Having entered orbit around Jupiter a year earlier, the Galileo spacecraft swooped within 692 kilometers of Europa's surface, returning images of a strange, fractured icy shell crisscrossed with dark linear features that scientists interpreted as evidence of a liquid water ocean below, transforming the moon into one of the solar system's most tantalizing targets in the search for extraterrestrial life.
Europa had long been a point of fascination since Voyager's flybys in 1979 revealed its strange, smooth, and crack-veined surface, utterly unlike the heavily cratered moons typical elsewhere in the solar system. Galileo's close pass allowed scientists to study these features in unprecedented detail, finding jumbled, chaotic terrain resembling pack ice on Earth's polar seas—a strong visual argument that a liquid or slushy ocean churned beneath the frozen shell.
The data gathered fundamentally reoriented planetary science's search for habitable environments beyond Earth. Rather than looking only at planets in a star's traditional habitable zone, scientists began seriously considering icy moons warmed by tidal flexing from a giant planet's gravity as plausible abodes for life. Europa's suspected ocean, kept liquid by tidal heating from Jupiter's immense gravitational pull, is now believed to contain more water than all of Earth's oceans combined, and it remains a top target for future missions, including NASA's Europa Clipper, designed to probe deeper into the question Galileo's flyby first crystallized.