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Scientists for the first time have gathered comprehensive evidence suggesting Enceladus may have all the necessary ingredients to harbor life in the ocean beneath its icy crust.
Particles in a large plume of water vapor emanating from the surface suggest the moon has an active ocean that circulates life-sustaining nutrients picked up from the rocky interior below.
"The plume is our smoking gun," said astrobiologist Christopher Parkinson of the University of Michigan. "It gives you a hint about what's going on inside."
Life could arise in these conditions, or it could arrive from elsewhere in the galaxy, Parkinson said Monday at the annual meeting of the American Geophysical Union.
"If we sent a probe with the idea that there was microbial life on it that we were going to infect the place with, it would likely be a successful experiment," he said. "I'm not suggesting we do it, but it would be very cool."
So far, space missions have found evidence in the solar system for liquid water and organic molecules on Mars, Jupiter's moon Europa and now Enceladus. Scientists hunting for life outside of the solar system are looking for planets with atmospheres with the right chemistry. But if Enceladus could host life, it shows that worlds without atmospheres could also be candidates.
Chemical analyses of the moon's vapor plume made by the Cassini-Huygens spacecraft in a close flyby in October showed that it was mostly water, but also contained methane, carbon dioxide, carbon monoxide and silicate dust.
Some of the vapor from the plume falls back to resurface the crust, Parkinson said. There the molecules could react with charged particles from the sun to create things like hydrogen peroxide and methanol - unappetizing to humans, but a nutritious buffet for microbes.
The plume emanates from an area of Enceladus that has long dark lines, known as tiger stripes, that may mark where the ice is slowly circulating up to the surface. That would mean that in between the stripes, the ice would make its way down to the ocean, taking the nutrients with it.
The circulation of the ocean could continue to bring more silicate dust from the rocky floor of the ocean up to the base of the ice where it would circulate through the tiger stripes to the surface and out into the plume, starting the process over. The whole cycle would take less than a million years - fast enough, Parkinson said, to keep a microbial colony alive.
"There could be ocean processes that could create a sustainable or complete geochemical cycle required for life," Parkinson said. "We have analogs on Earth that show this. Certainly we know from Lake Vostok in Antarctica (that) there's water and life stays fairly dormant, but it's there."
But going from the possibility of life on Enceladus to life actually taking hold is a leap, said planetary scientist Juergen Schmidt of Potsdam University, who has also studied Enceladus' plume.
"We say we need water and we need this chemistry, but that could just reflect our geocentrism," he said.
And even if we do know the right ingredients for life, Enceladus might not be the best candidate to host it.
"There are three candidates - Mars, Europa and now Enceladus, and all that is based on the presence of liquid water and organic molecules," Schmidt said. "But I don't think Enceladus is more likely than the others."
For now, Parkinson is content with just the possibility.
"I can show it's life friendly. Is there life? No guarantee," he said. "If we put it there, it could thrive. It's possible."
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