Enceladus’ ocean not too salty for life

Stark sunlit edge of barren moon with sprays coming up and diffusing into black space.
Scientists feel certain about a liquid ocean beneath the icy surface of Enceladus because, in 2005, the Cassini spacecraft found active geysers spewing through cracks at Enceladus’ south pole. Cassini also found water vapor, ice particles, salts, methane and a variety of complex organic molecules within the plumes. Image via NASA/ JPL/ Space Science Institute.

Saturn’s moon Enceladus has a global ocean of liquid water beneath its icy surface. In 2005, NASA’s Cassini spacecraft discovered water vapor plumes spewing from the little moon’s south polar surface. Cassini also showed that the subsurface ocean on Enceladus might be able to support some forms of life. It has heat, mineral nutrients and organic material. On July 20, 2022, researchers at the Massachusetts Institute of Technology (MIT) published a new study, suggesting Enceladus’ ocean is a little less salty than Earth’s oceans.

Coupled with the previous findings, this new finding bolsters the possibility of life in Enceladus’ ocean.

Wanying Kang led the research team at MIT. The researchers were able to estimate the saltiness by studying how the surface ice covers the ocean below. They published their peer-reviewed findings in Science Advances on July 20, 2022.

Karmela Padavic-Callaghan wrote about the tantalizing results in New Scientist on July 20, 2022.

Planet-like sphere with bluish cracks and craters on its surface.
View of Saturn’s moon Enceladus via the Cassini spacecraft. Image via NASA/ ESA/ JPL/ SSI/ Cassini Imaging Team/ Carnegie Science.

The salinity of Enceladus’ ocean

On Enceladus, the salinity of the ocean, the silicate core and the ice shell all have significant impact on the ocean dynamics and habitability. As the paper outlined:

Of profound astrobiological interest, Enceladus appears to have a global saline subsurface ocean, indicating water-rock reaction at present or in the past, an important mechanism in the moon’s potential habitability. Here, we investigate how salinity and the partition of heat production between the silicate core and the ice shell affect ocean dynamics and the associated heat transport, a key factor determining equilibrium ice shell geometry.

In particular, the researchers wanted to know how the thickness of the ice shell relates to the salinity of the ocean. Saltier subsurface oceans should, generally speaking, have thicker ice over a planet’s or moon’s poles. Less salty oceans would have thinner ice. The study showed that the salinity level on Enceladus is likely intermediate. As the New Scientist article explained:

The team devised a theoretical model detailing how ocean salinity, ocean currents and ice geometry affect each other on a planet or a moon, then tweaked it to best reproduce the properties of Enceladus’s ice.

From the paper:

Among scenarios explored here, the pronounced ice thickness variations observed on Enceladus are most consistent with heating that is predominantly in the ice shell and a salinity of intermediate range.

Cutaway view of ocean under ice, with ringed planet in background.
Cutaway view of Enceladus’ subsurface ocean. The salinity of the ocean appears to be suitable for life. There is also evidence for hydrothermal vents on the ocean floor, just like on Earth. Image via NASA/ JPL-Caltech/ SwRI.

Salinity not the only factor

Salinity isn’t the only factor when it comes to habitability, though. As the New Scientist article mentioned:

David Stevens at the University of East Anglia, UK, says that the behavior of ice and water on other planets is directly related to their habitability. At the same time, salinity is only one factor, he says.

Thinner ice over Enceladus’ poles

Interestingly, the results of the study indicate that the ice over Enceladus’ poles is thinner than at its equator. That would point to a less salty ocean. More specifically, the specific variation in ice thickness suggests that the ocean’s salinity could be as high as 30 grams of salt in a kilogram of water. That’s salty, but less than Earth’s oceans, which have 35 grams of salt per kilogram of water.

Overall, that sounds promising for the potential for life. The waters are salty, like oceans on Earth, but not too salty. If anything, Enceladus’ ocean may be a bit more benign than oceans on our planet.

Headshot of smiling woman with glasses and parted hair.
Wanying Kang at the Massachusetts Institute of Technology (MIT) led the study to determine how salty Enceladus’ ocean is. Image via MIT.

Heat from the seafloor of Enceladus’ ocean

Another intriguing finding seems to support earlier data from the Cassini mission. The team found evidence for heat emanating from the ocean floor on Enceladus. Previous analysis by Cassini of water vapor and organics in Enceladus’ famous plumes suggested the existence of hydrothermal vents on the ocean floor. This new evidence would seem to agree with that, although more study is needed.

On Earth, such hydrothermal vents create an oasis for many different types of organisms. They provide essential heat and nutrients in the otherwise dark and hostile depths of the oceans.

Last year, scientists also reported evidence in the Cassini data for a lot of methane in Enceladus’ ocean. Could this be an indication of methanogenic organisms in the ocean?

Enceladus' ocean: Moon-like body with a few craters, cracks and plumes of water vapor on the bottom.
Artist’s concept of Enceladus, the ocean moon of Saturn. Plumes of water vapor erupt from the South Pole. A new study suggests that the Enceladus’ ocean is salty like Earth’s oceans, but not too salty for life. Image via Science Photo Library/ Alamy/ New Scientist.

Bottom line: A new study shows that the subsurface ocean on Saturn’s moon Enceladus is less salty than oceans on Earth. This bolsters the chances of Enceladus’ ocean supporting some form of life.

Source: How does salinity shape ocean circulation and ice geometry on Enceladus and other icy satellites?

Via New Scientist

July 29, 2022

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