The Icy But Warm Moons of Jupiter

The three icy Galilean satellites of Jupiter—Europa, Ganymede, and Callisto—are of high geophysical and astrobiological interest, because each is inferred to hide a global subsurface ocean beneath an ice-rich surface and icy shell. Most of what we understand about these worlds is based on data from the Galileo spacecraft, which orbited Jupiter and made close flybys of these moons between 1996 and 2001. Callisto’s heavily cratered surface implies a geologically dead world, consistent with Galileo gravity data that suggests incomplete differentiation. Magnetometer data, which indicates an induced magnetic field, and morphologies of the largest impact structures are consistent with a salty ocean beneath an ice shell ~100 km thick. Salts may serve as antifreeze to permit an ocean beneath a conductive ice shell within Callisto today from radiogenic heating alone. Past tidal heating is likely for Ganymede, which is fully differentiated and possesses an intrinsic magnetic dynamo, implying a cooling iron core today. Ganymede also possesses an induced magnetic field, implying an internal salty ocean. The bright grooved terrain that covers 2/3 of Ganymede’s surface records evidence of past extensional tectonism and possibly icy volcanism, plausibly from a past increase in orbital eccentricity that amplified tidal heating. Europa currently experiences significant tidal heating from its forced orbital eccentricity, permitting an internal ocean beneath ~20–30 km of ice. The presence of an ocean is further supported by: induced magnetic field measurements, tectonic features that imply a large tidal amplitude, global tectonic patterns, and large impact structures that may have penetrated through the icy shell. Europa’s icy shell probably undergoes solid-state convection along with partial melting, based on numerical modeling and inferred surface manifestations including chaotic terrains. The probable oceans of Callisto and Ganymede are sandwiched between >100 km thick ice I above and denser ice polymorphs below; Europa's ocean is capped by a relatively thin and tidally heated ice shell and is likely in direct contact with its rocky mantle below. Consideration of the “ingredients” necessary for life—water, biogenic elements, chemical energy, and stable environment—suggests that the interior oceans of large icy bodies may be the most common habitats for life in the universe, with Europa as the prime target in the search for extant life in the outer solar system.