NASA’s Juno spacecraft has revealed that Jupiter’s moon Europa features an average of 29 kilometres of solid ice above its global subsurface ocean. While the icy shell is thicker than three Mount Everests combined, new computer simulations show that turbulent heat loss in deep ice fractures likely blocks ocean water from reaching the surface.
Jupiter’s moon Europa is smaller than Earth’s Moon, yet it hides a vast global ocean beneath its frozen exterior that may hold more than twice as much water as all of Earth’s oceans combined. Despite being located hundreds of millions of miles from the Sun, where surface temperatures plummet between -256 Fahrenheit (-160 Celsius) and -364 Fahrenheit (-220 Celsius), Europa stays warm enough internally to keep its ocean liquid. Gravitational tidal heating from Jupiter constantly stretches and squeezes the moon, generating enough internal energy to sustain the deep sea and warm the rocky ocean floor.
Juno’s Microwave Radiometer Measures a 29-Kilometre Ice Shell
New data collected by NASA’s Juno spacecraft during a close flyby on 29 September 2022 provides a clearer picture of that frozen crust. Passing within about 360 kilometres of Europa, Juno’s Microwave Radiometer (MWR) collected measurements across roughly half of the moon’s surface. The instrument, originally built to peer beneath Jupiter’s clouds, measured microwave emissions at six frequencies to estimate temperatures at different depths within the ice.
The analysis, led by Steve Levin and published in Nature Astronomy, produced a central estimate of an average 29 kilometres of cold, conductive ice in the region observed. To put that vertical barrier into perspective, three Mount Everests stacked end to end would reach about 26.55 kilometres, leaving the central estimate for Europa’s ice stretching another 2.45 kilometres beyond that imaginary summit.
Researchers noted important limitations in the model. The result carries a statistical uncertainty of plus or minus 10 kilometres, accounting for possible unmodelled changes across Europa’s surface. Furthermore, the model assumes pure water ice and excludes a warmer convective layer underneath the rigid crust. Dissolved salt can also make ice more opaque to microwaves, which could reduce the thickness estimate by about 5 kilometres, while a convective sub-layer could increase the overall solid barrier.
For years, planetary scientists hoped that narrow fractures known as dikes might allow deep ocean water to rise via cryovolcanism and collect in shallow reservoirs near the surface. These shallow pockets would give future space missions an accessible target to sample deep-ocean chemistry without drilling through tens of kilometres of ice.
Photo: Nature
However, new computational modelling led by planetary scientist Lujendra Ojha at Rutgers University indicates that direct fluid exchange between the deep ocean and the shallow subsurface is highly unlikely. Instead of moving upward in an orderly laminar flow, rising water through deep cracks experiences rapid, turbulent mixing.
“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, That’s really what we think we disproved.”
Photo: Open Access Government
Lujendra Ojha, planetary scientist at Rutgers University, via Open Access Government
As water rushes through the fractures, turbulent mixing forces it to churn constantly against the freezing walls of the ice channel, rapidly dissipating its internal heat. The water cools below its freezing point and becomes supercooled, forming microscopic ice crystals called frazil ice. These ice crystals accumulate quickly and freeze narrow fractures completely shut within hours, effectively blocking the pathway.
Any shallow pockets of liquid water discovered in Europa’s crust are therefore more likely formed by localized internal friction and melting within the ice shell itself rather than direct feeds from the deep ocean below. While localized melt pockets remain scientifically interesting, they may lack the organic chemistry and potential biosignatures trapped in the deep ocean environment.
Upcoming Flagship Missions Prepare to Explore Europa
These findings provide critical context for two flagship missions currently traveling toward the Jupiter system.
