News – buried beneath 20 kilometers of ice, the underground ocean of Enceladus – one of Saturn’s moors – appears to be churning with streams similar to those on Earth.
The theory, derived from the shape of the ice shell of Enceladus, challenges the current notion that the lunar ocean is homogeneous, as well as a few direct mixtures guided by the warmth of the lunar heart.
Enceladus, a tiny frozen ball about 500 kilometers in diameter (about 1 / 7th the diameter of Earth’s moon), is Saturn’s sixth largest moon. Despite its small size, Enceladus caught the attention of scientists in 2014 when a flyby of the Cassini spacecraft found evidence of its subterranean ocean and sampled water from geyser-like explosions that occurs through cracks in the ice at the south pole. It is one of the few places in the solar system with melt water (another is Jupiter Europa ‘s moon), which makes it a target of interest for astronauts looking for signs of life.
The ocean on Enceladus is almost completely unlike Earth. The Earth’s ocean is relatively thin (an average depth of 3.6 km), covering three-quarters of the planet’s surface, warmer at the top from the sun’s rays and colder in the depths near the seabed, and there are wind-affected streams; Enceladus, meanwhile, appears to be a spherical and completely subterranean ocean at least 30 km deep and cooled at the summit near the ice shell. and warmed at the base by heat from the heart of the moon.
Despite their differences, Caltech graduate student Ana Lobo (MS ’17) suggests that ocean currents on Enceladus are similar to those on Earth. The work builds on measurements by Cassini as well as research by Andrew Thomson, a professor of environmental science and engineering, who has studied how ice and water interact to move ocean mixtures around Antarctica.
The oceans of Enceladus and the Earth have one important feature: they are salty. And as can be seen by conclusions published in Geology of nature on March 25, changes in salinity could be drivers of ocean circulation on Enceladus, just as they are in the South Earth Ocean, which surrounds Antarctica.
Lobo and Thompson collaborated on the work with Steven Vance and Saikiran Tharimena of JPL, which Caltech manages for NASA.
Gravity measurements and heat calculations from Cassini had already shown that the ice shell is thinner at the poles than at the equator. The poles appear to have sections of thin ice associated with melting and sections of thick ice at the equator with freezing, Thompson says. This affects ocean currents because when salt water freezes, it releases the salts and makes the surrounding water heavier, causing it to sink. The opposite occurs in regions of melting.
“Experiencing ice circulation allows us to place constraints on circulation patterns,” Lobo explains. A very suitable computer model, based on Thompson’s studies on Antarctica, suggests that the regions of freezing and melting, characterized by the ice structure, connected by ocean currents.This would create a pole-to-equatorial circulation that affects the circulation of heat and nutrients.
“Understanding what regions of the underground ocean could be so hospitable to life because we know it could one day inform efforts to find signs of life,” Thompson says.
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The title of the paper is “A pole-to-equatorial belt crossing the circulation of Enceladus.” This work was supported by the JPL Strategic Development and Technology program; the Icy Worlds node of NASA’s Institute of Astrobiology; and the David and Lucile Packard Foundation.