils the bands beneath. Winds at the equator are nonetheless ferocious, up to 1,800 kilometres per hour. Every twenty or thirty years an enormous storm erupts and wraps around the entire planet.

At the north pole sits one of the strangest features in the solar system: a jet stream that forms a near-perfect hexagon, 30,000 kilometres across, with a hurricane at its centre. It has persisted for at least forty years. Laboratory experiments with spinning fluids can reproduce six-sided shapes, but why Saturn alone shows one so clearly is not fully understood.

The rings

Saturn's rings stretch about 280,000 kilometres from edge to edge, nearly three quarters of the distance from Earth to the Moon, yet they are astonishingly thin, in most places only ten metres or so, as proportionally flat as a sheet of paper the size of a city. They are not solid. They consist of countless pieces of almost pure water ice, ranging from dust grains to boulders the size of houses, each on its own orbit, occasionally colliding and clumping.

Gaps divide the rings into bands. The widest, the Cassini Division, is swept clear by the gravitational rhythm of the moon Mimas. Tiny moonlets embedded within the rings carve narrower lanes and raise waves along their edges. Where did the rings come from? Perhaps a moon that strayed too close and was torn apart, or a comet that met the same fate. The Cassini spacecraft found hints that they are young by cosmic standards, perhaps only 100 to 400 million years old, and that they are slowly raining down onto the planet. Saturn may have been ringless for most of its history and may be again.

Titan: a world with weather

Saturn's largest moon, Titan, is bigger than Mercury and the only moon in the solar system with a thick atmosphere. It is mostly nitrogen, like ours, but half again as dense, and laced with methane and other hydrocarbons that form an orange smog hiding the surface entirely. In 2005 the European Huygens probe parachuted through that haze and landed on a damp, pebbled plain.

Titan is the only world besides Earth with liquid on its surface, but at minus 180 degrees the liquid is not water. It is methane and ethane, filling lakes and seas near the poles, fed by methane rain and drained by rivers. Beneath the icy crust lies an ocean of water. Titan's chemistry is so rich that some scientists consider it a natural laboratory for how life's ingredients assemble. NASA's Dragonfly, a nuclear-powered drone, is due to fly across its dunes in the 2030s.

Enceladus: the ocean that leaks

Enceladus is small, barely 500 kilometres across, and brilliantly white. Cassini discovered that its south pole is cracked by long fissures from which geysers of water vapour and ice crystals spray hundreds of kilometres into space. The spray feeds one of Saturn's rings. Flying through the plumes, Cassini tasted salt, organic molecules and hydrogen, chemical evidence that beneath the ice lies a global salty ocean in contact with warm rock. That is precisely the combination that supports life at hydrothermal vents in Earth's deep sea. Enceladus may be the single most promising place in the solar system to look for alien microbes, and the ocean sample is free for the taking, already in space.

Why it matters

The Cassini mission orbited Saturn for thirteen years and ended in 2017 by plunging into the planet, deliberately, to avoid any chance of contaminating Enceladus or Titan with Earthly microbes. That decision says something: we now treat Saturn's moons as places where life might exist. A planet once admired for its beauty has become one of the most important destinations in the search for life.