As Europa — the sixth-closest moon of Jupiter and the smallest of its four Galilean satellites — orbits, its icy surface heaves and falls with the pull of Jupiter’s gravity, creating enough heat to support a subsurface ocean. Now, a new study suggests that this process, known as tidal dissipation, could create far more heat in the moon’s ice than planetary researchers had previously assumed.
The surface of Europa looms large in this color view; image scale is 1.6 km per pixel; north on Europa is at right. Image credit: NASA / JPL-Caltech / SETI Institute.
Europa was first discovered by Galileo Galilei in 1610. It was first examined by NASA’s Voyager mission in 1979 and was first seen in detail by NASA’s aptly-named Galileo orbiter in the 1990s.
“Scientists had expected to see cold, dead places, but right away they were blown away by their striking surfaces,” said lead author Christine McCarthy of Columbia University.
“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.”
The only way to create enough heat for these active processes so far from the Sun is through tidal dissipation.
“The effect is a bit like what happens when someone repeatedly bends a metal coat hanger,” McCarthy said.
However, the details of the process in ice aren’t very well understood, and modeling studies that try to capture those dynamics on Europa had yielded some puzzling results.
“People have been using simple mechanical models to describe the ice. While those calculations suggested liquid water under Europa’s surface, they weren’t getting the kinds of heat fluxes that would create these tectonics,” McCarthy said.
“So we ran some experiments to try to understand this process better.”
In deformation experiments, McCarthy and her colleague, Prof. Reid Cooper of Brown University, loaded ice samples into a compression apparatus.
They 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, scientists could infer how much heat is generated.
Modeling 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 the team found similar results even when they substantially altered the grain size in the 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 create more heat than would be expected from the grain boundaries.
“We 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,” Prof. Cooper said.
The findings of this study, published online March 25 in the journal Earth and Planetary Science Letters, could help planetary researchers to better estimate the thickness of Europa’s outer shell.
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Christine McCarthy & Reid F. Cooper. 2016. Tidal dissipation in creeping ice and the thermal evolution of Europa. Earth and Planetary Science Letters, vol. 443, pp. 185-194; doi: 10.1016/j.epsl.2016.03.006
