NASA’s Juno spacecraft has measured an average 29-kilometre-thick solid ice shell on Jupiter’s moon Europa, according to a study published in Nature Astronomy. This thick conductive barrier challenges existing models of cryovolcanism and complicates theories about how deep ocean water might reach the surface.
Jupiter’s moon Europa remains a prime target in the search for extraterrestrial habitability because a vast liquid ocean is trapped beneath its frozen outer shell, sustained by gravitational tidal heating from Jupiter. While the moon is smaller than Earth’s Moon, scientists calculate that its hidden global sea could hold more than twice the water of all Earth’s oceans combined.
Juno’s Microwave Radiometer Measurement of Europa’s Ice Shell
The new thickness measurement comes from data collected by NASA’s Juno spacecraft during a close flyby on 29 September 2022, when the probe passed within about 360 kilometres of the moon. Although Juno was built primarily to investigate Jupiter, its Microwave Radiometer (MWR) collected measurements across roughly half of Europa’s surface. An analysis led by Steve Levin and published in Nature Astronomy relied on 129 measurements in each of six frequency channels to estimate temperatures at varying depths beneath the crust.
Rather than sending a physical pulse through the ice, the instrument measured microwave radiation emitted by the ice itself, alongside incoming radio emissions from Jupiter’s radiation belts and the Galaxy that reflected off Europa. Because lower frequencies carry information from deeper layers, the team could model the vertical temperature gradient. The best-fitting model produced an average conductive shell thickness of 29 kilometres. To put that figure in perspective, the officially adopted height of Mount Everest is 8.84886 kilometres, meaning three stacked Everests would reach about 26.55 kilometres—leaving the central estimate for Europa’s ice shell stretching another 2.45 kilometres beyond that imaginary summit.
The researchers reported the measurement as 29 plus or minus 10 kilometres, accounting for potential unmodelled changes across the surface.
Turbulent Mixing and Frazil Ice Block Cryovolcanic Pathways
For years, scientists hoped that narrow fractures known as dikes might allow deep ocean water to rise via cryovolcanism, collecting in shallow pockets near the surface that would be easier for space probes to detect (Open Access Government). However, new computational modelling led by planetary scientist Lujendra Ojha at Rutgers University indicates that this direct fluid exchange is far less likely than previously assumed (Open Access Government).
Earlier models assumed that water moved upward through ice fractures in an orderly, laminar flow. Instead, simulations run by the Rutgers team revealed that fluid movement through these deep cracks is highly turbulent. Turbulent mixing forces rising water to churn constantly against the freezing walls of the ice fracture, rapidly dissipating its internal heat. As the water cools below its freezing point, it becomes supercooled and forms microscopic ice crystals called frazil ice, which quickly accumulate and block the pathway (Open Access Government).
“There’s all this speculation about how that water can come from deep underground and make its way all the way up without freezing en route,” said Ojha. “That’s really what we think we disproved.”
Lujendra Ojha, planetary scientist at Rutgers University, via Open Access Government
The simulations demonstrated that narrow fractures freeze completely shut within hours (Open Access Government). Consequently, if shallow pockets of liquid water do exist within Europa’s crust, researchers suggest they are likely generated by localized internal friction and melting within the ice shell itself rather than direct upwelling from the deep ocean (Open Access Government). While these localized melt pockets remain scientifically interesting, they might not contain the organic chemistry or biosignatures present in the deep ocean environment (Open Access Government).
Implications for NASA Europa Clipper and ESA JUICE Missions
These findings arrive as two flagship planetary missions travel toward the Jovian system to study the moon in detail (Open Access Government). NASA’s Europa Clipper, launched in October 2024, is scheduled to arrive at Jupiter in April 2030 to execute 49 close flybys of Europa (Open Access Government).
Meanwhile, the European Space Agency’s JUICE (Jupiter Icy Moons Explorer) probe launched in April 2023 and is due to arrive at Jupiter in July 2031 to study Europa alongside Ganymede and Callisto (Open Access Government). By demonstrating that Europa’s ice shell acts as a rigid barrier against direct fluid exchange, the new measurements and hydrodynamic models provide mission scientists with critical context to interpret the ice-penetrating radar and compositional data that Clipper and JUICE will collect during their upcoming flybys (Open Access Government).
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