NASA’s Galileo mission discovered strong evidence of a liquid-water ocean beneath Europa’s icy surface.
Galileo entered orbit around Jupiter in December 1995 and made repeated close encounters with Europa. Measurements of the moon’s induced magnetic field, along with geological observations, indicated a conductive layer beneath the ice, most plausibly a global salty ocean. The evidence made Europa one of the Solar System’s leading locations for searching for potentially habitable environments.
Galileo did not directly drill through Europa’s ice or photograph the ocean. Its findings are therefore evidence based on magnetic and surface data, not a direct sample. The later Europa Clipper mission is designed to conduct a much more extensive investigation of Europa, but it is not the mission that first identified the subsurface ocean evidence.