An ocean under the ice, and a spacecraft on its way

Europa is a little smaller than Earth’s Moon, and from a distance it looks like a cracked billiard ball. Its surface is water ice, cold enough to be as hard as rock and crossed by thousands of reddish-brown ridges and fractures. Beneath that shell, most planetary scientists are now confident, lies a global ocean of salty liquid water perhaps 60 to 150 kilometers deep.

The volume is easy to estimate and surprising when you do. A layer 100 kilometers deep on a world 1,560 kilometers in radius holds about 2.8 billion cubic kilometers, roughly twice all of Earth’s oceans, inside a body with less than a hundredth of Earth’s mass. It is one of the few places in the solar system where liquid water, chemical energy and the right elements might have been in contact for billions of years.

How we know it is there

No one has seen the ocean. The strongest evidence is an inference from magnetism, and it is an elegant one. Jupiter’s magnetic field is tilted relative to its rotation, so as the planet spins, every moon sees the field swing back and forth about once every eleven hours. A swinging field induces electric currents in any conductor, and those currents generate a field of their own that flips in step. NASA’s Galileo spacecraft, flying past Europa repeatedly between 1996 and 2000, measured exactly that flipping signature. Ice does not conduct well enough to produce it. A global layer of salt water does.

Other clues have accumulated since. Telescopes have found sodium chloride, ordinary table salt, in young regions of the surface. The James Webb Space Telescope has detected carbon dioxide concentrated in a disrupted region called Tara Regio, suggesting that carbon from below has reached the surface. Tentative detections of water plumes erupting into space have been reported several times, though none has been confirmed, and Webb’s targeted search did not see one.

Why it has not frozen

Sunlight at Jupiter is about one twenty-seventh as strong as at Earth, far too weak to keep an ocean liquid. The heat comes from gravity. Io, Europa and Ganymede are locked in an orbital rhythm: for every orbit Ganymede completes, Europa completes two and Io four. The repeated tugs keep Europa’s orbit slightly non-circular, so Jupiter’s tide squeezes and relaxes the moon on every 3.5-day circuit. Some of that flexing ends up as heat.

The same arrangement bears on whether anything could live there. A habitable ocean needs chemical energy as well as warmth, a supply of reactive compounds that organisms could exploit. Jupiter’s radiation does part of that job, splitting surface ice into oxidants. Those molecules are only useful to an ocean if they can get down to it.

The ice shell question

That makes the thickness of the ice the central question, and in the past year it got its first direct answer. Earlier estimates ranged from about 3 kilometers to about 45. Then scientists used Juno’s microwave radiometer, built to study Jupiter’s atmosphere, to look down into the ice during a 2022 flyby. Published in Nature Astronomy, the analysis finds that the data fit a conductive shell about 29 kilometers thick, give or take 10, for pure water ice, in the region Juno observed.

The result comes with qualifications. Modest salinity would thin the estimate by around 5 kilometers, while a warmer convecting layer below would thicken the total. Juno also detected small scatterers in the upper ice, cracks or pores no bigger than a few inches across and reaching depths of hundreds of feet, and the authors concluded that these alone are unlikely to carry material between the surface and the ocean. If the shell really is this thick over large areas, getting surface chemistry to the water takes something more dramatic, such as the churning that may have produced the chaos terrain.

Europa Clipper is not designed to detect life. It is designed to find out whether life would have a chance.

Forty-nine passes

Europa Clipper launched on 14 October 2024 and swung past Mars in March 2025 to gain speed. On 3 December 2026 it flies past Earth at about 3,200 kilometers, its final gravity assist, and a useful opportunity to calibrate its magnetometer against a known field. It reaches Jupiter in April 2030.

It will not orbit Europa, because the radiation there would destroy its electronics within months. Instead it will orbit Jupiter and make 49 close flybys, dipping in and retreating each time. Its instruments include an ice-penetrating radar to measure the shell’s thickness across much of the moon, a magnetometer to refine the ocean’s depth and saltiness, spectrometers to map surface chemistry, and a dust analyzer to sample any particles thrown up from the surface. Juno’s single strip of data will become a starting point for testing a global map.

Europa Clipper is not designed to detect life. It is designed to find out whether life would have a chance. That distinction matters for how to read its results. A thin spot in the ice, a plume or an unusual salt would each be important, and none would be a sign of biology. The harder lesson of recent claims on Mars is how long that next step can take.

It will not be alone. The European Space Agency’s JUICE mission, launched in 2023, will reach Jupiter in 2031 and make two Europa flybys before settling into orbit around Ganymede, another moon thought to hide an ocean. Between them, the two spacecraft will give the first detailed look at the ocean worlds of the outer solar system.

Sources

• Levin et al., “Europa’s ice thickness and subsurface structure characterized by the Juno microwave radiometer”, Nature Astronomy, 2025
• Kivelson et al., “Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa”, Science, 2000
• Trumbo and Brown, “The distribution of CO₂ on Europa”, Science, 2023
• Villanueva et al., “Endogenous CO₂ ice mixture on the surface of Europa”, Science, 2023
• NASA: Europa Clipper mission