Which moon of Jupiter has the strongest evidence for a subsurface ocean of liquid water?

• A. Europa
• B. Ganymede
• C. Callisto
• D. Io

Answer: (A)

Evidence for a subsurface ocean on Europa comes most directly from how its interior responds to Jupiter’s magnetic field. The Galileo mission detected an induced magnetic field that changes with Jupiter’s field as the moon orbits, which can only be produced by a global, conductive layer beneath the icy shell—most plausibly a salty liquid water ocean. This magnetic signal points to a widespread ocean rather than a localized pocket, making it the strongest single line of evidence.

Surface geology also supports the idea: Europa’s cracked, disrupted ice and telltale lineae suggest a mobile, interacting ice shell above a liquid layer, where tides keep the interior warm enough to prevent freezing solid. While other moons show hints—such as possible ocean indicators on Ganymede or a lack of ocean evidence on Callisto and Io—the combination of a clear magnetic induction signal plus compatible surface features makes Europa the best-supported case for a subsurface ocean.