A novel theory proposed by planetary scientists from Caltech and NASA’s Jet Propulsion Laboratory challenges the current thinking that the saltwater global ocean of Enceladus, the sixth largest moon of Saturn, is homogenous.
Enceladus’ tiger stripes are known to be spewing ice from the moon’s icy interior into space, creating a cloud of fine ice particles over the moon’s south pole and creating Saturn’s mysterious E-ring. Evidence for this has come from NASA’s Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown from a close flyby. Tiger stripes are visible in false-color blue. Image credit: NASA / ESA / JPL / SSI / Cassini Imaging Team.
In 2014, NASA’s Cassini spacecraft discovered evidence of a large subsurface ocean on Enceladus and sampled water from geyser-like eruptions that occur through fissures in the ice at the south pole.
It is one of the few locations in the Solar System with liquid water, making it a target of interest for astrobiologists searching for signs of life.
The ocean on Enceladus is almost entirely unlike Earth’s.
Earth’s ocean is relatively shallow, covers three-quarters of the planet’s surface, is warmer at the top from the Sun’s rays and colder in the depths near the seafloor, and has currents that are affected by wind.
Enceladus, meanwhile, appears to have a globe-spanning and completely subsurface ocean that is at least 30 km deep and is cooled at the top near the ice shell and warmed at the bottom by heat from the moon’s core.
Despite their differences, Caltech graduate student Ana Lobo and colleagues suggest that the Enceladus’ ocean has currents akin to those on Earth.
The oceans of Enceladus and Earth share one important characteristic: they are salty. And as shown by the team, variations in salinity could serve as drivers of the ocean circulation on Enceladus, much as they do in Earth’s Southern Ocean.
This artist’s rendering showing a cutaway view into the interior of Enceladus. A plume of ice particles, water vapor and organic molecules sprays from fractures in the moon’s south polar region. Image credit: NASA / JPL-Caltech.
“Gravitational measurements and heat calculations from Cassini had already revealed that the ice shell is thinner at the poles than at the equator,” said Professor Andrew Thompson, a researcher in the Division of Geological and Planetary Sciences at Caltech.
“Regions of thin ice at the poles are likely associated with melting and regions of thick ice at the equator with freezing.”
This affects the ocean currents because when salty water freezes, it releases the salts and makes the surrounding water heavier, causing it to sink. The opposite happens in regions of melt.
“Knowing the distribution of ice allows us to place constraints on circulation patterns,” Lobo said.
The team’s computer model suggests that the regions of freezing and melting, identified by the ice structure, would be connected by the ocean currents.
This would create a pole-to-equator circulation that influences the distribution of heat and nutrients.
“Understanding which regions of the subsurface ocean might be the most hospitable to life as we know it could one day inform efforts to search for signs of life,” Professor Thompson said.
The study was published in the journal Nature Geoscience.
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A.H. Lobo et al. A pole-to-equator ocean overturning circulation on Enceladus. Nat. Geosci, published online March 25, 2021; doi: 10.1038/s41561-021-00706-3
