Enceladus is a small moon of Saturn with an ocean under its surface and jets that blast water and ice out into space.

Scientists just looked at data from a spacecraft that flew through the plume of water last year and found traces of complex organic molecules.



The finding isn't proof of life elsewhere in the solar system, but it's a good sign that makes Enceladus even more intriguing for future missions to search for alien life.

"How cooperative Enceladus is," Nozair Khawaja says. The small moon of Saturn, already one of the prime places to look for life elsewhere in the solar system, just got a little bit more enticing.



Khawaja, a planetary scientist with the Institut für Geowissenschaften (Institute for Earth Science) at Heidelberg University in Germany, co-authored a new study in Nature that reveals complex organic molecules are present in the plumes of water erupting from Enceladus out into space. In other words, all the components in the recipe for life as we know it are present on the small, watery moon—although whether life has taken root is still unknown.



"Cautious optimism is in order."

The new research led by Khawaja and colleague Frank Postberg analyzes data from two instruments on the now-defunct Cassini spacecraft that crashed into Saturn last year. Before its fiery end, however, the spacecraft flew through the plumes of Enceladus to collect material ejected from the ocean of water beneath the moon's icy surface.

"The moon freely delivers its organic inventory to the spacecraft in high concentrations," Khawaja says. "You don’t need to drill, not even to land for that. It’s just amazing."



Hunting for Life's Building Blocks

When NASA's Cassini spacecraft arrived at Saturn in 2004, scientists thought Enceladus was nothing more than a frozen ice ball. A year later, when the team realized that the moon was in fact spewing water from a subsurface ocean out into space, the discovery was "jaw-dropping," Cassini project scientist Linda Spilker told Popular Mechanics last year.

NASA adjusted its mission to fly Cassini through the plumes of Enceladus to learn about the composition of the water. Because the spacecraft wasn't built for this purpose, however, it was not carrying instruments built for the task. Instead, it had to rely on the Cosmic Dust Analyzer (CDA) and the Ion and Neutral Mass Spectrometer (INMS), tools designed to study much smaller molecules. Even so, the shadows of larger organic compounds have been spotted.

Plumes of watery material erupting from the surface of Enceladus, imaged by the Cassini spacecraft on November 30, 2010. NASA/JPL-Caltech/Space Science Institute

Because Cassini's readings are at a much lower resolution than you'd ideally want to spot larger molecules, the researchers struggled to pin down exactly what they are looking at. Nevertheless, they can say with confidence that there are large carbon-based molecules in the watery material of Enceladus' plumes.



Hunter Waite, the principal investigator for the INMS instrument at the Southwest Research Institute (SwRI) and a contributing author of the new study, says that the Cassini data points to "complex organic compounds that contain benzene moieties, possibly linked by polymeric chains with evidence for hydroxyl, ethoxy, and/or carbonyl functional groups as well as nitrogen bearing functional groups."



"The new organics can be either building blocks (things the microbes consume) or biological remains of membranes (things the microbes produce)," Waite tells Popular Mechanics.

That said, the presence of complex organic molecules is not a smoking gun for alien life on Enceladus. These compounds could have been produced by other natural processes, such as heat and pressure from hydrothermal vents on the ocean floor, created by the churning of Enceladus' porous rocky core, or even brought to Enceladus by comets.

Contributing author Christopher Glein, a research scientist with SwRI who specializes in extraterrestrial oceanography, says via email:



"Because organic molecules are fundamental to the nature of life as we know it, this finding makes Enceladus an even more attractive astrobiological destination in the solar system. ... However, we know from studying meteorites, comets, and Titan’s atmosphere that organic compounds do not equal life. Cautious optimism is in order."



Life Bubbling Up from Below?

While we cannot say with any certainty where this stuff is coming from, a discovery last year from the Cassini team helps fill in a few pieces to the puzzle.

Before scientists were able to work out the complex organics in the plumes of Enceladus, NASA managed to determine that a smaller substance is abundant in the moon's waters: molecular hydrogen. This finding suggests that hydrothermal vents, driven by the gravitational tug of Saturn, are currently on the ocean floor of Enceladus, pumping heat and nutrients into the water.

Whether or not this is a good sign for life is an open question. On the one hand, hydrothermal vents on the ocean floor of Earth can support thriving communities of living organisms, from microbes to tube worms and even crabs. However, the heat and pressure of these ecosystems can also produce complex organic compounds chemically, without any help from life.

A diagram of the hydrothermal processes of Enceladus’ core and ocean floor. These processes could synthesize organics from inorganic precursors, or transform preexisting organics by heating, or generate geochemical conditions in the ocean of Enceladus that would allow possible forms of alien life to synthesize biological molecules. NASA/JPL-Caltech/Space Science Institute/LPG-CNRS/Nantes-Angers/ESA

"We proposed that the most plausible origin of these complex organic molecules are hydrothermal vents inside the highly porous rocky core of Enceladus," Khawaja says.



Glein adds: "We can get some guidance from analogous processes on Earth. On our planet, [hydrogen]-rich conditions in some hydrothermal systems create a strong drive to produce organic molecules, either abiotically [without life] via mineral-catalyzed reactions, or by biosynthesis where vent fluids mix with seawater."

So we have two different pictures of what could be happening on Enceladus. In one case, material on the ocean floor is churning and reacting to generate complex organic materials chemically. In the other case, dead alien microbes are decomposing and their leftover organic material is floating to the surface. From there, the organics are ejected into space in icy plumes that reach more than 10,000 above the ground, where our spacecraft can snatch them out of the void.

But to answer definitively one of the oldest questions in science—whether life exists someplace else—we need to send more spacecraft to the outer solar system to continue the search.

"We need to mount a serious campaign to methodically address the life question," Glein says. "A first step would be to fly much more capable mass spectrometers through the plume. This could provide the first hints of life, called biosignatures, such as a strange distribution of organic molecules or isotope ratios that are unlikely to be produced by geochemical processes." If the results from these scouting missions were promising, then additional missions could be launched to look for life directly with "landers, plume sample return, and submarines."

This process has already begun for another tantalizing watery world in the solar system: Europa, the fourth-largest moon of Jupiter. Like Enceladus, Europa is thought to have hydrothermal activity on the ocean floor and plumes erupting into space, so this moon is the target of NASA's next flagship mission to the outer solar system. Europa Clipper is slated to launch in the early 2020s carrying more advanced instruments to look for complex organic molecules and determine their origin.

"We don’t know that Europa has such organic compounds, and we have to wait until we go there," Khawaja says. "There is a need to go back to Enceladus."

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