Large molecules show Enceladus “clearly is habitable for life”
New findings support, but do not prove, the idea that life may exist on Saturn’s icy moon. Richard A Lovett reports.
Large organic molecules are spewing into space from the depths of Saturn’s icy moon Enceladus, scientists say.
Exactly what these molecules are is unclear, but they contain at least as many as 15 carbon atoms, and possibly as much as 10 times more, says Nozair Khawaja, a planetary scientist at Heidelberg University, Germany.
Several years ago, scientists detected organic molecules containing up to four or five carbon atoms in the vicinity of Enceladus and in Saturn’s tenuous “E ring”, created by Enceladus’s emissions.
“But we were completely shocked [to find] organic compounds much more complex than we had previously imagined,” says study team member Christopher Glein, a planetary scientist at the Southwest Research Institute in San Antonio, Texas, US, who specialises in the oceanography of outer moons.
The new work, Glein says, came from collaboration between scientists using two different instruments on the Cassini spacecraft.
One, the Ion and Neutral Mass Spectrometer, was designed to analyse the molecular composition of gases. The other, the Cosmic Dust Analyser, examined dust particles. These particles hit its target plate at speeds of between three and 20 kilometres per second — fast enough not only to vaporise them but to break large molecules into fragments. The fragments were then “weighed” by a “time of flight” spectrometer, in which large, heavy molecules move more slowly than small ones.
When the two teams compared notes, they realised they were seeing different aspects of the same thing: fragments of organic-rich dust grains that crashed into their instruments and broke into smaller pieces. Some of these pieces were the small molecules detected by the Ion and Neutral Mass Spectrometer. Others were larger fragments detected by the Cosmic Dust Analyser.
Putting it all together, the scientists concluded that the Cassini spacecraft was encountering dust particles rich in carbon in large, complex “macromolecular structures”.
The only place this material could have come from was the interior of Enceladus, from which ice, dust and gas is jetting out in geyser-like plumes. These plumes are fed by vapours escaping from a sub-surface ocean.
“So this is a direct sample of the Enceladus ocean,” Khawaja says.
What exactly the newly discovered organic materials are is open for debate, although Khawaja believes they most likely are made of large numbers of ring-like structures cross-linked by hydrocarbon chains.
An important hint comes from the fact that the organic-rich grains don’t contain much water, implying that the materials in them don’t easily mix with water. Khawaja hypothesises that they formed deep inside Enceladus, then rose to the top of its underwater ocean, where they formed a thin film akin to an earthly oil slick.
Gas bubbles rising from the interior then lifted tiny blobs of the material into the vents that feed Enceladus’s jets. There they solidified into flakes that become the cores of the organic-rich dust grains detected by Cassini.
But that only explains how the dust grains are formed. The origin of the organic materials within them is entirely different.
One possibility is that they were created by reactions between hot water and matter in the rocky core of Enceladus. But it’s also possible they were created by life.
“This could come from abiotic processes and biotic processes,” Khawaja notes.
They could also have found their way to Enceladus via meteorites or comets, both of which are known to contain abiotically produced organics.
Other scientists are enthusiastic. Carolyn Porco, a planetary scientist at the Space Science Institute in Boulder, Colorado, and a visiting scholar at the University of California, Berkeley, US, is skeptical about the oil-slick theory, but excited about everything else.
Organic-rich flakes, she says, could also be created by bubbles rising from the ocean’s depth. These bubbles could collect organics on their way up, then eject them when they reach the surface and pop.
“[That] can likely explain the results without the need for a thin film,” she says.
But that’s a fairly minor quibble. The big news, she says, is the discovery of organic molecules with masses at least 200 times that of hydrogen, or 200 atomic mass units (amu).
“The average weight of the 22 amino acids used by terrestrial life is about 110 amu,” she explains.
Astrobiologist Chris McKay, of NASA’s Ames Research Center in Moffett Field, California, agrees. “[This] further indicates that the ocean of Enceladus is an organic-rich soup and clearly is habitable for life,” he says.
For the moment, however, there is no way to determine if the chemicals are produced by abiotic processes, or by life. And even if the Cassini spacecraft carried instruments capable of making the distinction (which it didn’t), the mission ended last September, when the spacecraft ran out of manoeuvring fuel and made a final, fiery plunge into Saturn’s upper atmosphere.
What’s needed, the scientists agree, is a return to Enceladus with a new spacecraft equipped with instruments capable of studying molecular structures in greater detail, thereby distinguishing biologically produced molecules from abiotically produced ones.
Not that this will happen in the near future. “It takes a long time to plan,” says Glein, who adds that the earliest we could get there is around 2035. “But we have quite a few people who are actively working toward this, so it’s very exciting,” he says.
In the meantime, NASA’s Europa Clipper mission is now being designed and built to visit Jupiter’s icy moon, Europa.
Like Enceladus, Europa is known to have a subsurface ocean. Like Enceladus, it might be venting materials from that ocean into space, through cracks in its icy surface.
The Europa Clipper’s launch date is not yet set, but if it is heavily prioritised, it could get to its destination as early as 2024, Glein says, though he suspects it will be a few years later.
Regardless of when it is launched, however, that spacecraft will contain the same types of instruments that Cassini did. “So, if Europa is spewing organic materials,” he says, the scientists on its instruments will be able to team up again. “[And] these instruments will be much more capable than the ones on Cassini because they are modern instruments with 20 years of advancements.”
Glein and colleagues have reported their results in the journal Nature.