The frozen surface of Enceladus showing the curve of the moon with space in the backgroundImage credits: NASA.

Enceladus is hardly an inviting place.

It sits 1.5 billion kilometers from the Sun, covered by a thick frozen exterior. Yet this moon of Saturn is actually one of the likeliest places in our solar system to host life. Under the ice, there lies an ocean of liquid and salty water.

But there’s another plot twist. This ocean appears to be almost devoid of oxygen and starved of freely available carbon dioxide. By terrestrial standards, it sounds very unfriendly to life.

Or so we thought.

Now, scientists have shown that an Earth microbe can grow under surprisingly similar conditions. In a laboratory experiment, researchers recreated key aspects of Enceladus’s ocean chemistry and introduced a methane-producing microorganism normally found around deep-sea hydrothermal vents. It not only survived, but continued growing and making methane at conditions previously thought beyond its limits.

There’s even more good news. A second study published alongside this research suggests that if microscopic life (or even just chemical traces of it) really exist on Enceladus, the ocean will naturally concentrate them into individual ice particles before spraying them into space. In other words, the moon may make its own potential biosignatures easier for us to find.

A large opening in the frozen surface of Enceladus with curving markings all over the icy environment around itMethanothermococcus okinawensis, whose ability to survive on Saturn’s moon Enceladus was tested by researchers at LMU. Image credits: LMU.

An ocean we can sample without drilling

Enceladus is only about 500 kilometers across (for comparison, our moon is 3,475 kilometers across). But it’s got plenty going for it.

When Cassini flew by Enceladus, it discovered enormous plumes erupting from fractures near the moon’s south pole. The jets carry water vapor and ice from the global salty ocean beneath the crust, allowing spacecraft to effectively sample that ocean simply by flying through the spray.

When researchers analyzed years of Cassini flybys, they found salts, organic compounds, phosphorus and molecular hydrogen. Tiny silica particles and hydrogen in the plume also point toward ongoing reactions between liquid water and warm rock on the ocean floor — something reminiscent of hydrothermal systems on Earth.

“Enceladus is considered to be one of the most promising places to search for extraterrestrial life,” says Dr. Vanessa Helmbrecht, lead author of the study.

But something was missing.

Life needs carbon, too

Photo of Enceladus showing the surface and curve of the moon and the light of the sun behind itA photograph of the icy moon Enceladus taken by the space probe Cassini. Image Credit: NASA/JPL/Space Science Institute

Enceladus’s ocean is thought to be extraordinarily alkaline, perhaps around pH 10 or 11. The pH scale runs from acidic to alkaline, with 7 considered neutral. Lower numbers are more acidic and higher numbers more alkaline.

At such high pH, very little of the ocean’s dissolved inorganic carbon exists as free CO₂. That could pose a severe obstacle for organisms that combine hydrogen and carbon dioxide to produce methane.

But how big a problem is that, really?

Researchers led by Vanessa Helmbrecht at Ludwig-Maximilians-Universität Munich decided to test that directly.

They created an oxygen-poor “Enceladus simulant” containing carbonate salts and minerals intended to reproduce interactions between the moon’s ocean and its rocky floor. Then they introduced Methanothermococcus okinawensis, heat-loving methane-producing archaea found around Earth’s hydrothermal vents.

The result was pretty surprising. In conventional laboratory growth medium adjusted to pH 10 or 11, the microorganisms couldn’t grow. There simply wasn’t enough accessible CO₂.

But in the simulated Enceladus environment, they did.

The rocks can feed the microbes

The simulated water-rock reactions generated hydrogen, supplying the microbes with energy. This was the key difference. But the microbe’s cells also changed their metabolism to cope with the scarcity of carbon dioxide.

The organism grew fine at pH 11, surpassing what researchers expected, and its carbon and energy metabolism was powered by hydrogen generated through the simulated mineral-water reactions.

“Our experiments show that its unique geochemistry could create conditions that are even more favorable for microbial life than we had previously thought,” Helmbrecht mentioned.

“Our study doesn’t prove that life exists on Enceladus,” geomicrobiologist William Orsi said, “but it does show that key geochemical features of its environment can support one of life’s most ancient metabolisms.”

Meanwhile, the second study asked a different question: if biological material is there, how easy would it be to detect?

Great news for the search for life

Diagram showing a cross section of Enceladus with vents ejecting particles into the airEnceladus ejects ice particles hundreds of kilometers into space. This results in salts and organic materials being separated and distributed at different locations inside each droplet. Image credits: FUB / Marie Dannenmann

Frank Postberg and colleagues analyzed 961 salt-rich ice grains measured by Cassini. Some were rich in sodium chloride, others in carbonates, phosphates or potassium salts, and overall, there was a lot of diversity.

Which begs the question: why? Why were the drops so diverse?

Yet again, scientists turned to lab experiments. When Enceladus-like droplets froze slowly, different compounds separated into different parts of the ice. The researchers propose that something similar happens as ocean spray rises through cracks in Enceladus’s crust. The partly frozen droplets are then shattered into tiny grains as they accelerate through the vents.

That means one grain might be salt-rich, while another could concentrate organic molecules.

This is good news for future spacecraft. Instead of biological compounds being diluted evenly through the plume, Enceladus may naturally package them into a small number of unusually rich particles.

“That is great news in the search for life,” says Postberg. “Future spacecrafts will have to analyze many individual ice particles in the plume. But if they come across one with microbial material in it, they could identify biosignatures in the particle relatively easy with already available technology.”

Together, the two studies remove two obstacles from the search for life on Enceladus. Its ocean chemistry may be compatible with microbial metabolism, and its plumes may concentrate potential biosignatures into particles that spacecraft can sample.

For a world hiding its ocean beneath kilometers of ice, Enceladus is surprisingly good at throwing clues into space.

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