Saturn’s small icy moon Enceladus contains a global subsurface ocean. According to new research from the University of Washington, the ice-covered ocean probably has a more Earthlike pH level and higher than previously known concentrations of carbon dioxide and hydrogen, providing more favorable conditions for microbial life in the ocean than previous estimates.
A new study by Fifer et al suggests more favorable conditions for life in Enceladus’ ocean than previous estimates, and provides a potential upper limit on the amount of biomass in the ocean due to an abundance of biologically useful material. Image credit: NASA / JPL / Space Science Institute.
Enceladus is a small moon, an ocean world about 310 miles (500 km) 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 — spotted and studied by NASA’s Cassini spacecraft — erupting hundreds of miles into space from the ocean through cracks in Enceladus’ ice-encased surface provide a tantalizing glimpse into what the moon’s ocean might contain.
But University of Washington doctoral student Lucas Fifer and his colleagues found that the plumes aren’t chemically the same as the ocean from which they erupt at 800 mph (1,290 kmh), the eruption process itself changes their composition.
“The plumes provide an ‘imperfect window’ to the composition of Enceladus’ global subsurface ocean and that the plume composition and ocean composition could be much different,” the researchers said.
“That 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.”
Fifer and co-authors returned to Cassini data with a computer simulation that accounts for the effects of fractionation, to get a clearer idea of Enceladus’ ocean composition.
They found ‘significant differences’ between Enceladus’ plume and ocean chemistry.
Previous interpretations, they found, underestimate the presence of hydrogen, methane and carbon dioxide in the ocean.
This artist’s painting of the south polar region of Enceladus shows massive jets of water ice being blasted into space. Image credit: Karl Kofoed / NASA.
“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.”
“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 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,” Fifer said.
This graphic illustrates how water might be heated inside Enceladus. Over time, cool ocean water seeps into the moon’s porous core. Pockets of water reaching deep into the interior are warmed by contact with rock in the tidally heated interior and subsequently rise owing to the positive buoyancy, leading to further interaction with the rocks. The heat deposited at the boundary between the seafloor and ocean powers hydrothermal vents. Heat and rocky particles are transported through the ocean, triggering localized melting in the icy shell above. This leads to the formation of fissures, from which jets of water vapor and the rocky particles from the seafloor are ejected into space. In the graphic, the interior ‘slice’ is an excerpt from a new model that simulated this process. The orange glow represents the parts of the core where temperatures reach at least 194 degrees Fahrenheit. Tidal heating owing to the friction arising between particles in the porous core provides a key source of energy, but is not illustrated in this graphic. The tidal heating results primarily from the gravitational pull from Saturn. Image credit: NASA / JPL-Caltech / Space Science Institute / LPG-CNRS / University of Nantes / University of Angers / ESA.
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, 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?” Fifer said.
The scientists presented their work June 24 at the astrobiology conference AbSciCon2019 in Bellevue, WA.
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Lucas Fifer et al. High Gas Concentrations in Enceladus’ Ocean, Potential Fuel for Microbial Life. AbSciCon2019, abstract # 127-064
