Observations show that Jupiter’s icy moon Europa has a thick shell of ice covering its warm ocean. The ocean is rich in chemicals and may contain all the essential components for life. That’s why Europa is such an attractive target in the search for life, and why NASA’s European Clipper and ESA’s JUICE are heading out to study the moon in more detail.

Speculation that Europa may have an ocean dates back to the Voyager missions. Images from these spacecraft show cracked icy surfaces, leading some scientists to wonder if there is an ocean underground. The Galileo mission provided further evidence that Europa is not completely frozen. The Hubble Space Telescope also supported this idea when it observed plumes ejecting through the moon’s ice, where water may be leaking.

As interesting as these observations are, they are only the beginning of our understanding of our fascinating moon. There are several open questions that must be answered before proceeding with the habitability hypothesis. One question concerns the thickness of Europa’s icy shell and the cracks and fissures within it.

Scientists believe that for Europe’s oceans to support life, they need a pathway through the ice that allows the ocean and the surface to chemically “communicate.” Europa’s surface is exposed to Jupiter’s intense radiation, and the energy causes chemical reactions. This can produce antioxidants and other chemicals that help sustain life. If the ice is thin or has cracks that act as chemical pathways, these materials can reach the ocean.

*Charged particles from Jupiter collide with Europa’s surface, where they break up frozen water molecules. Scientists believe that some of the free oxygen flows through the ice into the moon’s underground ocean, where life can emerge. Image credit: NASA/JPL-Caltech/SWRI/PU*

It will be years before Europa Clipper and JUICE reach Europa, but NASA has spacecraft to study the Jupiter system. Juno. He has been researching Jupiter and its Galilean moons for nearly 10 years. Although Jupiter was the priority target, the spacecraft took several opportunities to point its instruments at the gas giant’s moons.

A recent study in Nature Astronomy features some of Juno’s observations of Europa. The title is “”Europa’s ice thickness and subsurface structure characterized by the Juno microwave radiometer.The lead author is Steve Levin, Juno project scientist and co-investigator at NASA’s Jet Propulsion Laboratory.

“Jupiter’s moon Europa is thought to have a brine ocean beneath its variously fractured icy shells, and it is therefore one of the highest priority targets for astrobiology in the solar system,” the authors write. “The ice shell is estimated to be between 3 kilometers and more than 30 kilometers thick, and observations by the Galileo spacecraft show extensive areas of ice fracture (chaotic topography), leading to speculation that the ice shell may contain subsurface cracks, faults, pores, or bubbles.”

To advance this issue, scientists used Juno to study Europa. microwave radiometer (MWR). The MWR has six different antennas, each monitoring a separate MW frequency. These six were chosen because they can penetrate Jupiter’s thick atmosphere and provide the mission with a way to measure the gas giant’s temperature at different atmospheric levels and create depth profiles. In a similar way, MWR can measure ice temperatures at different levels of the European ice sheet.

MWR made measurements that may impose constraints on the thickness of Europa’s ice. Current estimates range in thickness from 3 km to 30 km, which is a very wide range.

The MWR instrument measured different depths at each frequency, measuring ice temperatures ranging from just a few meters deep to several kilometers deep. The researchers pieced them together to create a temperature profile of the ice, which gives an indication of its thickness. These measurements not only improve our understanding of the thickness of Europa’s shell, but also reveal the nature of cracks and fissures that cause reflections during exploration.

“These observations of ice temperature and depth contain evidence of frequency-dependent scattering within the ice, and together with the size and depth of the observed reflection sources, provide constraints on the ice shell thickness,” the authors explain.

This figure shows six maps from Juno’s MWR observations. The circle at the bottom left of each image indicates the beam size, and the number at the bottom right indicates the approximate sounding depth if the ice is 50% water. Image credit: Levin et al. 2025.NatAstr.

Because the salinity of the ice is unknown, the researchers started with the baseline assumption that the ice is pure water when modeling their results.

“In the ideal case of pure water ice, the data are consistent with the presence of a thermally conductive ice shell 29 ± 10 km thick and the presence of cracks, pores, or other scatterers extending to depths of hundreds of meters below the surface and with characteristic sizes less than a few centimeters in radius,” the researchers explain.

It can be even thicker depending on the salinity or if there is an additional warm layer.

“The 18-mile estimate is related to the cold, hard, conductive outer layer of the pure water ice shell,” lead author Levin said in the paper. press release. “If there is also a slightly warmer convective layer inside, which is possible, then the total thickness of the ice shell would be even thicker. If the ice shell contains a moderate amount of dissolved salts, as some models suggest, the estimated shell thickness would be reduced by about 3 miles.”

This thick ice creates a more impermeable chemical barrier. The situation looks even more bleak when the cracks and holes are only a few centimeters in radius and extend only a few hundred meters into the ice.

These results put a damper on enthusiasm for Europa’s habitability. Other studies have also shown that the plumes observed coming from Europa may not have come from that ocean. Instead, they can originate from pockets of water trapped in voids and cracks in the ice. Therefore, the relationship between the ocean and the land surface is only provisional and may even be non-existent.

Is the lack of chemical pathways dooming Europe to be uninhabitable? Not necessarily. But this route may be the only way surface chemicals and nutrients reach the ocean. Life could still persist, but it would have to rely heavily on tidal heating and hydrothermal vents, and the primordial ocean would have been seeded with nutrients at the time of its formation. Still, without active exchange between the Earth’s surface and the ocean, it is difficult to understand how life could survive after the ocean’s nutrients are consumed.

The researchers point out several problems with their study. First, their measurements are not universal. “We note that our results are limited to the observed topography, and further mapping of Europa’s surface by radiometry or radar may reveal areas where the ice shell is thinner or thicker, or where the regolith contains unobserved variations,” they explain.

Also, their estimates of ice shell thickness are based on several assumptions about the purity of the ice. “Impurities in the ice, such as salts and other substances, increase the microwave opacity and potentially reduce the depth sounded by all channels, which reduces the depth difference between the 0.6 GHz and 1.2 GHz channels and has a roughly proportional effect on the ice shell thickness estimate,” the authors explain.

In any case, the researchers’ thickness estimate is on the thick side of estimates from other studies. But a more frightening result may concern pores and cracks in the ice.

“Due to their low volume fraction, shallow depth, and small size relative to ocean depth, our results suggest that the pores, voids, or fractures are probably not channels by themselves to supply nutrients to the ocean or provide ocean-surface communication,” the authors write.

“The thickness of the ice shell and the presence of cracks and pores within the ice shell are part of a complex puzzle for understanding Europa’s potential habitability,” said study co-author Scott Bolton, SwRI’s Juno principal investigator. “These provide important context for NASA’s European Clipper and ESA’s (European Space Agency) Juice (Jupiter ICy satellite probe) spacecraft, both of which are on their way to the Jupiter system.”

The two spacecraft will provide scientists with the measurements they need to further study the icy moon. European Clipper has ice penetrating radar This allows the ocean-ice boundary to be measured and mapped more closely. Clipper has completed nearly 50 flybys of Europe and is expected to create a near-global map of its ice. JUICE is also equipped with an ice-penetrating radar, but it is researching multiple satellites and is not just targeting Europa. Only a few flybys of Europa will take place.

The Europa Clipper will arrive first in 2030, followed by Juice the following year. Once the scientific observations start pouring in and the papers are published, we’ll get some much-needed answers to some of the intriguing prospects about Europa and its habitability.