Europa is slightly smaller than Earth's Moon. Underneath its bright, cracked, nearly crater-free ice shell is a global ocean of salty liquid water that is probably twice the volume of all Earth's oceans combined. Of every place outside Earth that humans or their instruments could plausibly reach, Europa is the place where "is there liquid water and a heat source and the right chemistry for life?" gets three tentative yeses.
How We Know There's an Ocean
Europa's surface is a near-pristine layer of water ice, criss-crossed by long dark cracks but almost entirely free of impact craters. That absence of craters means the surface is young in geological terms — resurfaced within the last few tens of millions of years, possibly within the last few — and something has to resurface it.
The clearest evidence for the ocean itself came from the Galileo orbiter in the 1990s. Galileo detected a weak induced magnetic field around Europa that could only be explained by a conductive layer under the surface. The best-fitting explanation is a global ocean of salty water, electrically conductive enough to be pushed around by Jupiter's much stronger rotating magnetic field. The same instrument confirmed similar, weaker signatures at Ganymede and Callisto.
A third line of evidence is surface chemistry. Europa's surface contains salts and sulfates that appear to have come from below and been deposited during resurfacing events. In at least a few cases, the Hubble Space Telescope and later missions have observed what look like plumes of water vapour erupting from Europa's surface, similar to but weaker than the Enceladus plumes. Plume observations have been inconsistent, and confirming them is one of the science goals of Europa Clipper.
The Habitability Case
Life as we know it needs three things: liquid water, an energy source, and the right chemistry (carbon, nitrogen, and so on). Europa has all three. The ocean is liquid, kept so by tidal heating driven by Jupiter's gravity and by resonant tugs from the other Galilean moons. The ocean floor is rock, probably with active hydrothermal vents that can drive chemistry of the kind that supports life at Earth's mid-ocean ridges. And the surface above the ocean is bathed in radiation that produces oxidants like molecular oxygen and hydrogen peroxide in the ice, which can then be delivered down into the ocean by resurfacing cycles.
Taken together, Europa has a more complete basic set of habitability ingredients than Mars does. What it lacks is accessibility. Any life there is probably in the ocean, under kilometres of ice, hard to reach in a way that Mars surface chemistry is not.
Europa Clipper
Europa Clipper launched on a Falcon Heavy in October 2024. It will arrive at Jupiter in 2030 and will not orbit Europa. Orbiting Europa directly is very difficult: you sit inside the worst of Jupiter's radiation belts constantly, and the perturbations from Jupiter's gravity make the orbit unstable without frequent station-keeping.
Clipper instead orbits Jupiter in a wide, looping orbit and makes roughly 50 close flybys of Europa over the primary mission. Most of each orbit is spent outside the heaviest radiation, letting the spacecraft cool and transmit data back to Earth. Each flyby passes close to Europa — a few hundred kilometres — and runs every instrument at once during the brief pass.
The instrument payload is designed to answer three questions: how thick is the ice shell, how deep is the ocean beneath it, and what is the chemistry at the ice-ocean interface? Ice-penetrating radar will probe the shell directly. Magnetometer measurements will refine the ocean's depth and salinity. Mass spectrometers will sample any tenuous atmosphere Europa has — including any plume material — and look for organic molecules.
Clipper is not a life-detection mission. It is designed to establish whether Europa is habitable, not whether it is inhabited. A later lander or ice-penetrating probe would be needed to look for biology directly.
JUICE's Europa Flybys
ESA's JUICE mission will do two close Europa flybys on its way to settling into Ganymede orbit. The Europa flyby science is secondary to the Ganymede and Callisto work, but JUICE carries its own instruments complementary to Clipper's, and the two missions operating in the Jupiter system at the same time is one of the more useful coincidences of scheduling in recent planetary science.
What Comes After Clipper
The obvious next step is a Europa lander or a cryobot — an ice-penetrating probe designed to melt its way down through the shell and into the ocean. Both have been studied; neither is funded. The engineering is genuinely hard: a cryobot has to be sterile to avoid contaminating a potentially habitable ocean, self-powered for a journey that could take years, and able to communicate back through kilometres of ice. These are not impossible requirements, but they are significantly harder than anything currently flying.
For now, the real ambition is one programme ahead. Clipper's job is to tell the field whether it is worth building that cryobot. If the answer is yes, a Europa surface or subsurface mission is the obvious successor flagship.