As the moon Europa orbits Jupiter, its icy surface constantly heaves and falls with the pull of gravity. This process, known as tidal dissipation, is thought to generate enough heat to support a global ocean beneath the moon's solid shell. But now researchers think these strong forces may produce even more heat than previously estimated.
First discovered by Galileo in the early 1600s, the largest Jovian moons - Io, Europa, Ganymede and Callisto - were thought to be nothing more than barren celestial bodies. But NASA expeditions to Jupiter in the 1970s and 1990s revealed the moons are full of surprises.
The recent study, led by geoscientists from Brown and Columbia universities, provides new insight that may ultimately help scientists better estimate the thickness of Europa's icy outer shell.
"[Scientists] had expected to see cold, dead places, but right away they were blown away by their striking surfaces," said Christine McCarthy, a faculty member at Columbia University who led this new research as a graduate student at Brown. "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."
As Jupiter's sixth-closest moon, Europa sits pretty far away from the sun. So, the only way to generate enough heat for such active processes is through tidal dissipation. The effect of such pushing and pulling the moon's icy shell is similar to what happens when repeatedly bending a metal coat hanger back and forth.
"If you bend it back and forth, you can feel it making heat at the junction," McCarthy added. "The way it does that is that internal defects within that metal are rubbing past each other, and it's a similar process to how energy would be dissipated in ice."
While previous models have tried to capture these dynamics on Europa, the exact details of the processes are largely unknown.
"People have been using simple mechanical models to describe the ice," McCarthy said. While those calculations indicate liquid water lies under Europa's surface, "they weren't getting the kinds of heat fluxes that would create these tectonics. So we ran some experiments to try to understand this process better."
Ice samples loaded into a compression apparatus were subjected to cyclical loads similar to those acting on Europa's icy shell. When the loads are applied and released, the ice deforms and then rebounds to a certain extent. Researchers were then able to calculate how much heat is generated, by measuring the lag time between the application of stress and the deformation of the ice.
Previous models demonstrated most of the heat generated by these processes comes from friction at the boundaries between ice grains. This suggests the size of the grains influences the amount of heat generated.
However, McCarthy's models yielded similar results even when she substantially altered the grain size in her samples, indicating grain boundaries are not the primary heat generators in the active processes.
Instead, the recent findings suggest most of the heat comes from defects that form in the ice's crystalline lattice as a result of deformation. Surprisingly, researchers found these defects create much more heat than would normally 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," explained Reid Cooper, co-author and professor of Earth, environmental and planetary sciences at Brown University. "The beauty of this is that once we get the physics right, it becomes wonderfully extrapolative. Those physics are first order in understanding the thickness of Europa's shell. In turn, the thickness of the shell relative to the bulk chemistry of the moon is important in understanding the chemistry of that ocean. And if you're looking for life, then the chemistry of the ocean is a big deal."
Researchers believe their new models will help future studies unravel the mysteries of Europa's hidden ocean.
Their findings were recently published in the journal Earth and Planetary Science Letters.
