Enceladus’ ocean even more habitable than thought

Stark sunlit edge of barren moon with sprays coming up and diffusing into black space.
These are water vapor geysers, erupting from the south pole of Saturn’s moon Enceladus. Scientists believe they originate in a global ocean below the moon’s icy surface. A new model suggests that Enceladus’ ocean might be moderately alkaline – not too acidic – reinforcing the idea it could be habitable. Image via NASA/ JPL/ Space Science Institute.

Could there be life on Saturn’s moon Enceladus? This icy world may be small (1/7 the size of our moon), but it has a global ocean of water beneath its outer ice crust. In fact, NASA’s Cassini spacecraft analyzed some of that water by flying through and sampling the moon’s water-vapor plumes. Those plumes originate from the ocean below and erupt onto the surface into space. The analysis results suggested that the ocean might well be suitable for some kind of life, at least microbes. Last month, NASA announced a new study from researchers at the University of Washington that further supports this possibility. It shows that the ocean is moderately alkaline (not too acidic) and may be even more habitable than previously thought.

The research team first published their peer-reviewed results in The Planetary Science Journal on August 11, 2022. NASA later posted the findings as a Research Highlight on November 30, 2022.

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A new model of Enceladus’ ocean

The researchers created a new model of Enceladus’ ocean. This model helps predict what conditions are like and how similar or dissimilar they are to those in Earth’s oceans. Thanks to Cassini, scientists already know that the ocean is fairly salty, like oceans on our planet. There is even evidence for hydrothermal vents on the ocean floor of Enceladus. On Earth, such vents provide heat and nutrients for a wide variety of organisms. The paper stated:

Enceladus harbors an ocean beneath its ice crust that erupts spectacular plumes from fissures at the south pole. The plume composition was measured by the Cassini spacecraft, and provides evidence for the ocean’s gas content, salinity, pH, and potential for life.

When Cassini analyzed the water-vapor plume spray, it found that the plumes contain water vapor, ice crystals, salts, ammonia, methane and various organic molecules. This isn’t proof of life yet, but it is tantalizing.

The new model now updates the effects of salinity, pH and gas content on the habitability of the ocean.

A moderately alkaline ocean

The results show that Enceladus’ ocean is likely even more potentially habitable than first thought. The model shows that the ocean is moderately alkaline with a pH (acidity) between 7.95 and 9.05. Oceans on Earth currently have an average pH of 8.1. Seven is considered to be a neutral pH. Anything below that is acidic and anything above that is basic (alkaline).

In addition, the model suggests that there is enough carbon dioxide and hydrogen in the ocean to support hydrogenotrophic methanogens (microorganisms that consume carbon dioxide and hydrogen, releasing methane in the process).

The paper said:

With new estimates for the gas content of the ocean, we find an ocean pH of 7.95 – 9.05, which encompasses the terrestrial ocean pH, high levels of NH4+ [ammonium] and inorganic carbon consistent with the accretion of Enceladus from comet-like planetesimals, and an abundance of chemical energy for potential Enceladean methanogens.

A comet-like origin for Enceladus’ ocean?

The analysis also provides possible clues about Enceladus’ origin. The model predicts the amount of bulk ammonia and inorganic carbon in the ocean. Those amounts, the model revealed, are consistent with the amounts that would come from comets. Researchers said that this shows Enceladus may have formed from comet-like materials.

Those materials contained a high concentration of volatiles, chemical elements and chemical compounds that can be easily vaporized. These can include nitrogen, carbon dioxide, ammonia, hydrogen, methane, sulfur dioxide, water and others. If the researchers are right, the ocean inside Enceladus is actually a remnant of those volatiles.

Enceladus' ocean: Cutaway view of interior of a planet-like object, with white text on black background.
Cutaway view of the interior of Enceladus showing the ice crust, global ocean, rocky core and water vapor plumes at the South Pole. Image via NASA/ JPL.

Not too salty, with lots of methane

If Enceladus’ ocean isn’t too acidic (moderately alkaline), that’s a good sign for possible life being able to exist. Earlier this year, researchers at the Massachusetts Institute of Technology (MIT) also reported that the ocean isn’t too salty, either. The ocean is salty, like Earth’s oceans, but too much salt can be a bad thing for living organisms.

Another study from a team of biologists in 2021 showed that Enceladus’ ocean also contains a lot of methane. Scientists don’t know yet, of course, if the methane is biological in origin or not, but it is certainly a possibility. The new ocean model discussed here, showing that there is enough carbon dioxide and hydrogen in the ocean to support hydrogenotrophic methanogens, could be a clue, perhaps?

Yet another study announced in 2020 indicated that the inner complexity of Enceladus is good for life. An even earlier study from 2019 showed that Enceladus’ ocean is also the ideal age – one billion years old – to support life. Very intriguing!

The proposed Enceladus Orbilander mission for late next decade would search for life on Saturn’s icy moon.

Enceladus' ocean: Smooth planet-like object with many long grooves on its surface, on black background.
NASA’s Cassini spacecraft captured this image of Enceladus on September 25, 2018. Image via NASA/ JPL-Caltech/ NASA Science.

Bottom line: NASA-supported researchers say that Enceladus’ ocean is moderately alkaline like Earth’s oceans, increasing the possibility of life existing in that dark abyss.

Source: Chemical Fractionation Modeling of Plumes Indicates a Gas-rich, Moderately Alkaline Enceladus Ocean


December 15, 2022

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