Published online 31 May 2011 | Nature | doi:10.1038/news.2011.337

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Enceladus named sweetest spot for alien life

Saturn's icy moon has all the key ingredients, scientists say.

EnceladusHome away from home: Enceladus has all the ingredients for extraterrestrial life.WALTER MYERS/SCIENCE PHOTO LIBRARY

Saturn's icy moon Enceladus is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it, scientists said last week at a meeting of the Enceladus Focus Group at the SETI Institute in Mountain View, California.

But it may be too late to get a mission there the fast way, via a gravity boost from Jupiter. This would cut the journey time from ten years to as little as seven, but the next Jupiter-assist window hits its peak in 2015-17, and then closes until the 2030s.

That leaves scant time to plan and build a mission, even if engineers start immediately – something that is unlikely, many scientists believe, given the current emphasis on Mars.

That's too bad, because Enceladus may trump Mars as the Solar System's most likely abode for extraterrestrial life.

"It has liquid water, organic carbon, nitrogen [in the form of ammonia], and an energy source," says Chris McKay, an astrobiologist at NASA's Ames Research Center in Moffett Field, California. Besides Earth, he says, "there is no other environment in the Solar System where we can make all those claims".

Better yet, geyser-like jets spew ice crystals and gases into space, allowing a spacecraft to sample the subsurface by flying overhead. The current Cassini mission has done that several times already, but it's only equipped to find the building blocks of life, not more complex molecules.

"We want biomarkers," says Larry Esposito, a planetary scientist at the University of Colorado, Boulder.

Signs of life

First, though, it's necessary to figure out what to look for. That begins by figuring out how Earth-like organisms might survive in Enceladus's underground lakes, ponds or oceans, where there is no sunlight and so no photosynthesis.

They might, McKay says, use hydrogen to synthesize methane from carbon dioxide — a trick evolved by the Earth microbes known as methanogens. The methane could even be recycled if geological processes raise its temperature above 500 °C. "The fact we see a lot of methane makes it interesting," says McKay

Ronald Oremland, a microbiologist with the US Geological Survey in Menlo Park, California, believes an even better food source would be acetylene.

On Earth, this chemical doesn't occur naturally, although humans make it as fuel for welding torches. But it exists in comets and there are hints it might be present in the Enceladus jets.

If so, it could be a "fast food" source for microbes, Oremland says.

Acetylene-eating microorganisms on Earth convert the molecule into ethanol and acetate. Their biochemistry is much simpler than that of methanogens, suggesting that acetylene might be the food of choice for the most primitive organisms.

But acetylene, methane, ethanol and acetate aren't biomarkers. What's needed, Oremland and McKay agree, is to examine the ratios of carbon's two stable isotopes: carbon-12 and carbon-13.

Chemical and geophysical processes don't distinguish between the two. But the biological processes we know tend to favour the lighter isotope. Thus, if the methane coming from Enceladus is relatively rich in carbon-12, it's a strong sign of biological activity. Similarly, if the plume is emitting ethanol and acetate rich in light carbon, acetylene-eating bugs may be at work.

Amino acids might also reveal life, especially if they reproduce, or reverse, Earth life's overwhelming preference for 'L' isomers over their mirror image 'D' isomers, a skew not seen in abiotic processes. Any such bias would be "persuasive evidence for a biological origin," says McKay.

There and back again?

Such tests would need new instruments. There are several options, said Nathan Strange, a mission architect at NASA's Jet Propulsion Laboratory in Pasadena, California.

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The simplest would be a spacecraft whose sole job is to sample Enceladus's plume. But a multi-purpose mission might be more efficient.

For example, Strange said, a future Enceladus mission could also fly past Saturn's giant moon Titan. Or it could tour some of Saturn's dozens of other moons.

It could go into orbit around Enceladus and stay there, or bring a sample of the plume back to Earth. It could even carry a lander - including a "hopper" capable of launching from one landing zone to another. "The low gravity makes this possible," Strange says.

Nobody wants to wait decades for such a mission, but not everyone thinks that missing the Jupiter-assist launch window is a disaster. Launching later adds only a couple of years to flight times, and planning a good mission is often better than planning a quick one. 

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