Launched three years before the new century... a spacecraft wound its way through the empty reaches of the solar system. On Earth, its progress was little noted, as it swung twice by the planet Venus, then our moon. And Earth. The asteroid belt. And Jupiter. Almost seven years later, on the first of July 2004, the Cassini probe entered the orbit of Saturn. It then began to compile what has become one of the greatest photographic collections of all time, of a giant gas planet, surrounded by colorful rings, guarded by a diverse collection of moons, and millions of tiny moonlets. Within this record, is a trail of clues... pointing to the energy sources and complex chemistry needed to spawn life. What are these mysterious worlds telling us about the universe, and Earth? In the outer reaches of the solar system, a billion and a half kilometers from the Sun... there is a little world known as Enceladus. Nearly all of the sunlight that strikes its icy surface is reflected back into space, making it one of the brightest objects in the solar system. At its equator, the average temperature is minus 198 degrees Celsius. It can rise about 70 degrees higher in grooves that stretch across the south pole like tiger stripes. Looming over it is the giant planet Saturn. In myth, Saturn - the Roman name for the primal Greek God Chronos - was the youngest son of Gaia, or Earth, and Uranus, sky. Wielding a scythe provided by his mother, the story goes, Saturn confronted his abusive father, castrating him. The blood of Uranus flowed into the seas, fertilizing the Earth and giving rise to Enceladus and other giant offspring. Saturn's moon Enceladus has its own tangled story. In 2005, the Cassini spacecraft spotted plumes of water vapor shooting out into space from its south pole. More recent close encounters have revealed jets of water, flavored by slightly salty chemical compounds, spewing out from vents in the rough, cracked polar terrain. That may mean that Enceladus harbors a remarkable secret below its frigid surface: A liquid ocean, and perhaps, a chemical environment that could spawn simple life forms. It's not the only promising stop in the realm of Saturn. The moon Titan is often said to resemble Earth in its early days. It is lined with volcanoes and a hazy atmosphere rich in organic compounds. While Enceladus is the size of Great Britain, Titan is ten times larger, 50% larger than our moon, and the second largest moon in our solar system. We've known about Titan since the astronomer Christian Huygens discovered it in 1655, and Enceladus since William Herschel spotted it in August 1789, just after the start of the French Revolution. Scientists began to investigate these moons in earnest with the launch of the two Voyager spacecraft in 1977. The lineup of outer planets in the solar system allowed the spacecraft to fly past each of them. They disclosed new details about their magnetic fields, atmospheres, ring systems, and inner cores. But what really turned heads were the varied shapes and surfaces of their moons. They've all been pummeled over the millennia by wayward asteroids and comets. A few appear to be sculpted by forces below their surfaces. Neptune's largest moon Triton has few craters. It's marked with circular depressions bounded by rugged ridges. There are also grooves and folds that stretch for dozens of miles, a sign of fracturing and deforming. Triton has geysers too, shooting some five miles above the surface. But on this frigid moon -- so far from the Sun -- the liquid that spouts is not water but nitrogen. Tiny Miranda, one of 27 known moons that orbit Uranus, wears a jumbled skin that's been shaped and reshaped by forces within. Jupiter's moon Io -- orbiting perilously close to the giant planet is literally turning itself inside out. Rivers of lava roll down from open craters that erupt like fountains. Flying by Europa, Voyager documented a complex network of criss-crossing grooves and ridges. In the 1990s, the Galileo spacecraft went back to get a closer look. It found that Europa's surface is a crazy quilt of fractured plates, cliff faces and gullies... amid long grooves like a network of superhighways. How did it get like this? Then, heat rising up through a subsurface ocean of liquid water cracks, and shifts, and spreads the icy surface in a thousand different ways. Europa's neighbors, Callisto and Ganymede, show similar features, suggesting they too may have liquid oceans below their surfaces. Crossing outward to Saturn, Voyager found a similar surface on the moon Enceladus. So when the Cassini spacecraft arrived in 2004, it came looking for answers to a range of burning questions: if this moon and others have subsurface oceans? Do they also have the ability to cook up and support life? And what could they tell us about the origin of life throughout the galaxy?

• published: 14 Apr 2011
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