Europa is smaller than Earth’s Moon, yet the ocean hidden inside it may hold more than twice as much water as every ocean on Earth combined.
That contrast has always been the part of Europa I find hardest to picture. A world only 3,120 kilometres across may contain a global sea roughly 60 to 150 kilometres deep, wrapped around rock and sealed from space.
Now add the new measurement from NASA’s Juno mission: an average 29 kilometres of cold, conductive ice in the region its microwave instrument observed.
Using the officially adopted height of Mount Everest, 8.84886 kilometres, three Everests would reach about 26.55 kilometres. The central estimate for Europa’s ice would continue another 2.45 kilometres beyond their imaginary summit.
It is a startling comparison, but it needs immediate qualification. The result is 29 plus or minus 10 kilometres, it applies to the sampled region, and it comes from a model with specific assumptions about the ice.
This is one measurement, not a final global map of Europa’s shell.
Even with those limits, the finding moves an old debate towards the thick-ice side. It also sharpens the question that matters most for Europa’s possible habitability: not simply whether a great ocean exists, but whether useful chemistry can travel between that ocean, the rock below and the irradiated surface far above.
Juno measured heat coming through the ice
Juno was built primarily to investigate Jupiter, not to map Europa’s interior. Its Microwave Radiometer, or MWR, was designed to look beneath Jupiter’s clouds by measuring microwave emission at six frequencies.
On 29 September 2022, the spacecraft passed within about 360 kilometres of Europa. During that flyby, MWR collected measurements across roughly half of the moon’s surface.
The resulting analysis, led by Steve Levin and published in Nature Astronomy, used 129 measurements in each frequency channel to estimate temperatures at different depths.
That last sentence can sound more direct than the method really was. Juno did not send a radar pulse through 29 kilometres of ice, detect the ocean and measure the distance with a stopwatch.
Ice emits microwave radiation. Lower frequencies can carry information from deeper layers than higher frequencies, depending on the ice’s temperature and opacity. The researchers modelled how brightness temperature changed across MWR’s channels and how some incoming radio emission from Jupiter’s radiation belts and the Galaxy was reflected by Europa.
The difference between the 0.6-gigahertz and 1.2-gigahertz measurements was particularly important because it constrained the vertical temperature gradient. A thick conductive shell has a different gradient from a thin one.
The best-fitting model produced an average conductive shell thickness of 29 kilometres. NASA’s account of the result calls it the first measurement able to discriminate between thin-shell ideas of less than a kilometre and models extending to tens of kilometres.
What impressed me here was the use of interference that initially looks inconvenient. Reflected radio emission from Jupiter complicated the signal, but its changing angle also helped the team constrain how reflective the ice was. Part of the noise became part of the measurement.
The 29-kilometre figure is a model, not a ruler
The formal statistical uncertainty in the team’s preferred model was much smaller than 10 kilometres. The large published uncertainty comes from a more realistic concern: the possibility that unmodelled changes across Europa’s surface could imitate part of the signal attributed to ice thickness.
The authors therefore reported 29 plus or minus 10 kilometres.
At the lower end, 19 kilometres, the shell would not be taller than three Everests. At the upper end, 39 kilometres, it would exceed four. The headline comparison describes the central estimate, not the whole allowed range.
The model also assumes pure water ice and no warmer convective layer beneath the rigid conductive ice. Those assumptions pull the interpretation in different directions.
Dissolved salt makes ice more opaque to microwaves. NASA says a modest amount of salt of the kind used in some Europa models could reduce the thickness estimate by about 5 kilometres.
A convective layer would do the opposite to the total barrier. The 29-kilometre result describes the cold outer layer transporting heat mainly by conduction. If warmer ice below is slowly overturning, the full solid shell between surface and ocean could be thicker.
There is also geography. MWR sampled about half of Europa during one close encounter, and the analysis used a laterally uniform model even though Europa’s ridged plains and disrupted chaos terrain are visibly different.
So “Europa has exactly 29 kilometres of ice everywhere” would be an overstatement. A fairer reading is that the microwave data favour a thick conductive shell in the observed region, with 29 kilometres as the central estimate under an idealised pure-ice model.
How a small moon can hide more water than Earth
The water comparison sounds impossible only if we picture oceans as surface features.
Earth’s oceans are broad but relatively shallow. Their average depth is under 4 kilometres, and continents interrupt them. Europa’s proposed ocean may be global and tens to more than a hundred kilometres deep.
NASA’s Europa overview gives a likely ocean depth of about 60 to 150 kilometres and says it may contain more than twice the water in all Earth’s oceans combined.
That volume remains an inference because nobody has seen the ocean directly. The strongest evidence comes from several independent clues rather than one photograph.
Europa’s young-looking surface contains few large impact craters and is crossed by ridges, bands and areas where blocks of ice appear to have shifted and refrozen. These features suggest a mobile interior, although geology alone does not prove a present global ocean.
The most persuasive evidence came from NASA’s Galileo spacecraft. As Jupiter’s magnetic field swept past Europa, Galileo measured an induced magnetic response. A deep layer of electrically conductive liquid provides a good explanation, and salty water is the leading candidate.
NASA’s summary of Galileo’s science results places Europa alongside Ganymede and Callisto as moons with evidence for subsurface saltwater layers.
In an earlier Space Daily article about icy moons and the habitable zone, I wrote about how Jupiter’s changing gravitational pull flexes Europa as it travels along a slightly eccentric orbit. Its resonance with Io and Ganymede prevents that orbit from becoming perfectly circular, helping tidal deformation continue.
Friction from that repeated flexing supplies heat. The same mechanism that makes Europa’s hidden ocean plausible also helps explain why a shell so far from the Sun is not merely frozen all the way to the rocky mantle.
A thick shell changes the chemistry problem
Liquid water is necessary for life as we know it, but a large ocean is not automatically a habitable one.
Europa also needs sources of chemical energy and biologically useful elements, along with processes that bring them together over long periods.
The bottom of the ocean may contact rock. Tidal flexing and retained interior heat could drive water-rock reactions, perhaps creating chemical gradients comparable in broad principle to those used by some ecosystems around Earth’s seafloor vents.
The surface offers another source. Jupiter’s radiation breaks apart water and other molecules in the upper ice, producing oxidants that could provide chemical energy if they reach the ocean.
The difficult part is connecting the two environments.
A thin, frequently fractured shell makes downward transport easier to imagine. Twenty-nine kilometres of cold ice makes every proposed pathway longer and more demanding.
Juno’s microwave data detected shallow scatterers that could be cracks, pores, voids or inclusions. The preferred model gave them a characteristic vertical scale of about 219 metres, and NASA says they extend only hundreds of metres below the surface.
That is geologically interesting, but it is a small fraction of 29 kilometres. The study’s authors concluded that the observed shallow features alone are unlikely to provide a major route for oxygen and nutrients all the way to the ocean.
This does not prove that surface-ocean exchange never happens. Large faults, subduction-like recycling, impact events, diapirs of warm ice and pockets of melt could all produce pathways the average model does not capture.
It does mean the visible cracks should not automatically be drawn as open pipes reaching a global sea.
A second 2026 study makes the barrier more consequential
A separate paper published in Nature Astronomy on 23 July 2026 approached the same problem from the movement of water rather than the penetration of microwaves.
Lujendra Ojha, Ankit Barik and Jacob Buffo used fluid and thermal simulations to test whether water from Europa’s deep ocean could rise through narrow dykes and collect in shallow reservoirs.
Their model of direct fluid exchange found that ascending water freezes too quickly to deliver the volumes needed to explain proposed shallow features. Turbulence made the limitation stronger by increasing heat loss, encouraging small ice crystals to form and accelerating blockage of the dyke.
The authors argue that, if shallow liquid reservoirs exist, local melting within the shell may be a more likely source than water arriving directly from the deep ocean.
This is also one modelling study, not a direct observation of a Europan fracture freezing shut. Still, it fits uncomfortably well with a thick-shell measurement.
It also adds a subtle warning for future exploration. A spacecraft that detects a shallow pocket of liquid may not be sampling ocean water that recently rose from below. The chemistry of that pocket could have developed locally and may not represent the global ocean.
The more I read about Europa, the more the shell stops looking like packaging around the interesting part. It is an active world in its own right, controlling heat, movement, chemistry and what any visiting spacecraft can learn.
Enceladus shows what access changes
Europa’s situation becomes clearer beside Saturn’s moon Enceladus.
Enceladus is much smaller, but fractures near its south pole release plumes containing material from its subsurface ocean. NASA’s Cassini spacecraft flew through those plumes and analysed the particles without drilling through the ice.
In my earlier article on the chemistry found at Enceladus, I looked at the significance of sodium salts, molecular hydrogen and phosphorus detected in material connected to that ocean.
Those measurements do not prove life exists there. They show the scientific advantage of an ocean that sends samples into space.
Europa may also produce water-vapour plumes, but the evidence has been intermittent and difficult to confirm. Its surface chemistry is heavily altered by radiation, which makes any material found above the ice harder to connect cleanly to the ocean.
A thick shell does not make Europa uninhabitable. It makes access and interpretation harder.
That distinction matters because habitability and detectability are different questions. An ocean could contain suitable conditions while remaining almost perfectly concealed from the instruments above it.
Europa Clipper was built to investigate the shell, not pierce it
NASA’s Europa Clipper launched on 14 October 2024 and is scheduled to reach Jupiter in April 2030.
It will not land, drill or attempt to melt through the ice. Instead, it will make about 50 close flybys, repeatedly examining Europa from different positions with radar, cameras, spectrometers, thermal imaging, magnetic measurements and gravity science.
The mission’s three main science objectives begin with the ice shell: determine its thickness and relationship with the ocean, characterise Europa’s composition, and understand its geology.
The REASON radar instrument will search for structures within the ice and possible liquid pockets. Magnetic and plasma measurements will improve estimates of the ocean’s depth, conductivity and salinity. Gravity measurements will reveal how Europa deforms under Jupiter’s pull.
Juno’s 29-kilometre estimate does not make those measurements redundant. It gives Clipper a stronger prior to test, while Clipper’s repeated, targeted flybys should show how much the shell varies from place to place.
I recently wrote about the difficulty of reaching protected environments beneath Mars, where even a few metres of rock can place potentially preserved material beyond every rover drill used so far.
Europa turns the same problem into a different scale entirely. Twenty-nine kilometres is not an engineering target for current drilling technology. It is the thickness of an environment that must be understood remotely.
Europa Clipper is not a life-detection mission, and it cannot tell us whether anything lives in the ocean. Its job is to work out whether the physical and chemical setting could support life and where future missions might obtain the most informative samples.
The barrier is part of what makes Europa interesting
The easy version of Europa is a huge ocean hidden under ice.
The more accurate version is a coupled system: irradiated surface, rigid conductive lid, possibly convecting deeper ice, local pockets of melt, global saltwater ocean, rocky seafloor and an orbit that continually supplies mechanical energy.
Every layer affects the others, and the 29-kilometre estimate belongs to a model of one part of that system.
The measurement does not close the case on Europa’s ice. Its 10-kilometre uncertainty is large, salt and convection change the interpretation, and one flyby cannot reveal every regional difference.
What it does is make the scale of the barrier harder to wave away.
Europa may hold more liquid water than our planet, yet volume is not access. A world can possess an enormous ocean and still keep its history, chemistry and possible biology hidden behind a distance that, on Earth, would rise well above the highest mountains.
The ocean is the reason Europa draws us there. The ice is the reason understanding it will take time.
