All but two of our solar system's planets have natural 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, we take 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 neighbors. 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.
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 313 miles (504 km) and a rock/ice composition, Enceladus should have frozen solid billions of years ago, like many of its neighbors in the Saturnian system. But tidal forces caused by a gravitational 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 donut-shaped E Ring — the outermost and sparsest of Saturn's major rings.
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