By David Warner 
In the cold, dark, outer reaches of our solar system, orbiting the giant planet Jupiter, is a small, bright moon called Europa. At a glance, it looks like a smooth, white billiard ball, covered in long, dark cracks. But for decades, scientists have looked at this icy world and become more and more convinced that it is hiding one of the most exciting secrets in the solar system. Beneath that frozen, fractured crust of ice, Europa almost certainly has a huge, global ocean of liquid, salty water.
This is not just a wild guess or a science fiction dream. The evidence has been building for years, turning this moon from a cold, dead piece of rock and ice into the most promising place to search for a habitable environment outside of Earth. The story of Europa is a story of a hidden world, powered by a strange internal heat, that could have all the right ingredients for life. But finding proof for an ocean we cannot see, millions of miles away, is a difficult task. So, what clues make scientists so sure that a moon smaller than our own is hiding an ocean bigger than all of Earth’s?
Europa is one of the four largest moons of Jupiter, first seen by the astronomer Galileo Galilei back in 1610. These four moons are known as the Galilean moons: Io, Europa, Ganymede, and Callisto. Europa is the smallest of this group, about the same size as Earth’s Moon. But that is where the similarity ends. Our Moon is a dry, dead, dusty world, covered in billions of years’ worth of craters. Europa’s surface is the exact opposite. It is the smoothest solid object in the entire solar system. It has very few large impact craters, which tells scientists that the surface is very young, geologically speaking. Something must be actively erasing or “repaving” the surface, covering up craters as they form. This was the very first clue that Europa was not just a frozen, inactive ball of ice. This repaving process could be caused by new ice or slushy water from below rising up through cracks and freezing, like a natural ice-rink resurfacer. The surface itself is a bright water ice, but it is crisscrossed by massive, reddish-brown streaks and strange, jumbled patches of ice. These features are the next big clue, pointing to a dynamic and active world.
The single most powerful piece of evidence for Europa’s ocean came from a spacecraft that flew past it in the 1990s, NASA’s Galileo. This spacecraft carried a special instrument called a magnetometer, which measures magnetic fields. Jupiter has an incredibly powerful magnetic field, the largest of any planet. As the Galileo spacecraft flew close to Europa, it measured Jupiter’s field. But it also found something else, something that should not have been there: a second, weaker magnetic field coming from Europa itself. Europa is too small and too cold to generate its own magnetic field deep in its core the way Earth does. So, what was causing this? The scientists realized that Jupiter’s powerful, rotating magnetic field was sweeping over Europa, and this was inducing a magnetic field inside the moon. This process, called induction, can only happen if the object contains a material that is a very good electrical conductor. After studying all the possibilities, the best fit for this material was a global, underground layer of liquid, salty water. Just as salt water on Earth conducts electricity, a vast, salty ocean inside Europa would perfectly explain the strange magnetic field that the Galileo mission discovered. This discovery shifted the idea of an ocean from a possibility to a high probability.
This is perhaps the most important question of all. Europa is more than five times farther from the Sun than Earth is. Its surface temperature is incredibly cold, around minus 160 degrees Celsius (minus 260 degrees Fahrenheit). At that distance, any water should be frozen solid from the surface all the way down to the rocky core. The tiny amount of sunlight that reaches Europa is not nearly enough to melt the ice. The answer, scientists believe, is heat generated from the inside of the moon. This internal heat source is called tidal heating. Europa’s orbit around Jupiter is not a perfect circle. It is a slight oval, or ellipse. This is because Europa is constantly being pulled by the gravity of Jupiter’s other large moons, Io and Ganymede. This gravitational tug-of-war means that as Europa orbits Jupiter every 3.5 days, its distance from the giant planet changes. When it gets a little closer, Jupiter’s massive gravity pulls on it, stretching the entire moon. When it moves a little farther away, the pull weakens, and the moon relaxes back into its shape. This constant stretching and relaxing, over and over, creates a huge amount of friction deep inside Europa’s rock and ice. This friction generates heat. It is just like bending a metal paperclip back and forth in your hands. After just a few seconds, the metal at the bend gets very hot. Europa is a “cosmic paperclip,” and the constant tidal flexing from Jupiter’s gravity generates enough internal warmth to keep a massive ocean in a liquid state, all hidden beneath a thick, insulating shell of ice.
When we look at pictures of Europa, the most obvious features are the long, dark cracks that run for thousands of miles, crisscrossing the entire globe. These are called lineae, which is Latin for “lines.” These are not just simple cracks; they are massive geological features. In many places, scientists have observed that the ice on one side of a crack seems to have broken apart and shifted, like pieces of a jigsaw puzzle. It appears that the ice shell itself has broken into large “plates” that can move around. The cracks are places where these plates have pulled apart, allowing warmer, “dirty” ice or slush from the ocean below to rise up and freeze, filling the gap. This process is very similar to plate tectonics on Earth, where our continents drift around. On Earth, the plates are rock floating on a molten mantle. On Europa, it seems to be plates of ice floating on a liquid water ocean. This “ice-tectonics” theory is another strong piece of evidence that a liquid layer exists just below the surface, allowing the ice shell to move and break.
Besides the long, neat lines, there are other regions on Europa’s surface that are messy, jumbled, and chaotic. These areas, known as “chaos terrain,” look as if someone took a giant hammer to the ice, smashed it into huge, iceberg-sized blocks, and then let them float around and refreeze in random positions. These chaos regions are some of the most compelling visual evidence for a subsurface ocean. The leading theory is that these are places where a large plume of warm water, or perhaps a “diapir” of warmer, buoyant ice, rose from the ocean below and melted the ice shell from the bottom up. As the shell thinned, it became unstable and collapsed. The huge blocks of surface ice then drifted around on the liquid water (or thick slush) underneath before the area froze over again. This process would also explain why the surface is so young; these chaos-forming events would completely destroy and remake large patches of the surface, erasing any craters that were there before. This suggests the ocean is not just passively sitting there, but is actively interacting with the ice shell above it.
This is one of the biggest unanswered questions about Europa, and it is a topic of great debate among scientists. The answer is extremely important, as it determines how easily material from the ocean can get to the surface, and how difficult it would be for a future mission to ever drill through it. There are two main theories. The “thin shell” model suggests the ice could be just a few miles (perhaps 5 to 10 kilometers) thick. In this model, the ocean would be very connected to the surface, with cracks potentially opening up all the way to the liquid water, and chaos terrains forming easily. The “thick shell” model suggests the ice is much thicker, perhaps 15 to 30 kilometers (about 10 to 20 miles) or more. In this model, the ocean is more isolated. The chaos terrains and cracks might be caused by warmer ice churning within the shell, rather than the ocean melting it all the way through. Recent studies analyzing the shapes of Europa’s few large craters seem to support the “thick shell” model. The way the ice slumped after a large asteroid impact suggests the solid ice layer must be at least 20 kilometers thick. This is a key mystery the next mission to Europa is designed to solve.
One of the most exciting discoveries in recent years came from the Hubble Space Telescope. While observing Europa, astronomers detected faint signs of what appeared to be giant plumes, or geysers, of water vapor erupting from the moon’s southern hemisphere. These plumes were seen shooting high into space, over 100 miles (160 kilometers) above the surface, before falling back down. This was a stunning discovery. It was very similar to the famous geysers on Saturn’s moon Enceladus. Later observations confirmed the presence of water vapor in Europa’s very thin atmosphere, supporting the idea that these plumes are real, though they may not be constant. This is a potential game-changer for exploring Europa. If Europa is actively spraying its own ocean water into space, a spacecraft could one day fly through that plume and “taste” the water. It could sample the ocean’s contents and analyze them for salt, minerals, and other chemicals without ever needing to land on the surface or drill through miles of solid ice. This gives scientists a direct pathway to studying the ocean’s composition.
The numbers are difficult to imagine, but they are a huge part of why Europa is so fascinating. Based on the magnetic field data, scientists believe the ocean is global, meaning it covers the entire moon under the ice. It is also believed to be very deep, perhaps 60 to 100 miles (100 to 160 kilometers) deep. Let’s compare that to Earth. Earth’s oceans are, on average, about 2.3 miles deep. Even the deepest point on Earth, the Mariana Trench, is only about 7 miles deep. Europa’s ocean is so vast and so deep that this tiny moon, which is smaller than our own Moon, is estimated to contain more than twice the amount of liquid water as all of Earth’s oceans combined. All of Earth’s rivers, lakes, and oceans together make up a massive amount of water, and Europa has double that. This makes it a true “ocean world,” with the vast majority of its water hidden from view in a dark, liquid realm beneath the ice.
The existence of a vast, warm ocean is exciting, but it is not the only thing needed for life. Astrobiologists, the scientists who search for life in the universe, look for three key “ingredients” that life as we know it requires. The amazing thing about Europa is that it seems to have all three.
First, liquid water. Europa has this in abundance, as we have seen. Water is a “universal solvent,” meaning it can dissolve nutrients and transport chemicals, allowing the chemistry of life to happen.
Second, the right chemical elements. Life on Earth is built from a few key elements, especially carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (often called CHNOPS). Scientists believe Europa’s ocean floor is not ice, but rock. The same tidal heating that warms the ocean is likely also warming this rocky seafloor, creating hydrothermal vents, just like the “black smokers” we find at the bottom of Earth’s oceans. These vents could pump heat and chemical nutrients (like sulfur and other minerals) from the moon’s rocky interior up into the ocean, providing the raw chemical “food” for life.
Third, a source of energy. Sunlight cannot reach this dark ocean, so life could not use photosynthesis. But it could use chemical energy, or “chemosynthesis.” Jupiter’s intense radiation belts constantly blast Europa’s surface. This radiation splits water ice (H2O) into hydrogen and oxygen. The hydrogen floats away, but the oxygen and other related compounds (oxidants) can get trapped in the ice. Through the cracks and the churning “ice-tectonics,” this surface ice could slowly get mixed down into the ocean. When these oxidants from the surface meet the chemical “fuel” (like sulfur) from the hydrothermal vents on the seafloor, it creates a chemical reaction that releases energy. A microbe could “eat” this energy, just as plants on Earth eat sunlight. The combination of water, seafloor nutrients, and surface radiation could create a stable, energy-rich “chemical battery” in the ocean, capable of powering a large ecosystem for billions of years.
Because the evidence is so strong, humanity is already on its way back to Europa. Two major missions are set to explore this ocean world. The first is the European Space Agency’s JUICE (JUpiter ICy moons Explorer) mission. It launched in 2023 and will arrive at Jupiter in 2031. While JUICE’s main goal is to study Jupiter’s largest moon, Ganymede, it will perform two very important flybys of Europa. It will use its instruments, including an ice-penetrating radar, to get a first look at the ice shell and confirm the ocean’s depth.
The second, and most dedicated, mission is NASA’s Europa Clipper, which launched in October 2024 and is scheduled to arrive at Jupiter in 2030. The Clipper is a huge, sophisticated spacecraft loaded with nine powerful instruments. It is not a “life-detection” mission; its goal is to find out if Europa is habitable. To do this, it will make about 50 close flybys of the moon, some as low as 16 miles (25 kilometers) above the surface. Its ice-penetrating radar will map the ice shell and find out how thick it is. Its cameras will map the surface in high definition. Its spectrometers will “taste” the reddish-brown ice and any plumes it finds to determine their chemical composition. And its magnetometer will take much more precise measurements of the magnetic field to confirm the ocean’s depth and, most importantly, its salinity (how salty it is). The Clipper will, for the first time, give us definitive answers to the biggest questions about this mysterious moon.
Europa is one of the most compelling worlds in our solar system. It is a place of stark contrast, with a surface frozen in a deep cold, but an interior kept warm and active by the relentless pull of its parent planet. All the evidence we have—from its mysterious magnetic field, to its young, cracked, and jumbled surface, to the hints of water plumes spraying into space—points to the existence of a vast, salty ocean hidden from sight. This ocean, possibly twice the size of all of Earth’s, may have been liquid for billions of years. Most importantly, it appears to have the three essential ingredients for life: liquid water, essential chemical elements, and a source of chemical energy. We are now on the threshold of finding out for sure, as the Europa Clipper mission speeds on its way.
If this hidden ocean does indeed have all the right conditions for life, what do you think that life might look like, thriving in a dark, cold sea so far from the Sun?
The reddish-brown material in Europa’s cracks and chaos terrain is a major mystery. Scientists believe these are salts and sulfur compounds that have been altered by the intense radiation from Jupiter. The exciting possibility is that these “stains” are material from the ocean below that has welled up, meaning we are “seeing” the chemical composition of the ocean on the surface.
It would be extremely difficult. The surface of Europa is one of the most dangerous places in the solar system due to the intense, deadly radiation from Jupiter. Any human base would have to be built many meters beneath the ice to be protected. The frigid temperatures also make it incredibly hostile.
Both are icy moons with subsurface oceans and geysers. The main difference is their parent planet and size. Enceladus is a tiny moon of Saturn, and its plumes spray directly from its ocean into space. Europa is a large moon of Jupiter, and its ice shell is much thicker, making its plumes (which are still being studied) more mysterious.
It takes a long time, about six years. Because Jupiter is so far away, spacecraft like the Europa Clipper cannot fly there directly. They must use gravity assists, which means they fly past other planets (like Mars and Earth) to use their gravity as a “slingshot” to gain enough speed to reach the outer solar system.
We do not know. Right now, there is no evidence of life on Europa. However, it is considered the most promising place in our solar system to search for a currently habitable environment, and possibly life, because of the strong evidence for its liquid water ocean, internal heat, and chemical ingredients.
The surface of Europa is extremely cold. The temperature at its equator averages about minus 160 degrees Celsius (minus 260 degrees Fahrenheit), and it can get even colder at its poles. The ocean below, however, is kept warm by tidal heating and insulated by the ice shell.
This is a science fiction scenario, but if a crack opened all the way to the ocean, you would fall through the ice shell. After falling for miles through the ice, you would splash into a dark, cold, and utterly black liquid water ocean, under immense pressure from the ice and water above.
In the future, yes, but it is an enormous engineering challenge. A future mission, often called a “lander,” would have to carry a heat-based drill (like a “cryobot”) that could melt its way down through many miles of ice. It would also need to communicate back to Earth through that thick ice.
It has few craters because its surface is geologically young and active. Something is constantly “repaving” the surface, erasing craters. This is likely the movement of the ice plates (“ice tectonics”), the welling-up of slushy ice in cracks, and the formation of chaos terrain, all of which destroy old surfaces and create new ones.
Based on the magnetic field data and the reddish material on the surface, scientists believe the ocean is salty. It might be similar to Earth’s oceans, but it could also be rich in other types of salts, like magnesium sulfates (similar to Epsom salts). This means it would likely taste very salty and perhaps bitter.