Europa is under a constant gravitational assault. As it orbits, Europa's icy surface heaves and falls with the pull of Jupiter's gravity, creating enough heat, to support a ocean beneath the moon's solid shell, researchers said.

Experiments by geoscientists from Brown and Columbia universities suggest that this process, called tidal dissipation, could create far more heat in Europa's ice than scientists had previously assumed.

The work could ultimately help researchers to better estimate the thickness of moon's outer shell.

"There was clearly some sort of tectonic activity - things moving around and cracking. There were also places on Europa that look like melt-through or mushy ice," said Christine McCarthy, a faculty member at Columbia University who led the research as a graduate student at Brown.

The only way to create enough heat for these active processes so far from the Sun is through tidal dissipation.

Working with Reid Cooper, professor at Brown, McCarthy loaded ice samples into a compression apparatus. She subjected the samples to cyclical loads similar to those acting on Europa's ice shell.

When the loads are applied and released, the ice deforms and then rebounds to a certain extent. By measuring the lag time between the application of stress and the deformation of the ice, McCarthy could infer how much heat is generated.

The experiments yielded surprising results. Modelling approaches had assumed that most of the heat generated by the process comes from friction at the boundaries between the ice grains.

That would mean that the size of the grains influences the amount of heat generated. But McCarthy found similar results even when she substantially altered the grain size in her samples, suggesting that grain boundaries are not the primary heat-generators in the process.

The work suggests that most of the heat actually comes from defects that form in the ice's crystalline lattice as a result of deformation. Those defects, the research showed, create more heat than would be expected from the grain boundaries.

"Christine discovered that, relative to the models the community has been using, ice appears to be an order of magnitude more dissipative than people had thought," Cooper said.

The research was published in the journal Earth and Planetary Science Letters.

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