Juno Spacecraft Measures Europa Ice Shell at 29 Kilometers Thick

The picture of the cosmos we receive in school is wonderfully neat. Put a star in the middle, draw a narrow band around it, and call that the habitable zone where liquid water can exist. A planet inside the band might support life; a world outside it is simply too hot or too cold. Far beyond the reach of life-sustaining sunlight, icy moons hold massive oceans liquid beneath frozen crusts, warmed not by the sun but by the relentless gravitational flexing of the giant planets they orbit.

Among those distant worlds, Europa stands out as one of the most promising candidates for life in our solar system. The moon is roughly the size of Earth’s Moon, yet it harbors a global ocean 60 to 150 kilometres deep, wrapped around rock and sealed from space. On the surface, temperatures plummet to between -256 and -364 Fahrenheit. Stand on that barren plain of ice, and the sky overhead is nearly black, with Jupiter hanging motionless and enormous in the sky for observers on one side, while remaining entirely invisible from the far side.

Juno’s Microwave Radiometer Measures a 29-Kilometre Ice Shell

Until recently, scientists debated whether Europa’s icy shell was a thin crust less than a kilometre thick or a thick barrier extending for tens of kilometres. A definitive measurement has now pushed the debate firmly toward the thick-ice side.

While Juno was built primarily to investigate Jupiter, its Microwave Radiometer (MWR) collected measurements across roughly half of the moon’s surface. Led by Steve Levin and published in Nature Astronomy, the resulting analysis used 129 measurements across six frequency channels to model temperatures at varying depths.

The instrument did not send a physical pulse straight through the ice with a stopwatch. Instead, ice naturally emits microwave radiation, and lower frequencies carry information from deeper layers. By modelling brightness temperature changes and accounting for radio emission reflected from Jupiter’s radiation belts, the team arrived at a central estimate of an average conductive shell thickness of 29 kilometres.

That headline figure requires caution. The researchers reported a formal result of 29 plus or minus 10 kilometres to account for unmodelled surface variations. At the lower end, the shell measures 19 kilometres; at the upper end, it reaches 39 kilometres—exceeding four Mount Everests. Furthermore, the model assumes pure water ice without a warmer convective layer underneath. Modest amounts of dissolved salt could reduce the estimate by about 5 kilometres, while a convective sub-layer could make the full solid barrier even thicker.

Gravity, Resonance, and the Energy Budget of Tidal Heating

The heat keeping the subsurface ocean liquid originates from Jupiter’s immense gravitational pull. Because Europa travels on a slightly elliptical orbit, the strength of the tide changes as its distance from the gas giant varies.

Those gravitational encounters prevent Europa’s orbit from circularizing. As the moon is continually stretched and relaxed, internal friction turns orbital energy into heat.

Simulations Show Deep Ocean Water May Struggle to Reach the Surface

Reddish-brown lines and curved cracks called cycloids rake across Europa’s surface, opening and closing as Jupiter squeezes the moon. Scientists long theorized that shallow pools of water observed near the surface could represent a direct access route, allowing probes to study deep ocean water without drilling through tens of kilometres of ice.

A study published in Nature Astronomy challenges that assumption. Led by Lujendra Ojha, an earth scientist and first author at Rutgers University, researchers used computer simulations to test whether water from the deep ocean could travel upward through ice fractures.

The team modeled water velocity, crack size, turbulence, and supercooling. Their findings suggest that rising water flows rapidly and turbulently, causing it to lose heat quickly to the freezing surrounding ice walls. Small ice crystals form rapidly, clogging the fractures within hours and blocking the water from reaching the surface.

“In particular, we asked whether the water could travel this distance before losing enough heat to freeze and whether the fractures could remain open long enough to transport a meaningful amount of water.”

Lujendra Ojha, earth scientist at Rutgers University

Gizmodo highlights Ojha’s explanation that earlier models often treated water rising through the ice as an orderly process comparable to volcanic activity, overlooking the fundamental differences of ice and turbulence.

What Upcoming Spacecraft Missions Will Test Next

The realization that shallow surface pools likely originate from local melting rather than deep-sea upwelling shifts how scientists view future exploration. Ojha cautioned against assuming surface water reflects underlying ocean chemistry.