For nearly two decades Enceladus, a 500-kilometer-wide moon of Saturn, has been a top target in the hunt for extraterrestrial life. In 2005, shortly after arriving in orbit around the ringed planet, the joint NASA–European Space Agency (ESA) Cassini mission found clinching evidence that Enceladus harbored a liquid-water ocean beneath its bright-white icy crust—plumes of seawater spraying up from the moon’s south pole. Astrobiologists have become ever more enthralled by Enceladus ever since, as further studies of the ice grains in the plumes have revealed multiple molecular building blocks of life blasting out from the hidden ocean.
Now scientists revisiting data from Cassini—which ended its mission in 2017—have spied even more tantalizing ingredients in the plumes: suites of complex organic molecules which, on Earth, are involved in the chemistry associated with even bigger molecules considered essential for biology. Published Wednesday in Nature Astronomy, the discovery bolsters the case for follow-up missions to search for signs of life within Saturn’s enigmatic, ocean-bearing moon.
The findings show “there is chemical complexity in Enceladus’s subsurface ocean,” says Nozair Khawaja, a planetary scientist at the Free University of Berlin in Germany, who led the Nature Astronomy study.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
“These new results are very intriguing and raise the question of what, exactly, is the true nature and origin of organics within Enceladus’s ocean,” says Kevin Hand, a planetary scientist and director of the Ocean Worlds Lab at NASA’s Jet Propulsion Laboratory, who was not involved in the study.
Besides its remoteness from Earth, Enceladus has kept so many of its secrets for so long because the Cassini orbiter wasn’t really designed for such deep scrutiny of a single, specific object. “The goal of the mission was to understand Saturn, its rings and its moon systems,” Khawaja says. Cassini launched nearly 30 years ago carrying instruments built in the 1980s or 1990s, back when the moon’s subsurface ocean and south polar plumes were unknown. Repurposing that vintage kit for in-depth astrobiology was difficult—not least of all because of how hard the resulting data were to work with.
“Cassini’s instruments were made to analyze the chemical composition of dust and ice particles, but they weren’t meant to explore the subsurface material of Enceladus,” Khawaja says. One particular problem was the relatively low resolution available from a mass spectrometer on Cassini called the Cosmic Dust Analyzer (CDA), which parsed the chemical composition of puffs of dust from ice grains striking its detectors each time the spacecraft swooped through a plume. The plumes proved so thick with material, Khawaja explains, that the CDA would be overwhelmed during Cassini’s Enceladus flybys.
The result was that countless different types of particles with similar masses blurred together in the CDA’s detections, making it nearly impossible for scientists back on Earth to discern them. They could clearly see that ordinary water molecules comprised the vast majority of collected material—nearly 98 percent, Khawaja says. Piecing together the nature of the remaining 2 percent, however, required many carefully choreographed flybys and tweaks to the CDA’s operations across several years. The flyby that eventually hit a bull’s-eye was a maneuver on October 9, 2008, code-named E5. It wasn’t the first and it wasn’t the closest, but E5 was special because of its higher-than-average speed and a fortuitously timed eruption from Enceladus.
Cassini’s speed during E5 was nearly 18 kilometers per second (km/s)—about 6 km/s faster than most other flybys—which translated into massive improvements in the CDA data. “The impact speed was higher, and at such high speeds, water molecules shatter. They don’t survive. But other species like organics remain,” Khawaja explains. E5 was also lucky because it sent Cassini plowing through a plume that had been ejected mere minutes beforehand. This ensured the material came fresh out of Enceladus’s subsurface and had not been altered or degraded by cosmic radiation. “The curtain went up,” Khawaja says. But years of painstaking data analysis were still to come.
A view of Enceladus against the backdrop of Saturn.
NASA's Goddard Space Flight Center
Some of the co-authors of the new study published a paper in 2011 analyzing the E5 flyby results. “Back then, we clearly saw the features of organic molecules in the mass spectra produced by the CDA, but we were unable to nail down the type of these organics. We just knew they were there,” Khawaja says. Based on exhaustive experiments examining how differences in the ice grain impact speeds affect the CDA data, he and his colleagues think they’ve now tracked down most of what’s within the plumes, with major implications for the moon’s possibility of hosting life.
“I think it makes a lot of sense that it would take diligence and patience to fully understand the CDA data. I applaud them for taking such care in their analyses,” says Shannon MacKenzie, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory, who wasn’t involved in the study.
The team’s work revealed the plumes contain several chemical compounds Cassini previously detected in the E ring, a torus of ice and dust Enceladus makes as it spews material in its orbit around Saturn. “There were complex organics in the signal which had a benzenelike structure, with many compartments connected with side chains with some oxygen and nitrogen plugged in. They were like hydrocarbons—massive and complex,” Khawaja says. The analysis found other materials that had been seen in the E ring as well: amines, aromatics and oxygen-bearing molecules. Their presence in the freshly ejected plumes, Khawaja argues, confirms they all originated in Enceladus’s subsurface ocean. Most excitingly, the study also revealed new, never-before-seen compounds lurking in the plume, sourced from somewhere within the moon.
“In these fresh grains, we’ve got molecules like esters and ethers, which were carrying oxygen in themselves and had double bonds,” Khawaja says. Another new finding was the presence of compounds where oxygen and nitrogen were probably combined. “We suspect these are sort of intermediates to make further, complex organics, maybe potentially organics that are biologically relevant,” he adds. Certainty is elusive because the organics collected by CDA were shattered into multiple tiny fragments; researchers are still figuring out how to piece these fragments back together.
“This work shows that some of the fragments are indeed derived from quite large and complex organic compounds,” Hand says. “But maybe those compounds originated from even larger compounds. What exactly would we find if we dove into the ocean below—are the compounds reported here just the tip of the astrobiological iceberg?”
Khawaja already has ideas about what follow-up missions might find by delving deeper with better, state-of-the-art instruments. The newly revealed cocktail of compounds, he says, could feed into a “network of reactions” to create pyrimidines—a class of molecules necessary for the formation of DNA. (And, here on Earth, DNA is what leads to fish, lions, humans and life as we know it.) This network of reactions could yield lipids, too—molecules that can arrange themselves into cell membranes. Even so, Khawaja notes, “we don’t have a clue about any actual biological relevance yet.”
For now, in the absence of a follow-up mission at or en route to Enceladus, the team is developing an advanced computer model of the entire Enceladus subsurface system in hopes of pinpointing the probable sources and interactions of the moon’s rich assortment of chemical compounds. There’s also some room left for discovery in the Cassini data. “There are still certain spectral types that I see and don’t understand,” Khawaja says.
Ultimately, most of the hope for definitive answers about life on Enceladus in the near term lies with a mission still on the drawing board at ESA. Such a mission would most likely include an orbiter, albeit one far more advanced than Cassini, with a lander as a possible addition. “In a mission like that, a lander and an orbiter should complement each other,” Khawaja says.
But not all are equally sold out on the lander idea. “The results of this study corroborate mission concepts that don’t even need to land—we could just continually fly through the plumes and collect fresh material,” Hand says. “Why risk landing when Enceladus is handing out free samples?”
Regardless of such logistical debates, what’s clear is that Enceladus remains one of the most alluring destinations to look for extraterrestrial life.
“Water, energy and the right chemicals—all three keystones of habitability are there,” Khawaja says. Even if future studies fail to find life, he argues, the implications would be enormous. “If it’s not there despite those three keystones, it would mean that life needs something more.”
