Over the past four billion years, Jupiter’s ice-covered moon Europa has been stretched and squeezed like taffy. For any potential life, that’s a good thing.

The kneading of Europa is caused by gravity, and is known as tidal heating. Tugs from the giant parent planet, with help from the moons Io and Ganymede, heat up Europa’s interior.

“Just as with a rubber ball, squishing and pulling it, the ball will get warm,” says Cynthia Phillips, project staff scientist for the Europa Clipper mission at NASA’s Jet Propulsion Laboratory in Southern California.

Tidal heating has likely allowed a vast ocean beneath Europa’s icy shell to remain liquid for the whole history of our solar system. That means the same forces that set the stage for the development of life on Earth – a steady heat source, liquid water and plenty of life-friendly chemistry – also could have existed on Europa for the same amount of time.

“Chemicals plus heat ­– that energy can serve as fuel for biospheres,” Phillips said. “If tidal heating is in place in the rock layer on Europa, it’s possible that hydrothermal systems exist there, too.”

Time to find out. In 2024, Europa Clipper will be launched to Jupiter, packed with a suite of sensitive instruments. Once it reaches orbit around our system’s largest planet, Europa Clipper will begin a series of flybys past Europa, turning its array of cameras and detectors on the ice-covered moon.

In a sense, revealing the ins and outs of tidal heating – and how it affects Europa’s ice shell, its rocky seafloor, and the ocean sandwiched in between – is Europa Clipper’s main mission. These details will tell us whether or not Europa might have habitats suitable for life.

“Without tidal heating, we wouldn’t be sending Europa Clipper,” Phillips said.

This color image of Europa was acquired by Voyager 2 during its close encounter on July 9, 1979. The complex array of streaks indicate that the crust has been fractured and filled by materials from the interior. The lack of relief, any visible mountains or craters, is consistent with a thick ice crust. In contrast to its icy neighbors, Ganymede and Callisto, Europa has very few impact craters. The relative absence of features and low topography suggests the crust is young and warm a few kilometers below the surface. The tidal heating process suggested for Io may be heating Europa's interior at a lower rate.
This color image of Europa was acquired by Voyager 2 during its close encounter on July 9, 1979. The complex array of streaks indicate that the crust has been fractured and filled by materials from the interior. The lack of relief, any visible mountains or craters, is consistent with a thick ice crust. In contrast to its icy neighbors, Ganymede and Callisto, Europa has very few impact craters. The relative absence of features and low topography suggests the crust is young and warm a few kilometers below the surface. The tidal heating process suggested for Io may be heating Europa's interior at a lower rate.

Scientists began speculating that the tidal heating process might be warming Europa’s interior even before Voyager 2’s close encounter with the moon in 1979. The images received from Voyager 2 showed that Europa, in contrast to its icy neighbors Ganymede and Callisto, has few impact craters. The relative absence of features and low topography suggested a crust that is relatively young and warm.

While scientists largely agree that this "push-and-pull" heating is probably happening on Europa, how exactly that works could be critical to determining whether life exists there today.

“Where is the vast majority of tidal heating?” Phillips asks. “Is it mostly in the surface ice layer, or the ice-water interface? Is it primarily at the top of the ocean, or mostly at the bottom of the ocean, in the rock layer?”

The answer will have to come from Europa Clipper. Computer models, or mathematically based simulations, are increasingly powerful ways to unlock the inner workings of planets and moons. In this case, however, data collected so far is not yet nuanced enough to properly inform the models.

“Current models can’t prove which one is right,” Phillips said. And in the search for life beyond Earth, the answer isn’t trivial.

If the heating is happening in the rock layer on Europa’s ocean floor, it could give rise to deep-sea hydrothermal vents. We have hydrothermal vents on Earth at the bottom of our own ocean; scientists have proposed that they might well have been the chemical cauldrons that, billions of years ago, yielded some of the first experiments in the origin of life on our planet.

Such vents, typically in mid-ocean ridges – the seams between continental plates – remain active sites for thriving ecosystems.

“There is abundant life at the bottom of Earth’s oceans, almost independent of sunlight,” Phillips said. “There’s enough heat and interesting chemistry going on, with chemical gradients as sea-water circulates through hot rock and then is ejected back out at high temperatures.”

Research and computer modeling show that volcanic activity may have occurred on the seafloor of Europa in the recent past – and may still be happening. Computer modeling shows how internal heat produced by tides – warping Europa's shape as it changes distance from Jupiter during its orbit – could partially melt its rocky layer, a process that could feed volcanoes on the ocean floor.
Research and computer modeling show that volcanic activity may have occurred on the seafloor of Europa in the recent past – and may still be happening. Computer modeling shows how internal heat produced by tides – warping Europa's shape as it changes distance from Jupiter during its orbit – could partially melt its rocky layer, a process that could feed volcanoes on the ocean floor.

Something similar might have happened on Europa, whose ocean is completely cut off from the Sun by its thick, icy shell – even from the comparatively feeble sunlight that hits its surface. Like some deep-sea organisms on Earth, any Europan lifeforms would be completely dependent on chemical or other sources of energy, not on sunlight-driven photosynthesis.

“It might be a story in which, either through tidal heating or through intense internal heating during Europa’s formation, volcanic activity occurred at some point,” said Steve Vance, an astrobiologist and geophysicist at JPL who studies ocean worlds and creates computer models of their interior.

If so, then “high-temperature hydrothermalism had a role in the evolution of the ocean, potentially supporting the sustenance or origin of life on Europa,” he said.

Modeling to which Vance contributed showed that the primary heating locations would be in the high-latitude, polar regions of Europa, where volcanic activity still might be going on.

“We don’t know how much Europa is flexing, or how much energy in the Jupiter-Europa satellite system is being dissipated as tidal heating,” he said. “We need a lot more information about Europa, what it’s made of, and its interior structure.”

To that end, Vance will be especially interested in data from Europa Clipper’s geophysical instruments, which will remotely measure structural characteristics of the moon. Radar combined with gravitational measurements, for example, could tell us things like how warm the interior is, if are there seamounts (underwater mountains), how thick the ice is, and how squishy it is, he said.

Europa Clipper’s whole suite of instruments will work together to capture a full picture of the effects of tidal heating, Phillips said. That includes the Europa Thermal Emission Imaging System (E-THEMIS).

“The E-THEMIS thermal imager will be able to look for hot spots, to study surface and near-surface properties at thermal wavelengths,” she said. “If there are any places where the ice layer is thinner, or where there is liquid water within the icy crust, it’s possible that the E-THEMIS instrument will be able to detect those places.”

The Europa Thermal Emission Imaging System (E-THEMIS) uses infrared light to distinguish warmer regions on Europa where warm liquid water may be near the surface or might have erupted onto the surface. It will also measure surface texture to understand the small-scale properties of the surface.
The Europa Thermal Emission Imaging System (E-THEMIS) uses infrared light to distinguish warmer regions on Europa where warm liquid water may be near the surface or might have erupted onto the surface. It will also measure surface texture to understand the small-scale properties of the surface.

A frozen-over ocean world with hot spots below is probably more likely to be habitable for microorganisms than for large, multi-cellular creatures – things like whales, giant squid, or fish – Phillips said. But even that would be extraordinary.

“If we found single-celled organisms on Europa, it would be mind-blowing – Earth-shattering – in terms of importance,” she said. “It would be a second origin of life. In our dinky little, undistinguished solar system, if life started not once but twice, that’s revolutionary. It means life is abundant – life is everywhere. It gives me chills.”

By Pat Brennan

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