Europa holds an ocean containing more than twice the water of every ocean on Earth, and a new measurement says 29 kilometers of solid ice sits on top of it. Three Mount Everests, stacked end to end. NASA's Juno spacecraft measured it during a flyby at 360 kilometers on September 29, 2022, using a microwave radiometer originally designed to probe Jupiter's atmosphere. The result, published in Nature Astronomy in December 2025, settled a decades-long debate between thin-shell models proposing less than a kilometer of ice and thick-shell scenarios proposing tens of kilometers. The thick shell won.
Then last week, a second paper in the same journal closed the workaround. Lujendra Ojha at Rutgers, working with Ankit Barik at Johns Hopkins and Jacob Buffo at Dartmouth, ran fluid and thermal simulations to test whether ocean water could rise through fractures in the ice and pool in shallow reservoirs near the surface. The answer was no. Water ascending through narrow cracks moves turbulently, churning against walls at roughly minus 160 degrees Celsius. Rapid cooling pushes the liquid below its freezing point while it remains fluid, and tiny ice crystals called frazil ice nucleate, build up, and clog the pathway. Narrow fractures seal shut within hours.
## The Promise
Scientists had studied Europa's surface for evidence that the ocean touches it. Double ridges, chaos regions, and dome-like features called lenticulae were read as traces of shallow water fed from below. A 2022 Nature Communications study found an analog in Greenland's ice sheet, where fractures refroze and pressurized in patterns matching Europa's double ridges, and concluded that shallow liquid water is "spatially and temporally ubiquitous" beneath the surface. The logic ran from surface to ocean: features on top look like water made them, so water must be reaching the top.
Ojha's simulations break that chain. Any shallow reservoir that exists likely formed from local melting inside the ice, not from ocean upwelling. The water did not rise. It thawed in place.
## The Instrument
NASA launched Europa Clipper in October 2024 on a $5.2 billion trajectory that reaches Jupiter on April 11, 2030. The spacecraft carries nine instruments across 49 planned flybys at altitudes of 25 to 100 kilometers. The one most relevant here is REASON, a dual-frequency ice-penetrating radar operating at 9 and 60 MHz, built to resolve subsurface layers, buried water pockets, and the ice-ocean interface down to 30 kilometers.
REASON can see through the shell. But what it was built to find has been reframed since launch. The instrument was designed to map shallow reservoirs that scientists expected would sample the global ocean. If those reservoirs contain only locally melted ice, they carry no information about the ocean's chemistry or whether anything lives in it. The radar will still image a 29-kilometer cross-section of one of the most studied surfaces in planetary science. The cross-section now tells a different story than the one the mission was designed to read.
## The Shield
Jupiter bathes Europa in radiation sufficient to sterilize the surface. Charged particles at 100 MeV penetrate 60 to 80 centimeters of ice. At one meter depth, the dose falls to 0.3 gray per year. At 29 kilometers, it falls to nothing. The ice does not merely separate the ocean from the surface. It absorbs every particle that would destroy whatever molecular complexity the water has accumulated over billions of years.
Read together, the two Nature Astronomy papers describe a single structure with two functions. The shell that blocks access to the ocean is the shell that makes the ocean habitable. The thickness that seals fractures before water can rise is the thickness that absorbs Jupiter's radiation before it reaches the water. A thinner shell would be easier to study and harder to survive in. A permeable shell would deliver ocean chemistry to the surface and deliver sterilizing radiation to the ocean.
The fractures heal in hours. The ice is not a wall built to keep us out. It is a membrane built to keep the ocean alive.