NASA’s Juno spacecraft has revealed Europa’s ice shell may be up to 39 kilometers thick — a finding that reshapes how scientists model the moon’s hidden ocean and its potential to host life.
Europa has long been considered one of the most promising locations in the solar system to search for extraterrestrial life. Its global subsurface ocean, potentially holding more water than all of Earth’s oceans combined, is rich in organic compounds and salts — the basic ingredients for biology. But until now, one critical variable remained murky: the thickness of the ice shell separating the surface from the ocean below.
Now, a new study led by S. M. Levin and colleagues from JPL, Caltech, Stanford, and Purdue universities, using data from NASA’s Juno spacecraft, has provided the clearest estimates yet — suggesting the ice shell is between 19 and 39 kilometers thick. This finding not only refines our understanding of Europa’s structure but also redefines how scientists assess the moon’s habitability.
A Moon of Promise, With a Deepening Mystery
For decades, Europa has captivated scientists with its combination of extreme cold and abundant organic chemistry. The moon’s fractured ice crust conceals a salty, global ocean that may be more voluminous than Earth’s entire oceanic reservoir. That combination has fueled speculation that Europa could harbor life — possibly even microbial ecosystems.
Yet one key uncertainty has persisted: how deep the ocean lies beneath the ice. No spacecraft has directly drilled into Europa’s shell or mapped it with radar. Instead, researchers have relied on indirect clues — such as the shapes of impact craters and how the surface flexes under stress — to estimate ice thickness. These methods yielded wildly varying results, with some studies suggesting the ice might be just a few kilometers thick, while others argued for shells tens of kilometers deep.
This uncertainty made it difficult to model how materials, including potential biosignatures, might move between the surface and the ocean below. The new Juno analysis offers a more precise answer — and it may be more complex than anyone expected.
Repurposing Juno’s Microwave Radiometer for Europa
Juno, designed to study Jupiter’s atmosphere and interior, has completed several close flybys of Europa over the past two years. During those flybys, its Microwave Radiometer (MWR) — originally built to measure microwave emissions from Jupiter’s deep atmosphere — was repurposed to probe Europa’s ice.
“The same physical principles apply to icy surfaces,” explained S. M. Levin to The Brighter Side of News. “Ice emits microwave radiation that varies with temperature. Different frequencies can penetrate to different depths. Juno’s MWR measures six frequencies, allowing us to infer temperatures from just below the surface down to several kilometers deep.”
By analyzing these signals, the team reconstructed temperature profiles across parts of Europa’s ice shell. These profiles reveal how heat moves through the ice — both vertically and sideways — and allow scientists to identify where ice transitions to liquid water. That boundary marks the top of the subsurface ocean.
Thick Ice, But Not a Barrier to Life
The analysis points to an ice shell roughly 19 to 39 kilometers thick — a range that rules out the idea of a shallow ocean lying just beneath the surface. This finding narrows a long-running debate and adds new context to the question of whether Europa’s buried ocean could support life.
At first glance, thicker ice might seem to hinder habitability. A deep shell could limit how surface materials reach the ocean. But the relationship is more complex. Life on Earth relies on chemical reactions that move electrons between molecules — reactions that often depend on oxidants and reductants being brought together.
On Europa, intense radiation from Jupiter breaks apart surface ice, creating oxidizing chemicals. The surface itself is too hostile for life, but those oxidants could serve as energy sources if they reach the ocean. A thick ice shell may slow this transport, but it may not stop it. Thick ice can undergo convection — with warmer ice rising and cooler ice sinking — helping to move oxidants downward over time.
The Challenge of Detecting Life
Detecting life on Europa remains difficult. Any signs of biology in the ocean would need to travel upward through kilometers of ice before reaching the surface, where spacecraft could detect them. Heat conduction in the upper ice may block this movement. However, surface features suggest that liquid water has reached the surface in the past — possibly through localized melting or ocean upwelling.
Such events raise the possibility that biosignatures could sometimes be delivered closer to the surface. Still, scientists do not yet understand how often this happens or under what conditions. The Juno data doesn’t directly address this, but it provides a critical baseline for future missions.
What Comes Next: Europa Clipper and Beyond
Future answers will likely come from NASA’s Europa Clipper mission, set to arrive in the Jupiter system in 2030. Europa Clipper carries a suite of instruments designed to study the moon’s ice shell and ocean, including a ground-penetrating radar — a tool that will provide more direct measurements than Juno’s microwave data.
Unlike Juno, Europa Clipper does not include a microwave radiometer. Even so, the creative use of Juno’s data provides a valuable foundation. As Europa Clipper maps the ice in detail, scientists will be able to reinterpret the microwave results with greater confidence — and build a more accurate picture of Europa’s internal structure.
Together, these missions will help clarify how Europa’s ice works and how it affects the moon’s potential for life. The research findings are available online in the journal Nature Astronomy.
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