Europa is a terrible place to go looking for an ocean.
There is no beach. There is no sunlight reaching the water. There is, in fact, somewhere between 15 and 25 kilometers of ice in the way. The surface temperature can fall below 100 kelvin, Jupiter’s radiation environment is vicious, and the whole moon is being periodically squeezed by the gravity of the largest planet in the Solar System. Yet Europa probably has a global ocean beneath its frozen exterior, and that ocean may contain roughly twice as much water as all of Earth’s oceans combined. That’s a pretty strange thing for a moon to have.
Europa is only about 3,100 kilometers across, slightly smaller than Earth’s Moon, and it orbits Jupiter every 3.55 days. From a distance, its most obvious feature is the enormous network of dark lines covering its surface. Some extend for thousands of kilometers; others form tangled bands and ridges, cutting across one another in ways that make Europa look less like a frozen world and more like a piece of glass that has been repeatedly cracked and put back together.
Those fractures were one of the early clues that Europa might be geologically active. The surface has remarkably few impact craters compared with many other ancient bodies in the Solar System, suggesting that something has been resurfacing it. The ice is moving. Cracks open, close, shift, and sometimes fill with new material. There are also regions called chaos terrain, where blocks of the surface appear to have broken apart and moved relative to one another. Whatever is happening underneath Europa, the ice shell is participating in it.
The obvious question is where all that energy comes from.
Europa is much too far from the Sun for ordinary sunlight to keep a deep ocean liquid. At Jupiter, sunlight is roughly 25 times fainter than it is at Earth, so solar heating contributes very little to the temperature of the moon’s interior. Instead, Europa gets its heat from Jupiter, although “gets its heat from Jupiter” makes the process sound rather more straightforward than it is.
Europa’s orbit is slightly eccentric, meaning that its distance from Jupiter changes as it travels around the planet. Io and Ganymede help maintain this eccentricity through their gravitational interactions with Europa, preventing the orbit from becoming perfectly circular. As Europa moves through that orbit, Jupiter’s enormous gravity repeatedly deforms it. The amount of stretching changes throughout the orbit, and friction inside the moon converts some of the associated mechanical energy into heat. This is tidal heating, and it is one of the most useful phenomena in explaining Europa’s story.
Io demonstrates the same phenomenon in a much more dramatic fashion. Its proximity to Jupiter and stronger tidal interactions make it the most volcanically active world in the Solar System. Europa is farther out and experiences less heating, but the principle is the same. Jupiter is effectively kneading the moon like dough, except that the dough is several thousand kilometers wide and the kneading has been going on for billions of years.
That heat has consequences for Europa’s interior. The simplest picture is a rocky mantle and metallic core surrounded by a thick layer of water ice, with a global saltwater ocean sandwiched between the ice and rock. Estimates for the ocean’s depth vary substantially, but it could be tens of kilometers deep and contain more water than Earth’s oceans despite Europa being vastly smaller than Earth.
The ice therefore acts less like a permanent lid and more like the upper boundary of an active planetary system. Heat has to move through it. Material may move through it. Tidal stresses continually deform it. If the ocean and ice shell exchange material, the surface could preserve information about an environment that is otherwise buried beneath kilometers of solid water. This is where Europa’s surface chemistry becomes especially useful.
The reddish-brown material along many of its fractures has attracted considerable attention, but its exact composition remains a subject of investigation. Sulfur compounds, salts, and radiation-processed materials have all been considered as possible contributors. Europa sits inside Jupiter’s intense magnetosphere, so energetic particles constantly strike its surface and drive chemical reactions in the ice.
There is a particularly interesting possibility here. If material from the ocean can reach the surface, then some of the chemistry we see on the outside might tell us something about what is happening underneath. The reverse could also happen: oxidants and other compounds produced at the irradiated surface might eventually be transported downward, potentially supplying chemical energy to the ocean. In other words, Europa’s ice may be doing more than keeping the ocean frozen. It might also be helping connect the ocean to the rest of the moon.
That matters because water by itself is a fairly poor definition of habitability. Mars has water. Comets have water. The outer Solar System is full of water ice. To make a genuinely interesting environment for life, you need more: an energy source, useful chemistry, and enough stability for those processes to persist.
The ocean provides liquid water. Tidal heating supplies energy to the interior. And the interaction between seawater and the rocky seafloor could produce chemical gradients. On Earth, organisms living around hydrothermal vents exploit chemical energy rather than sunlight, allowing ecosystems to flourish in places where photosynthesis is impossible. We have no evidence that Europa has hydrothermal vents, but the moon’s ocean-rock boundary is exactly the sort of environment where planetary scientists would like to look for them.
This is also why the phrase “Europa has an ocean” undersells the issue. The question is not simply whether there is liquid water somewhere under the ice. We want to know what the water is made of, how salty it is, how it exchanges heat with the surrounding ice and rock, whether oxidants from the surface reach it, and whether chemical reactions at the seafloor can provide a persistent source of usable energy.
Those are difficult measurements to make when the thing you want to measure is several kilometers underground. Fortunately, Europa has a magnetic field problem.
Europa does not generate a strong global magnetic field of its own. Instead, its electrically conductive ocean responds to Jupiter’s changing magnetic field, producing an induced magnetic field around the moon. By measuring how Europa responds as it moves through Jupiter’s magnetosphere, spacecraft can infer properties of the hidden ocean, including its conductivity and, indirectly, its extent.
It is a remarkably indirect way to find an ocean. Nobody has to drill through the ice. Nobody has to lower a probe into the water. You measure magnetism outside the moon and use electromagnetic physics to work backward toward the thing you cannot see.
Europa Clipper is going to do considerably more of this detective work. NASA launched the spacecraft in October 2024, and it is scheduled to reach the Jupiter system in 2030. Rather than orbiting Europa itself, Europa Clipper will orbit Jupiter and make 49 close flybys of Europa, repeatedly passing over different regions of the moon. Its nine scientific instruments will investigate the surface, atmosphere, magnetic environment, composition, and interior structure. An ice-penetrating radar instrument will be particularly important because it can probe beneath the surface and help determine the structure of the ice shell.
The mission is sometimes described as a search for life, although that description is also a bit misleading. Europa Clipper isn’t going to land, drill through the ice, scoop up ocean water, and announce whether anything is swimming around down there. Its immediate goal is to determine whether Europa possesses the physical and chemical conditions that could support life. That means characterizing the ocean, understanding the ice shell, identifying surface compounds, investigating possible exchanges between the surface and interior, and looking for evidence of an active seafloor. That’s still only a small step in determining whether Europa contains life or not. We need to first understand the environment before we begin making claims regarding the presence of life.
That distinction is important because habitability and life are very different claims. A world could have liquid water, organic molecules, chemical energy, and billions of years of geological activity and still be sterile. We have exactly one confirmed example of life in the universe, so there is a considerable amount we are assuming when we decide which planetary environments “should” be habitable. Europa gives us a chance to test some of those assumptions.
Earth’s biosphere is powered overwhelmingly by sunlight, but Europa’s ocean receives essentially none. If organisms exist there, they would need another source of energy. The most obvious candidates involve chemical reactions, perhaps powered by differences in the oxidation state of materials entering the ocean or by reactions between seawater and the rocky mantle.
That would make Europa especially interesting in the broader search for life beyond Earth. We have spent much of the modern era looking for planets in the habitable zones of other stars, where temperatures could allow liquid water on the surface. Europa belongs to a different category entirely. Its potentially habitable environment exists far from the Sun, beneath a frozen surface, powered by tidal interactions rather than starlight. A planet does not have to look like Earth to contain an environment worth investigating.
If Europa turns out to contain life, the implications would extend far beyond Jupiter. We would have two examples of biology arising in the same Solar System but under dramatically different conditions: one on the surface of a rocky planet, the other potentially in a dark ocean beneath the ice of a moon. That would tell us that life can emerge in environments very different from the one we happen to inhabit (which we already can theorize about, but this would provide solidified evidence of that).
If Europa turns out to be lifeless, that result will be interesting too. Finding an ocean with abundant water, available energy, active chemistry, and billions of years of stability but no evidence of biology would open up the question as to why it wasn’t able to sustain life, or whether it is in the early stages of developing it.
Right now, our information regarding Europa is very minimal. We know that its surface is geologically young. We have strong evidence for a subsurface ocean. We understand the basic mechanism that can keep that ocean liquid, and we have plausible sources of chemical energy within it. We can even use Europa’s response to Jupiter’s magnetic field to study the ocean. What we cannot yet do is look beneath the ice and see underneath it directly.
That is probably the most compelling thing about Europa. From the outside, it is a frozen moon covered in cracks. Those cracks reveal tidal stresses; the stresses reveal an active interior; the interior points toward a global ocean; and the ocean raises the possibility of chemistry that has been running for billions of years in complete darkness. The interesting part of Europa is therefore the part we cannot see. Which, inconveniently for anyone trying to photograph it, is also the part that matters most.
I like to wonder about the possibility that, even if we confirm that Europa isn’t able to contain life, we can still send microorganisms or other simple forms of life, and see how they evolve on the moon. Basically, artificially kickstart the process of life on another world that has all the necessary components to sustain it. But, that could probably be the topic for another blog post.
Thank you for listening to my rant.