Saturn's icy moon could support life, but we'd need to send a probe to be sure

Scientists from the University of Arizona and the Université Paris Sciences et Lettres decided last year that if life started on Enceladus, it might explain why the moon burps methane.

"To know if that is the case, we must go back to Enceladus and look," said Régis Ferrière, senior author of the new paper and associate professor in the Arizona Department of Ecology and Evolutionary Biology. 

Any life is probably microscopic

In their most recent paper, Ferrière and his colleagues say that even though the total number of living microbes in Enceladus' ocean would be small, all it would take is a visit from a spacecraft in orbit to know for sure if Earth-like microbes live in the ocean under the moon's shell.

"Sending a robot crawling through ice cracks and deep-diving down to the seafloor would not be easy," Ferrière said, explaining that more realistic missions have been designed that would use upgraded instruments to sample the plumes like Cassini did or even land on the moon's surface.  

"By simulating the data that a more prepared and advanced orbiting spacecraft would gather from just the plumes alone, our team has now shown that this approach would be enough to confidently determine whether or not there is life within Enceladus' ocean without actually having to probe the depths of the moon," he said. "This is a thrilling perspective." 

Enceladus orbits Saturn once every 33 hours. The moon isn't as large as Arizona, but its surface shines like a frozen pond in the sun. At least 100 huge water jets erupt from the moon's south pole like lava from a volcano.

Scientists believe these geyser-like phenomena contribute to one of Saturn's rings. Cassini sampled this ejected mixture from Enceladus' ocean.

Cassini's excess methane in the plumes recalls hydrothermal vents in Earth's oceans. At the boundaries of two adjacent tectonic plates, heated magma below the seafloor heats the ocean water through porous bedrock, forming "white smokers." Without sunlight, organisms rely on chemical substances emitted by white smokers.

"On our planet, hydrothermal vents teem with life, big and small, despite the darkness and insane pressure," Ferrière said. "The simplest living creatures there are microbes called methanogens that power themselves even in the absence of sunlight."  

They probably use methane as an energy source

Methanogens convert dihydrogen and CO2 to energy, generating methane. The calculations of Ferrière's team were based on the idea that Enceladus has methanogens that live in hydrothermal vents like those on Earth. Researchers figured out how many methanogens there were on Enceladus and if their cells and other organic molecules could be released in plumes.

"We were surprised to find that the hypothetical abundance of cells would only amount to the biomass of one single whale in Enceladus' global ocean," said the paper's first author, Antonin Affholder, a postdoctoral research associate at Arizona, who was at Paris Sciences & Lettres when doing this research.

"Enceladus' biosphere may be very sparse. And yet our models indicate that it would be productive enough to feed the plumes with just enough organic molecules or cells to be picked up by instruments onboard a future spacecraft," he added. 

Enceladus has gained attention as a place to revisit and study further. The "Enceladus Orbilander," designed by the Johns Hopkins Applied Physics Laboratory, would arrive in an orbit around Enceladus starting in the 2050s.

"Our research shows that if a biosphere is present in Enceladus' ocean, signs of its existence could be picked up in plume material without the need to land or drill," said Affholder, "but such a mission would require an orbiter to fly through the plume multiple times to collect lots of oceanic material." 

The paper says that the smallest plume material needed to find microorganisms and organic compounds should be used. Observable cells exhibit life.

"The possibility that actual cells could be found might be slim," Affholder said, "because they would have to survive the outgassing process carrying them through the plumes from the deep ocean to the vacuum of space – quite a journey for a tiny cell."  

Instead, organic substances like amino acids could be used to prove or disprove the existence of a rich environment for life.

"Considering that according to the calculations, any life present on Enceladus would be extremely sparse, there still is a good chance that we'll never find enough organic molecules in the plumes to unambiguously conclude that it is there," Ferrière said. "So, rather than focusing on how much is enough to prove that life is there, we asked, 'What is the maximum amount of organic material that could be present in the absence of life?'

According to the authors, if all measurements were to come back above a certain threshold, it could signal that life is a serious possibility.

"The definitive evidence of living cells caught on an alien world may remain elusive for generations," Affholder said. "Until then, the fact that we can't rule out life's existence on Enceladus is probably the best we can do."

The study was published in The Planetary Science Journal.

Study abstract:

"Saturn's moon Enceladus is a top candidate in the search for extraterrestrial life in our solar system. Ecological thermodynamic modeling of the plume composition data collected by NASA's Cassini mission led to the hypothesis that a hydrogenotrophic methanogenic ecosystem might exist in the putative hydrothermal vents at Enceladus's seafloor. Here we extend this approach to quantify the ecosystem's expected biomass stock and production and evaluate its detectability from the collection of plume material. We find that although a hypothetical biosphere in Enceladus's ocean could be small (<10 tons of carbon), measurable amounts of cells and organics might enter the plume. However, it is critical that missions be designed to gain meaningful insights from a negative outcome (no detection). We show that in order to sample a cell from the plume with 95% confidence, >0.1 mL of material needs to be collected. This would require material from more than 100 fly-bys through the plume or using a lander. We then consider amino acid abundance as an alternative signature and find that the absolute abundance of amino acids, such as glycine, could be very informative if a detection threshold of 1 × 10−7 mol L−1 could be achieved. Altogether, our findings set relatively high bars on sample volume and amino acid detection thresholds, but these goals seem within the reach of near-future missions."