Searching for life on other celestial bodies, or at the very least the necessary components to support it, has been fascinating scientists and enthusiasts for centuries. While planets are the obvious choice, their moons can also harbor the chemical ingredients for life.
Saturn is orbited by 146 moons, with Enceladus being the sixth largest at approximately 500km in diameter. This small, icy moon is characterized by its highly reflective white surface and geyser-like jets releasing ice and water vapor hundreds of kilometers into space from its south pole.
NASA’s Cassini spacecraft identified these jets in 2005, before going on to sample them in 2008, 2009 and 2015. Consequently, scientists found that the hot mineral-rich waters possess the necessary components for life, despite the moon’s surface reaching extreme temperatures of -201°C.
A large subsurface ocean, spanning approximately 20 million km3, has commonly been thought to be the primary source of Enceladus’s geysers, erupting through fractures in the crust. This is due to the saltiness of the sampled material and the cyclicity of the plumes matching the moon’s orbit around Saturn, with associated heating and cooling.
However, new modeling research by Professor Colin Meyer, of Dartmouth College, U.S., and colleagues, published in Geophysical Research Letters, has offered support for an alternative explanation.
“The Cassini spacecraft flew through one of Enceladus’s plumes and measured organics, a possible sign of life, making these geysers unique and important to astrobiology. This plume material emanates from a potentially habitable ocean below the ice shell, so we want to understand how the geysers form to determine if Enceladus is habitable,” Professor Meyer explains of the research’s significance.
“We feel that it is important to fully explore an alternative to the dominant explanations for the geysers because it will allow us to either strengthen the ocean source hypothesis or determine what data are required to distinguish between the two mechanisms.
“The two primary weaknesses for the ocean source, in my opinion, are 1) the difficulty for the fracture to break through the entire shell and 2) the mechanism by which ocean material makes it through the fractures.”
Instead of the subsurface ocean hypothesis, the researchers suggest that shear heating occurs, whereby heat is produced due to the friction of layers within a material moving at different speeds. This occurs due to tidal forcing, Professor Meyer states.
“Tidal forces from Saturn pull on the shell of Enceladus as the moon orbits the planet, just as the sun and moon cause ocean tides on Earth. Variation in the tidal pull laterally across the ice shell induces stress that results in ice deformation.
“The two sides of the south pole cracks are not coupled and there can be a difference in how they deform. The slip difference across the crack is akin to an earthquake and causes the two sides of the crack to rub together and generate heat.”
This shear heating can warm the moon’s ice above the eutectic temperature, the lowest temperature at which liquid brine is stable. When areas of the shell rise above the eutectic temperature, the salts are dissolved within the liquid brine, which fills up the spaces between ice crystals.
For Enceladus, this could take place in fractures within the salty ice shell—which scientists affectionately call tiger stripes—generating a “mushy zone” reservoir comprised of ice and liquid brine. The Cassini samples identified water vapor, carbon dioxide, methane, ammonia, carbon monoxide, nitrogen, salts and silica within the geysers.
This liquid brine results from salts within the icy shell reducing the temperature at which the shell melts, causing localized partial melting and subsequent escape from the surface as geysers. Professor Meyer’s simulations suggest 300kg of ice and vapor could be expelled via the plume every second. This mechanism relies upon there being sustained ice melt rates and sufficient volumes of liquid brine.
While Enceladus’s icy shell may be up to 25km thick globally, it could be just 6km thick over the south pole, making melting more likely. When fractures are shallow, the mushy zone is mostly absent, but as the fracture depth increases, the mushy zone can reach all the way through the icy shell.
“In the latter case, the fractures become an important conduit for life,” Professor Meyer says, as the “ocean-to-surface exchange could allow for materials that form the building blocks of life to make their way to the surface, therefore chemically increasing the moon’s potential habitability.”
Beyond Enceladus, this research also aids our understanding of the geophysical processes of other icy moons within our solar system—such as Neptune’s Triton, Saturn’s Titan and Jupiter’s Europa—and their potential to harbor life. https://phys.org/news/2025-02-geysers-saturn-icy-moon-enceladus.html
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