Enceladus is a small moon, an ocean world about 310 miles (500 kilometers) across. Its salty subsurface ocean is of interest because of the similarity in pH, salinity and temperature to Earth’s oceans. Plumes of water vapor and ice particles — detected by the NASA’s Cassini spacecraft — erupting hundreds of miles into space from the ocean through cracks in Enceladus’s ice-encased surface provide a tantalizing glimpse into what the moon’s subsurface ocean might contain, possibly providing conditions favorable to life, according to new research presented at the recently concluded astrobiology conference, AbSciCon2019.
The presence of high gas concentrations could provide fuel — a sort of chemical “free lunch” — for living microbes, said University of Washington astrophysicist Lucas Fifer. Or, it could mean “that there is hardly anyone around to eat it.”
The new information about the composition of Enceladus’ ocean gives planetary scientists a better understanding of the ocean world’s capacity to host life, including research on how long Enceladus’ ocean might have existed, says Marc Neveu, a planetary scientist and astrobiologist at NASA’s Goddard Space Flight Center.
For life to exist there must be a source of energy for its biochemistry, and the most likely source of this is from chemicals created by reactions between the water and the moon’s hot, rocky core. Similar reactions are what fuel flourishing ecosystems surrounding hydrothermal vents deep beneath the surface of Earth’s.
To figure out how old the ocean might be, Neveu modeled the evolution of Enceladus over billions of years, looking to see how its interactions not only with Saturn’s stupendous gravity, but also with other moons and the rings, would cause its interior to either warm or cool. Based on this, he says, Enceladus’s ocean appears to be about a billion years old. “It could be two,” he says, “[or] it could be a half billion years.”
If the ocean is too old, Neveu says, the reactions creating them may have subsided, depriving organisms within it of an energy source. If the ocean is too young, it may not have existed long enough for life to emerge – or at least not to detectable levels – which, he notes, “is all we really care about if we are going to send a spacecraft”. But the bottom line is that it’s neither super-old (compared to the solar system’s 4.5-billion-year history, nor super young. “So it could be ‘just right.”
“Future spacecraft missions will sample the plumes looking for signs of life, many of which will be affected just by the eruption process,” said Fifer of plumes erupting hundreds of miles into space from the ocean through cracks in Enceladus’s ice-encased surface. providing a tantalizing glimpse into what the moon’s subsurface ocean might contain. “So, understanding the difference between the ocean and the plume now will be a huge help down the road.”
Scientists didn’t know why Enceladus is the brightest world in the solar system, or how it related to its E ring. Cassini found that both the fresh coating on its surface, and icy material in the E ring originate from vents connected to a global subsurface saltwater ocean that might host hydrothermal vents.
Complex organic molecules have been discovered in the plumes. The data transmitted back to Earth by the Cassini Saturn orbiter, which ended its service above the ringed world on Sept. 16, 2017.
But Fifer and colleagues found that the plumes aren’t chemically the same as the ocean from which they erupt at 800 miles an hour; the eruption process itself changes their composition, providing an “imperfect window” to the composition of Enceladus’s global ocean. The plume composition and ocean composition could be much different. That, they find, is due to plume fractionation, or the separation of gases, which preferentially allows some components of the plume to erupt while others are left behind.
This in mind, the team returned to data from the Cassini mission with a computer simulation that accounts for the effects of fractionation, to get a clearer idea of the composition of Enceladus’s inner ocean’s. They found “significant differences” between Enceladus’s plume and ocean chemistry. Previous interpretations, they found, underestimate the presence of hydrogen, methane and carbon dioxide in the ocean.
“It’s better to find high gas concentrations than none at all,” said Fifer. “It seems unlikely that life would evolve to consume this chemical free lunch if the gases were not abundant in the ocean.”
Those high levels of carbon dioxide also imply a lower and more Earthlike pH level in the ocean of Enceladus than previous studies have shown. This bodes well for possible life, too, Fifer said.
“Although there are exceptions, most life on Earth functions best living in or consuming water with near-neutral pH, so similar conditions on Enceladus could be encouraging,” he said. “And they make it much easier to compare this strange ocean world to an environment that is more familiar.”
There could be high concentrations of ammonium as well, which is also a potential fuel for life. And though the high concentrations of gases might indicate a lack of living organisms to consume it all, Fifer said, that does not necessarily mean Enceladus is devoid of life. It might mean microbes just aren’t abundant enough to consume all the available chemical energy.
The researchers can use the gas concentrations to determine an upper limit for certain types of possible life that could exist in the icy ocean of Enceladus. In other words, Fifer said: “Given that there’s so much free lunch available, what’s the greatest amount that life could be eating to still leave behind the amount we see? How much life would that support?”
Thanks to Cassini, he said, we know about Enceladus’ ocean and the types of gases, salts and organic compounds that are present there. Studying how the plume composition changes as well as its age can teach us yet more about this ocean and everything in it.