ENCELADUS

The ring bearer

Mass: 1.1 x 1020kg (2.4 x 1020lbs) Diameter: 504km (313 miles) Parent planet: Saturn Discovered: 1789, William Herschel

Since NASA’s Cassini probe arrived at Saturn in 2004, the ringed planet’s small inner satellite, Enceladus, has become one of the most intensely studied and debated worlds in the entire Solar System. It owes its new-found fame to the discovery of huge plumes of water ice erupting into space along fissures in its southern hemisphere – a sure sign of liquid water lurking just beneath the moon’s thin, icy crust.

The strange activity of Enceladus was suspected before Cassini’s arrival thanks to earlier images that showed the moon has an unusually bright surface and craters that look like they are blanketed in snow. Nevertheless, the discovery of the ice plumes – initially made when Cassini flew straight through one – was a spectacular confirmation that Enceladus is an active world.

With a diameter of 504 kilometres (313 miles) and a rock/ice composition, Enceladus should have frozen solid billions of years ago, like many of its neighbours in the Saturnian system. But tidal forces caused by a tug of war between Saturn and a larger moon, Dione, keep the moon’s interior warm and active, making it a prime target in the hunt for life in the Solar System. While much of the water ice falls back to cover the surface, a substantial amount escapes from the weak gravity and enters orbit around Saturn. Here it spreads out to form the doughnut-shaped E Ring – the outermost and sparsest of Saturn’s major rings.

“The discovery of the ice plumes was a spectacular confirmation that Enceladus is an active world”

1 Inner rings The broad Cassini Division separates the A Ring from the inner B, C and D Rings. 2 A Ring Saturn’s brightest ring is the A Ring, divided by the narrow Encke Gap. The ultrafine F Ring runs around its outer edge. 3 E Ring The E Ring is a broad, largely transparent ring of scattered icy particles. 4 Orbit of Enceladus Enceladus’ orbit around Saturn coincides with the densest part of the E Ring. 5 Geyser faults Tidal forces create an ocean that erupts as icy plumes along weak fault lines in the southern hemisphere. 6 Fragmented material Small ice fragments trailing behind Enceladus are ground down into ever-finer particles as they orbit Saturn.