Space & Innovation

Europa's Deforming Ice Is a Surprising Heat Generator

Laboratory test on ice compression has revealed that Jupiter's icy moon's crust could generate a surprising amount of heat, opening a new window into understanding the moon's potentially life-giving ocean.

As excitement builds for NASA's Juno mission that will enter Jupiter orbit this summer, and future missions that will investigate ice moon Europa's life-supporting potential, new research into the strange qualities of the moon's cracked crust could reveal some fascinating science about Europa's sub-surface ocean.

Researchers of Brown University in Providence, R.I., have melded observations of Europa with computer models and laboratory experiments to reveal the tidal compression caused by Jupiter's hefty gravitational field could cause the moon's fragmented ice to generate more heat than thought, creating exciting new implications for the search for Europan life.

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Before NASA's Voyager and Pioneer flyby missions in the 1970s and then the Galileo mission in the 1990s, we had little clue about the dynamic nature of the Jovian satellites. "(Scientists) had expected to see cold, dead places, but right away they were blown away by their striking surfaces," said Christine McCarthy, of Columbia University who carried out research into Europa's ices while a graduate student at Brown 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."

It is now known that Europa possesses an extensive sub-surface ocean of water, protected by a fragmented, icy crust that appears to move much like the continental plates on Earth. Tidal pressures created during Europa's orbit around Jupiter create an internal dynamo, which gently heats the moon from the core, maintaining the ocean in a liquid state. In addition, the motion of the icy plates are thought to generate their own heat through frictional processes at the boundaries. Much like the heat produced when repeatedly bending a wire coat hanger, heat is dissipated through the repeated tidal flexing of Europa's crust at these boundaries.

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But the small-scale processes behind this tidal dissipation are poorly understood and may have been woefully underestimated.

"People have been using simple mechanical models to describe the ice," said McCarthy, "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."

To simulate what might be going on in Europa's crust, McCarthy headed a project to simulate the tidal pressures that would be felt by Europa's ice in the lab. Loading ice samples into a compression apparatus at Brown University, the amount of deformation and heating could be measured.

Until now, it has been assumed that the majority of heating comes from friction between individual ice grains, so that would suggest the frictional heating is directly related to the size of the grains. But on varying the ice grain size in her samples, McCarthy noticed no difference in heat flux. Instead, she realized that the bulk of the heating comes from microscopic defects in the ice's crystalline structure as the ice was deformed. The greater the deformation, the more heat is generated.

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"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," said collaborator Reid Cooper of 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."

In short, the realization that the microscopic structure of the ice is generating the heat and that heat generated is way more than can be produced by frictional heating alone, scientists can learn a lot more about the physics of Europa's icy crust and therefore open a new window into the chemistry of the liquid water ocean below.

As NASA plans what instrumentation its future "Europa Clipper" mission should carry to study one of the most fascinating worlds in the solar system, it's fundamental research such as this that could be used to better understand the habitable potential of Europa's mysterious ocean.

This research is published in the June 1 edition of the journal Earth and Planetary Science Letters.

Source: Brown University

This past week

, scientists found extensive evidence of a magma ocean underneath the surface of Io, a moon of Jupiter. It now looks like all four moons discovered by Galileo in the 1600s -- Io, Europa, Callisto and Ganymede -- could have oceans of some kind, magma or liquid water. Although they may have dramatically different qualities -- from the hellish lava flows of Io to the potentially life-giving sub-surface ocean of Europa -- these oceans are all driven by tidal interactions with Jupiter. While NASA studied the Jovian moons in detail in the 1990s and early 2000s with the Galileo mission, scientists have been anxious for another visit to Jupiter that would be longer than just a flyby.

NASA's Juno spacecraft

may get distant glimpses of the moons after it arrives in 2016. For a close-up view, space fans will need to wait for a

European mission called JUICE (JUpiter ICy moons Explorer)

, which should launch in 2022 and arrive in 2030.

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Starting with the most extreme, Io is a volcanic planet, but what has intrigued scientists for decades is the volcanoes appear to be offset from where computer models predict. A newly released study suggests that a magma ocean -- a great conductor of heat --

could explain why the hotspots are in a different location than expected

. This follows on from research in 2011 from the Galileo spacecraft, showing that there likely is a magma ocean under the moon.

Europa's surface is covered with cracks, something the Voyager spacecraft noted in the 1970s and that the Galileo spacecraft looked at up close in the 1990s and 2000s. It turns out these cracks are not in spots where they are expected to be, based on how Europa's crust flexes as it moves around Jupiter. But if there was an ocean underneath, this would explain them. Also, the surface of Europa is young, indicating something is resurfacing it. In 2013, scientists spotted a watery plume erupting from Europa but

have not been able to view another one since

. It's unclear whether the ocean is liquid water and if it has the potential to support life, although films like The Europa Report are already tackling the subject.

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Earlier this year, the Hubble Space Telescope carried out observations of Ganymede. The moon has its own magnetic field and is also affected by Jupiter's magnetic environment. By looking at how two types of aurora from these magnetic fields move on the planet,

the scientists concluded that it must have some kind of ocean underneath

-- likely saltwater and, like Europa, potentially habitable for basic life forms.

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In 1998, scientists found evidence of an ocean under Callisto after being pointed there by observations of Europa. When the Galileo spacecraft found evidence of electrical currents being conducted by a liquid ocean at Europa,

scientists looked for the same phenomenon

at Callisto and struck gold -- they found varying electrical currents that could be explained by ocean activity. Callisto, however, is further from Jupiter and does not get the tidal energy Europa gets. That tidal energy may make life more favorable on Europa.

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There are other bodies in the solar system that could have oceans, most prominently Enceladus (a moon of Saturn). We've seen plumes at Enceladus and there has been talk of a small ocean on the moon, but now there's something even more intriguing: a global ocean. Earlier this month, observations from the Cassini spacecraft showed the moon's orbit around Saturn wobbles in a way consistent with an ocean under its ice shell. Another strong possibility is Titan (a moon of Saturn), which is already known to host vast lakes of liquid methane and ethane.

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