Could there be life under the icy surface of Saturn's moon Enceladus?

Scientists have found a promising sign.

NASA announced on Thursday that its Cassini spacecraft mission to Saturn has gathered new evidence that there's a chemical reaction taking place under the moon's icy surface that could provide conditions for life. They described their findings in the journal Science.

"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment," Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate in Washington, said in a statement.

However, the scientists think that because the moon is young, there may not have been time for life to emerge.

In 2015, researchers said that there was evidence of a warm ocean under the moon's surface, as NPR's Geoff Brumfiel reported.

This posed an exciting prospect — researchers wondered whether that warm ocean might be interacting with rock to create a form of chemical energy that could be used by some forms of life.

If true, it would be analogous to ancient organisms on Earth fueled by the energy in deep-sea ocean vents.

On Thursday, NASA scientists said they have detected evidence that this kind of chemical reaction is likely occurring under the surface of Enceladus. By flying through a plume spraying out of its icy shell, Cassini was able to detect molecular hydrogen.

NASA said in a press release that the presence of hydrogen in the sub-surface ocean "means that microbes – if any exist there – could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in water." Called methanogenesis, it's a reaction that it says is "at the root of the tree of life on Earth."

As the lead author Hunter Waite put it, the reaction would basically provide a "candy store for microbes."

So what exactly could be lurking under the surface?

"Most of us would be excited with any life, and certainly when we're talking about the sources of energy, this is to feed the base of a food web. So we're going to start with bacteria and if we get lucky, maybe there's something that's larger," NASA astrobiology senior scientist Mary Voytek said at a news conference.

The hydrogen found "speaks to the habitability" of the moon, Voytek says. But paradoxically, the large amount detected could make finding life less likely. She explains:

"[The] fact that that we can measure such high concentrations of hydrogen and carbon dioxide mean that there might not be life there at all, and if there is life, it's not very active. ... We have this buildup of food that's not being used. And part of that could be that we think Enceladus might be fairly young."

The findings were announced along with observations by the Hubble Space Telescope of another, much older moon — evidence of plumes spraying out of the surface of Jupiter's Europa.

Europa is potentially billions of years older than Enceladus, and life takes time to emerge. And there's "no reason" why the same process wouldn't be happening on the moon orbiting Jupiter, Voytek says.

That's why she thinks it's actually more likely that they'll find life on Europa than Enceladus. "My money for the moment is still on Europa," she says.

It's going to take years for scientists to definitively determine whether there is life on either moon. The Europa Clipper mission is set to launch to Europa in the 2020s.

Cassini will end its mission in September and as NPR's Nell Greenfieldboyce reports, it "doesn't have the instruments needed to look for life itself" on Enceladus.

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