If you wanted to pick an example of how NASA’s deep space missions of exploration have changed our understanding of the solar system over just one lifetime, Enceladus’ would be a great choice. Voyager 2’s observations of the Saturnian moon showed a surface that might have been shaped by cryovolcanism but provided relatively few details. The arrival of Cassini changed everything. In 2005, the Saturn probe detected liquid water vapor being expelled from the surface of Enceladus, over the moon’s south pole. Since then, scientists have studied this vapor to determine its composition and original contents. While Cassini has long since plunged into Saturn, that work continues today — and continues to yield results.
Geyers erupting from Enceladus. Credit: NASA
Last year, some of the same research team reported finding complex organic macromolecules within the water vapor that were likely floating on the surface of Enceladus’ ocean. This year, they followed up with a more sophisticated analysis of what sorts of molecules were dissolved into the ocean water. The compounds found within Enceladus’ water vapor plumes, which are responsible for most of the content of Saturn’s E ring, are believed to be present in the liquid subsurface ocean that exists underneath the south pole rather than being the result of contamination as the water escapes from its subsurface prison. That’s significant because many of the nitrogen and oxygen-based compounds the researchers detected are also essential to amino acids here on Earth.
Abiogenesis is the process by which life arises from non-living matter, beginning with the presence of simple organic compounds. While there are those who argue that life might not have arisen on Earth but instead arrived here from elsewhere, this really just kicks the can down the road. Life, wherever it came from, had to evolve from non-life at some point. Since the 1950s, we’ve known that amino acids can be synthesized from inorganic compounds under conditions intended to replicate the early Earth. The discovery of so-called “black smokers” (undersea vents in the seafloor) and the Lost City hydrothermal vent system in 2000 both illustrated how life could arise without the need for photosynthesis. Black smokers are rich areas of life, while the Lost City hydrothermal vent system is rich in abiotically produced methane and hydrogen — two materials fundamental to life.
The nitrogen and oxygen compounds found within the water vapor indicate that some of these same processes are active within Enceladus as well, and therefore imply that its subsurface ocean contains the ingredients required for the formation of life. The detected compounds were initially dissolved in the oceans of Enceladus before evaporating and condensing on the surface of the moon. When the moon began emitting jets of water, the compounds were carried skyward and into Cassini’s path.
“If the conditions are right, these molecules coming from the deep ocean of Enceladus could be on the same reaction pathway as we see here on Earth,” said Nozair Khawaja, who led the research team of the Free University of Berlin. “We don’t yet know if amino acids are needed for life beyond Earth, but finding the molecules that form amino acids is an important piece of the puzzle.” Khawaja’s findings were published Oct. 2 in the Monthly Notices of the Royal Astronomical Society.
There’s still a great deal we don’t know about Enceladus. The reason it periodically emits jets of water is related to the mechanisms that keep the water liquid. As the moon orbits Saturn, tidal flexing from Saturn and other moons may generate enough energy to allow the planet’s water reserves to remain liquid. The ridges at the South Pole from which water escapes are thought to fill with ice when the planet is under comparatively little stress, then flex open once more as its orbit changes. But the tidal flexing of Enceladus isn’t thought to generate enough energy to account for the liquid ocean Cassini observed — only about 1.1GW of energy is thought to be produced, whereas ~4.7GW is required to generate the effects we observe. Radionuclide decay within the core is also unlikely; Enceladus may have been hot enough to maintain a liquid ocean using radionuclide heating early in its existence, but the short-lived elements that powered this reaction will have decayed by now. Investigations into how Enceladus’ vast subsurface ocean remains liquid are ongoing.
One big difference between how Enceladus was viewed when I was a kid versus now is the chance that the moon could harbor life. While Mars may still possess a subglacial lake, and water is theorized to exist on Ganymede and under the ice sheets of Europa, we know it exists under the surface of Enceladus as well, which might make it the best environment to search for signs of life. Multiple follow-up missions to the planet are under study and it may one day be targeted for an exploratory mission in its own right.