Is friction heating Saturn’s icy moon?

The Cassini mission revealed Saturn's moon Enceladus to be a hive of hydrothermal activity, but what is generating the activity? The answer may be found deep in the moon's core.

This image captured by the Cassini spacecraft shows plumes of water ice erupting from the surface of Enceladus. Do similar plumes exist on Jupiter’s moon Europa? Credit: NASA/JPL-Caltech/SSI

Heat generated by friction inside Saturn’s icy moon Enceladus may be driving its hydrothermal activity, and could do so for billions of years, according to a study using data from NASA’s Cassini mission.

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The Cassini mission ended in September 2017, but scientists continue to analyse its data to learn more about Saturn and its moons.

Cassini revealed plumes of water vapour and icy particles spray out from beneath the surface of Enceladus, and that the moon has a global ocean beneath its icy crust.

Evidence pieced together from Cassini data suggests hydrothermal activity – hot water chemically interacting with rock – is taking place on Enceladus’s sea floor.

A digram showing the inner workings of Enceladus, and how its hydrothermal activity may be generated. Copyright Surface: NASA/JPL-Caltech/Space Science Institute; interior: LPG-CNRS/U. Nantes/U. Angers. Graphic composition: ESA
A digram showing the inner workings of Enceladus, and how its hydrothermal activity may be generated.
Copyright Surface: NASA/JPL-Caltech/Space Science Institute; interior: LPG-CNRS/U. Nantes/U. Angers. Graphic composition: ESA

One clue is the detection of tiny grains of rock, which scientists believe could be the product of hydrothermal activity occurring at temperatures of about 90°C.

The amount of energy required to produce such high temperature could not be provided by the decay of radioactive elements, ruling this possibility out.

Gaël Choblet from the University of Nantes in France worked within a team that calculated a loose, rocky core in Enceladus could generate the required energy.

Their study found that as Enceladus orbits Saturn, rocks in the moon’s porous core could rub together, generating heat.

Enceladus’s cratered crust, as seen by the Cassini spacecraft. What lies below, in the moon’s subsurface ocean?Credit: NASA/JPL-Caltech/Space Science Institute
Enceladus’s cratered crust, as seen by the Cassini spacecraft. What lies below, in the moon’s subsurface ocean?
Credit: NASA/JPL-Caltech/Space Science Institute

The loose interior of the core would also allow water from the ocean to penetrate deep down, before heating up and then rising to react with the rocks.

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Plumes of water, now warm and containing minerals, hurtle upward, thinning the moon’s icy shell and bursting through fractures in the ice.