If water is the elixir of life, it's no wonder that Earth which is 70% ocean simply teems with living things. The other planets and moons in the solar system don't have it so good. They're forbidding places that are hydrological deserts, and thus biological ones too.
That, at least, had long been the conventional wisdom, but in recent years, scientists have come to learn that by some measures, the solar system fairly sloshes with water. Mars, we now know, was once as wet as Earth and still harbors ice and perhaps liquid water. The moon is thought to have water locked in permafrost at its poles. Jupiter's moon Europa is probably home to a globe-girdling ocean beneath a thin rind of ice, and its Jovian sisters Callisto and Ganymede appear to be icy and wet too. Now, according to new findings by the Cassini spacecraft, one more name can be added to the list of water worlds: Enceladus, a small moon orbiting Saturn. What's more, Enceladus' water might be unusually hospitable to the emergence of life.
The Cassini probe, which was launched from Earth in 1997 and arrived at Saturn in 2004, had a big job to do: principally, studying the planet's elaborate ring system and taking a census of its litter of moons of which 53 have been found and named. Of those, Enceladus, discovered in 1789, held some of the deepest secrets.
Even viewed from Earth, the 310-mile-diameter moon appears bright white, almost as if covered in ice or snow; when the Voyager 1 spacecraft arrived at Saturn in 1981, it confirmed that long-distance impression. More intriguing was the way Enceladus behaved. Embedded inside Saturn's E ring the outermost of the eight bands that make up the ring system Enceladus seemed to orbit with a thick clump of ring matter trailing behind it, almost as if it were dragging the material in its gravitational wake. What astronomers suspected instead and what Voyager confirmed was that Enceladus was not dragging matter but expelling it, chugging through its orbits like a locomotive and leaving a vapor trail behind it. What astronomers couldn't know for sure was what the exhaust was made of.
"Potential plume sources on Enceladus are an active area of research," says Linda Spilker, a Cassini project scientist at the Jet Propulsion Laboratory in Pasadena, Calif.
In 2005, Cassini helped advance that research when it endeavored to determine the composition of the exhaust in the most straightforward way possible: by flying through it and registering the thousands of high-speed pellets that collided with its skin. The speed and density of the pellets confirmed that they were ice. Analyzing the precise composition of that ice has taken years, but the results, published this week in the journal Nature, were worth the wait.
Not only is the ice made of ordinary water, but it's salt water, with sodium turning up in the samples no matter how many times the ring material was retested. "Our measurements imply that besides table salt, the grains also contain carbonates like soda," says Frank Postberg, a Cassini scientist working at the Max Planck Institute in Heidelberg, Germany.
For biologists, that's huge. The only way to account for that particular chemistry is if the salts have dissolved out of rocks in the interior of Enceladus into a large quantity of standing water, as would occur if the moon had a subsurface ocean. "Both components [table salt and carbonates] are in concentrations that match the predicted composition of an Enceladus ocean," says Postberg. "The carbonates also provide a slightly alkaline pH value, which could provide a suitable environment on Enceladus for life precursors."
How a moon that hangs in the frigid depths of the solar system could keep water in a liquid state is not much of a mystery. Too small to have a molten core and too far from the sun to feel even a flicker of its heat, Enceladus does have other moons principally outlying Tethys and Dione orbiting nearby. Each time those moons pass, they give Enceladus a gravitational tug, which causes it to flex slightly. Do that enough times and the 4 billion years the solar system has been around is more than enough and the pulsing moon heats up in much the way a wire hanger does if you bend it repeatedly back and forth. That explains both why the water stays liquid and why it's repeatedly squeezed up through cracks and into space, where it flash-freezes into icy mist.
The book on Enceladus is by no means closed, and Cassini has two more flybys of the moon scheduled for November. Scientists aren't expecting to find proof of biology there anytime soon but now, at least, they've got a good reason to look.