Europa, one of Jupiter’s largest moons, is considered one of the most promising places to find life beyond Earth. It almost certainly harbors a global saltwater ocean beneath its icy surface, and that ocean holds roughly twice the volume of all Earth’s oceans combined. Whether anything actually lives there remains unknown, but Europa checks every box scientists use to define a habitable environment: liquid water, chemical energy, and the raw ingredients for biology.
What Makes a World Habitable
When astrobiologists evaluate whether a place could support life, they look for three things: liquid water, a source of chemical energy, and the right elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur). Europa appears to have all three. Liquid water exists in enormous quantities beneath the ice. Jupiter’s radiation bombards the surface and creates oxygen and other reactive chemicals that could fuel biological processes if they reach the ocean. And the rocky seafloor likely interacts with seawater in ways that release additional energy and nutrients.
All known life on Earth extracts energy from chemical gradients, specifically the difference between oxidizing and reducing compounds. Europa seems to produce both: oxidants form on the irradiated surface, while the seafloor is expected to be chemically reducing. That combination is strikingly similar to the chemistry that powers microbial ecosystems at hydrothermal vents on Earth.
A Hidden Ocean Deeper Than Earth’s
Europa is slightly smaller than Earth’s Moon, yet it holds an astonishing amount of water. Its ocean averages about 100 kilometers deep, compared to Earth’s average ocean depth of roughly 4 kilometers. In total volume, Europa’s ocean is estimated at around 3 billion cubic kilometers, dwarfing Earth’s 1.4 billion. All of this water sits beneath a global ice shell estimated to be anywhere from 3 to 30 kilometers thick, though tidal heating models suggest the shell is likely no more than 12 to 17 kilometers on average.
The evidence for this ocean comes from multiple sources. NASA’s Galileo spacecraft, which orbited Jupiter from 1995 to 2003, detected a magnetic field signature consistent with a salty, electrically conductive liquid layer beneath the ice. The moon’s surface geology, covered in fractured ridges and jumbled “chaos terrain,” also points to an ocean that shifts and reshapes the crust above it.
How Tidal Heating Keeps the Ocean Liquid
Europa orbits Jupiter at a distance where sunlight alone provides almost no warmth. The ocean stays liquid because of tidal heating. Jupiter’s enormous gravity, combined with gravitational tugs from neighboring moons Io and Ganymede, keeps Europa’s orbit slightly elliptical. As Europa moves closer to and farther from Jupiter during each orbit, the planet’s gravity flexes the moon’s interior, generating heat through friction.
This process is self-sustaining. Tidal dissipation would normally circularize Europa’s orbit and shut off the heating, but the gravitational resonance with Io and Ganymede continuously maintains the orbital eccentricity. The heat generated in Europa’s rocky interior appears sufficient to drive high-temperature hydrothermal activity on the seafloor, similar in principle to the volcanic vent systems found along mid-ocean ridges on Earth.
What the Ocean Might Contain
Europa’s ocean is not pure water. Spectral observations of the surface reveal salts including magnesium sulfate, sodium chloride, and various sulfur compounds. Models of how water would leach elements from a rocky mantle predict that magnesium, sulfate, sodium, and chloride are the ocean’s major dissolved components. In some ways, it resembles a very salty version of Earth’s seawater, though the exact proportions differ.
The ocean’s acidity is a major open question. If surface-generated oxidants have been delivered to the ocean over billions of years and reacted with dissolved compounds like hydrogen sulfide, the pH could be as low as 2.6, roughly as acidic as lemon juice. That would be a serious challenge for most forms of life. On the other hand, if the rocky seafloor dissolves enough alkaline minerals to buffer the acid, or if oxidant delivery from the surface is limited, the ocean could be neutral or even slightly alkaline. Scientists have proposed pH values spanning the full range from highly acidic to above 10. Resolving this uncertainty is one of the key goals of future exploration.
Carbon From Below, Oxidants From Above
In 2023, the James Webb Space Telescope identified carbon dioxide concentrated in a specific region of Europa’s surface called Tara Regio, a geologically young area of disrupted “chaos terrain.” Analysis indicated the carbon likely originated in the subsurface ocean rather than arriving from meteorite impacts or other external sources, and it was deposited recently in geological terms. Previous Hubble observations had already found ocean-derived salts in the same region, so the carbon dioxide discovery added another piece of evidence that material cycles between the ocean and the surface.
Meanwhile, Jupiter’s intense radiation continuously breaks apart water ice and other molecules on Europa’s surface, producing oxygen, hydrogen peroxide, and other oxidants. If geological processes transport this irradiated material downward through the ice shell and into the ocean, those oxidants could serve as a chemical food source for microorganisms. On Earth, some bacteria thrive entirely on the energy released by combining oxygen with reduced chemicals from hydrothermal vents. A similar energy pathway is theoretically available on Europa.
Evidence for Water Plumes
One of the most tantalizing discoveries is evidence that Europa occasionally vents water vapor into space. In 2012, the Hubble Space Telescope captured ultraviolet images suggesting plumes above the moon’s surface. Then, in 2018, researchers reanalyzed data from a 1997 Galileo flyby that had passed about 200 kilometers above Europa. Using advanced 3D plasma modeling, they found that a brief, localized bend in the magnetic field data, along with plasma wave measurements, matched what a water plume would produce. The simulated plume aligned with both the Galileo data and the Hubble observations.
If confirmed, these plumes would mean a future spacecraft could potentially sample Europa’s ocean material without ever drilling through the ice. Robert Pappalardo, project scientist for NASA’s Europa Clipper mission, described the combined evidence as a “tipping point,” saying the plumes seem increasingly real.
What Europa Clipper Will Look For
NASA’s Europa Clipper spacecraft launched on October 14, 2024, and is currently en route to Jupiter. Once it arrives, it will orbit Jupiter and perform nearly 50 close flybys of Europa, swooping as low as 25 kilometers above the surface. Each pass will cover a different location, eventually scanning nearly the entire moon.
The mission is designed specifically to assess Europa’s habitability. Its instruments will measure the ice shell’s thickness, map the surface composition in detail, search for plume activity, and characterize the ocean’s depth and salinity using radar and gravity measurements. Europa Clipper will not detect life directly. Instead, it will determine whether the ocean environment has the conditions that could support it, giving scientists the clearest picture yet of whether this frozen moon hides a world where biology is possible.