STRANGEST MOONS in the solar system
All About Space|Issue 109
STRANGEST MOONS in the solar system
Some of the most fascinating worlds in our cosmic neighbourhood are not planets, but the moons that orbit around them
Giles Sparrow

All but two of our Solar System’s planets have satellites of one sort or another. Earth’s own Moon, a beautiful but stark, dead world shaped by ancient volcanoes and countless impact craters, is undoubtedly the most familiar, but it’s far from being the most interesting.

Each of the outer Solar System’s giant planets is accompanied by a large retinue of satellites, many of which formed at the same time and from the same ice-rich material as the planets that host them. Although far from the Sun and starved of solar heat and light, they nevertheless show as much variety as the planets themselves.

Here All About Space takes a trip to visit some of the strangest and most exciting of these astonishing worlds. Some, such as Jupiter’s Callisto and Saturn’s Mimas, have been frozen solid for billions of years but bear extraordinary scars from exposure to bombardment from space. Others, such as Saturn’s shepherd moons Pan and Atlas and Neptune’s lonely Nereid, have been affected throughout their history by interactions with their neighbours.

Most excitingly, some of these exotic worlds have been heated by powerful tidal forces from their parent planets, triggering phases of violent activity like those which shaped Miranda, Uranus’ Frankenstein moon. In some cases these forces are still at work today, creating fascinating bodies such as Jupiter’s tortured Io and Saturn’s icy Enceladus, whose placid exterior may even hide the greatest secret in the Solar System: extraterrestrial life itself.

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.

CALLISTO The most cratered world

Mass: 1.1 x 1023kg (2.4 x 1023lbs)

Diameter: 4,821km (2,996 miles)

Parent planet: Jupiter

Discovered: 1610, Galileo Galilei

The outermost of Jupiter’s Galilean moons, Callisto is the third-largest moon in the Solar System and is only slightly smaller than Mercury. Its main claim to fame is the title of most heavily cratered object in the Solar System; its dark surface is covered in craters down to the limit of visibility, the deepest of which have exposed fresh ice from beneath and scattered bright ‘ejecta’ debris across the surface.

Callisto owes its cratered surface to its location in the Jupiter system – the giant planet’s gravity exerts a powerful influence, disrupting the orbits of passing comets and often pulling them to their doom, most spectacularly demonstrated in the 1994 impact of Comet Shoemaker-Levy 9.

Jupiter’s larger moons are directly in the firing line and end up soaking up more than their fair share of impacts, but Callisto’s inner neighbours – influenced by greater tidal forces – have all experienced geological processes that wiped away most of their ancient craters. Callisto’s surface, however, has remained essentially unchanged for more than 4.5 billion years, developing its dense landscape of overlapping craters across aeons.

DACTYL The asteroid moon

Mass: Unknown

Diameter: 1.4km (0.87 miles)

Parent planet: 243 Ida

Discovered: 1993, Galileo space probe

243 Ida’s moon is tiny, just 1.6 kilometres (0.99 miles) on its longest axis. Thanks to the larger asteroid’s weak gravity, Dactyl is unlikely to be an object captured into orbit, but the alternative – that Ida and Dactyl formed alongside each other – raises as many questions as it answers.


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Issue 109