Juno Peered Beneath Europa’s Surface: New Measurements Narrowed the Debate on Ice Thickness and Real “Pathways” to the Ocean
NASA’s Juno spacecraft, which has been orbiting Jupiter since 2016, has provided the most direct constraint yet on how thick Europa’s ice crust is – a moon that has been at the top of the list of targets for the search for habitable environments beyond Earth for years. Analysis of measurements collected during a close flyby on September 29, 2022, shows that in the observed region, the cold, rigid, and thermally conductive outer part of the ice shell is on average
about 29 kilometers thick, with an estimated uncertainty of about 10 kilometers. This estimate is the first that can reliably distinguish between “thin” and “thick” crust scenarios based on direct microwave measurements, as earlier models and interpretations ranged from a few kilometers to several tens of kilometers.
Europa is slightly smaller than Earth’s Moon, but scientifically far more intriguing: a global salty ocean likely lies beneath the frozen surface, and it is in this combination of ice, water, and energy that scientists look for prerequisites for possible forms of life. Ice thickness is not just a geophysical curiosity. It determines how realistic the transport of oxidants and other chemical compounds from the surface to the ocean is, how deep signals from within can be “read,” and how demanding the interpretation of data that specialized missions are soon expected to deliver will be. The new measurements from Juno are therefore not just “another number,” but a key piece of the puzzle of how Europa functions as a system.
Microwave Radiometer, Designed for Jupiter, Turned Out to Be a Tool for an Ice “X-ray”
It is interesting that the instrument that enabled this measurement was originally designed for a completely different problem: the
MWR (Microwave Radiometer) was developed to penetrate beneath the tops of Jupiter’s clouds and measure the thermal structure of the atmosphere. But the same principle – detection of thermal radiation at multiple microwave frequencies – can also be used for ice. Different frequencies “see” to different depths: shorter wavelengths mostly capture shallow layers, while longer frequencies can penetrate deeper, to kilometer scales, depending on ice purity, temperature, and salt content.
During the flyby on September 29, 2022, Juno approached Europa to about 360 kilometers above the surface. The spacecraft is spin-stabilized, so instruments “scan” a strip of terrain during one rotation, and through multiple rotations, a map of microwave emissions is created. From such data, the team estimated what the temperature profile of the ice must be and what its efficiency in absorbing and scattering microwaves is to obtain the observed signal. In a paper published online on December 17, 2025, in the journal
Nature Astronomy, the authors clearly state that the obtained estimate refers to the cold, rigid, and thermally conductive part of the ice shell, i.e., the so-called conductive layer that transfers heat primarily by conduction.
This is precisely what is important for interpreting the results: Juno does not claim that the “overall” ice shell is exactly 29 kilometers, but that microwave measurements agree best with such a thickness of the conductive layer in the area the instrument covered during the pass. For a global map and variations by region, much more data will be needed, including radar profiles and gravity measurements, which is one of the reasons why Europa Clipper is conceived as a multi-year mission with a large number of close passes.
Ice Can Be Both Thicker and Thinner: Heat, Layers, and the Role of Salt
In public interpretations, it is easy to lose sight of the fact that Europa’s ice crust is likely layered. If beneath the cold outer part there is also an
inner, warmer convective layer – a part where the ice slowly “mixes” and transfers heat by convection – then the total thickness of the ice shell could be greater than what Juno has constrained here. Such a convective layer is often associated with the idea that Europa has more geologically active ice than it seems at first glance, because flow in the ice can maintain stresses and encourage the formation of structures on the surface.
On the other hand, the chemical composition of the ice can “lower” the estimate. If the ice contains dissolved salts, the microwave signal changes because salt affects the electrical and absorption properties of the ice. The authors state that in a scenario of moderate salinity, as suggested by certain models based on analogies with sea ice on Earth, the estimate of the conductive layer thickness could be smaller by
about 5 kilometers. This does not reverse the conclusion – we are still talking about a thick crust – but it shows why the combination of physics, chemistry, and temperature is key in this story, and not a single number.
What makes this measurement particularly useful is that it narrows the room for speculation in the region Juno actually observed. The range “from 3 to over 30 kilometers,” which was often cited as a framework, shifts with this result towards a scenario in which the cold outer layer is closer to the upper limit of earlier estimates. This directly changes the way surface geology is interpreted: many structures – double ridges, chaos terrains, bands, and networks of cracks – must be explained by processes acting in ice that is, at least locally, massive and thermally stable in the outer part.
What Thicker Crust Means for “Habitability”: Oxidants from the Surface Have a Longer and Harder Path
Europa is astrobiologically interesting because the salty ocean beneath the ice can be in contact with the rocky core, which opens the possibility of chemical reactions and energy sources comparable to hydrothermal systems on Earth. However, along with water and energy, a chemical “inventory” is also needed: oxidants, nutrients, and building blocks that could power metabolism. On Europa’s surface, exposed ice and surface material are constantly bombarded by particles from Jupiter’s magnetosphere. This process creates oxidized compounds, and part of the scientific community has been debating for years whether such compounds can, through geological processes, be transported to the ocean and participate in chemistry there that could potentially support life.
If the conductive layer is about 29 kilometers thick, then the path of such compounds to the ocean is, on average, long and likely interrupted by numerous layers and phases of ice. This does not mean transport is impossible. It means that, if it happens, it will likely depend on rare or localized events: episodic upward intrusions of brine, local melting and refreezing, or tectonic processes that occasionally create deeper cracks. That is precisely why recent discussions increasingly emphasize the need to map not only ice thickness but also places where heat and salinity could alter the mechanical properties of the crust.
Official NASA reviews of Europa also emphasize that there are indications of possible water ejections (plumes) and that surface chemistry can in certain conditions be connected to the internal ocean, although these signals are not universally confirmed and remain a subject of debate. Juno’s new ice thickness estimate does not prove gases or water intrusions, but it provides a geophysical framework: if we look for transport mechanisms, we must look for them in a world where the ice barrier in the observed area is thick and where “quick solutions” are not realistic without additional, strong processes.
“Scatterers” in the Ice: Cracks and Pores Exist, but They Are Small and Shallow
Another important part of the study relates to the structure immediately below the surface. Microwave measurements point to the presence of so-called
scatterers – irregularities that scatter microwaves on their return to the instrument, similar to the way light scatters in an ice cube. This can include tiny cracks, pores, bubbles, or cavities, i.e., heterogeneities in the ice that “spoil” the ideally smooth signal and create a characteristic signature in radiometric data.
Modeling in the paper suggests that these irregularities are small, on the order of centimeters, and that they extend to depths of
hundreds of meters below the surface. Such a picture has a clear consequence: although Europa looks like a network of cracks, ridges, and broken plates on the surface, the microstructure observed in the shallow subsurface is likely not “permeable” enough to be a main channel by itself through which oxygen and nutrients would travel to the ocean. In other words, the existence of cracks is not in dispute – what is in dispute is their capacity to act as a continuous transport system through tens of kilometers of ice.
This, however, does not close the question of communication between the surface and the depths. Surface structures recorded over decades, from the Galileo mission to modern telescopic observations, still point to geologically active ice. In 2024, Juno also released high-resolution images of Europa indicating details such as wide depressions and zones of disrupted ice, as well as possible traces of activity on the surface. Such visual clues and microwave signals together suggest a scenario in which Europa is dynamic, but in which the transport of matter and energy occurs selectively, locally, and likely in episodes, rather than as a constant “circulation” between the surface and the ocean.
Europa Clipper and JUICE Enter the Story with a Clearer Starting Point
In the coming years, Europa moves from a phase of broad assumptions to a phase of systematic mapping. NASA’s
Europa Clipper was launched on October 14, 2024, and the official NASA timeline notes that the spacecraft already performed a gravity maneuver past Mars on March 1, 2025, and that a flyby past Earth follows in December 2026. According to the same plan, Europa Clipper arrives in the Jupiter system in 2030 and then conducts almost 50 flybys of Europa to measure ice thickness, ocean properties, surface composition, and the moon’s interaction with Jupiter’s magnetic environment. The key advantage of such an approach is repetition: Europa will not be observed “once in passing,” but through a series of geometries, altitudes, and locations, which enables comparisons and the building of global models.
In parallel, ESA’s
JUICE (JUpiter ICy moons Explorer), launched on April 14, 2023, according to ESA’s mission overview, is scheduled to arrive in the Jupiter system in
July 2031. On its way, it already performed a complex double gravity maneuver past the Moon and Earth in August 2024, and ESA also announced that the flyby past Venus on August 31, 2025, was successful, after a communication problem that temporarily interrupted contact with the spacecraft was resolved earlier during the summer of the same year. JUICE is primarily focused on Ganymede and Callisto, but in the broader context of the Jupiter system, it will provide an important comparative framework: how ice crusts of different moons behave and what signals oceans beneath them leave.
For both missions, Juno’s results provide context and a “calibration” of expectations. If a conductive layer thickness of about 29 kilometers fits best in the observed area, then radar profiles, magnetometry, and gravity measurements will get a more realistic framework for interpretation: where to look for deviations, how strong they must be to imply thinner ice or warmer zones, and what signature saltier brine in the subsurface might leave. And equally important: results on scatterers suggest that shallow, tiny heterogeneities will have to be distinguished from potentially deeper structures that could be relevant for matter transport to the ocean.
Europa as a Laboratory of Subsurface Oceans: Questions Are Becoming More Concrete, and Answers More Measurable
Europa is not the only world with an ocean beneath ice, but it is among the most attractive because it is located relatively “close” and because its extreme radiation environment on the surface creates chemistry that can be relevant for potential life. In the last twenty years, Europa has become a symbol of so-called ocean worlds: places where liquid water hides beneath an ice crust, and where energy can be created by tidal interaction, friction in the ice, and chemical reactions in the depths. In such a picture of the universe, the most important question is not just where there is water, but where water, energy, and chemistry meet in a sufficiently stable way.
But with the new microwave measurement, that symbolism is increasingly getting hard edges. Instead of the question “does Europa have an ocean,” the focus shifts to a series of precise tasks: how thick is the conductive layer and how does it vary by region; does a convective layer exist and how much does it contribute to total thickness; how salty is the ice and how does that change physics and chemistry; what are the heterogeneities below the surface; and do convincing mechanisms exist that bring surface chemistry to the ocean. Answers to these questions are not a spectacle for headlines in themselves, but they are key so that one day, based on data, we can speak of real habitability, and not just an impression.
Juno, paradoxically, arrived at this shift with an instrument that was not intended for Europa. That is precisely why the results are important: they show how much can be gained by smart use of existing tools and how complex Europa is even when observed “in passing.” When Europa Clipper and JUICE begin sending series of detailed profiles and maps at the beginning of the next decade, Juno’s new findings will be one of the reference points. And then, perhaps for the first time, it will be possible to answer with greater certainty the question Europa has been posing for decades: not only does it hide an ocean, but how connected is that ocean to the surface at all, and does a stable environment exist in the depths that could be suitable for life.
Sources:- Nature Astronomy – scientific paper on the thickness and structure of Europa’s ice shell based on Juno’s MWR (PDF: link)- NASA Science – official timeline of the Europa Clipper mission, including launch on October 14, 2024, and planned arrival in 2030 (Mission Timeline: link)- NASA JPL – overview of the Europa Clipper mission and start of the Jupiter system tour in 2030 (Press kit/mission: link)- ESA – JUICE: mission overview, launch on April 14, 2023, and planned arrival in July 2031 (Overview: link)- ESA – report on successful gravity maneuver past Venus on August 31, 2025, and earlier communication anomaly (Operations update: link)- NASA – Europa: official facts and scientific context on the ocean, surface chemistry, and possible water ejections (Europa Facts: link)- NASA/JPL – data on Juno’s close flyby of Europa on September 29, 2022, and distance at closest approach (JPL news: link)- NASA – Juno: official mission overview and note on mission extension until September 2025 (Mission overview: link)- NASA – Juno and Europa: high-resolution images and interpretations of surface features released on May 15, 2024 (NASA news: link)- PDS Atmospheres – table of Juno perijoves (flybys) through 2025 (PDF: link)
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