Saturn’s moon Enceladus may have weak interior

Chuck Bednar for – @BednarChuck

Geysers of ice and water vapor emanating from the surface of Enceladus experience an unusual delay that could indicate that the sixth-largest moon of Saturn lacks a strong interior, researchers claim in a new study published this week in the journal Nature Geoscience.

According to, scientists believe that beneath the surface of this icy moon, there is an ocean of liquid water that could potentially harbor life. By studying its geysers, they hope to find out more detail about what the subterranean surface of Enceladus is really like.

In their study, a team of researchers from the US, France, and the Czech Republic explained that “eruptions of water vapor and ice emanate from warm tectonic ridges” at the moon’s south pole, and that observations in the visible and infrared spectra have revealed “an orbital modulation of the plume brightness,” suggesting that the eruptions are “influenced by tidal forces.”

However, that activity appeared to be experiencing a delay of several hours compared to what simple tidal models predicted. This prompted the authors to “simulate the viscoelastic tidal response of Enceladus with a full three-dimensional numerical model and show that the delay in eruption activity may be a natural consequence of the viscosity structure in the south-polar region and the size of the putative subsurface ocean.”

What lies beneath the surface

They compared plume brightness data to simulations of varying normal stress levels along faults, and found that the activity was reproduced in two different interior models: one involving a low-viscosity convective region above a polar sea along the south pole at depths as little as 30km, and one involving a 60km to 70km thick convecting ice shell resting above a global ocean.

“Previous predictions were too simple, in that they ignored important details of the structure of Enceladus,” said Francis Nimmo, a planetary scientist at the University of California, Santa Cruz and co-author of the new study. “Enceladus experiences tides from Saturn, which provide a force on the ice shell. The ice shell flows in response to these forces, and because the flow is quite slow, the response is delayed by several hours.”

The research centers around the viscosity of fluids on the moon – its thickness, or the degree to which it resists flow. For instance, water is a relatively low-viscosity fluid while honey is a relatively high-viscosity one. Nimmo’s team found that, while a strong, high-viscosity ice shell would instantly react to tidal forces, a weaker, low-viscosity one would react more gradually, which could be explained by either of their two models.

“The timing of geyser activity gives us an insight into the interior of a rather complicated planetary body,” lead author Marie Běhounková, a planetary scientist at Charles University in Prague, told However, more data from the Cassini spacecraft is required before the researchers could try to determine which is the more likely of those models.


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