1979

An Ocean on Europa?

Since its discovery in 1610 by Galileo, very little was known about Jupiter’s second large satellite, Europa, except that it was locked into a 4:2:1 (Io:Europa:Ganymede) orbital resonance, and thus likely to experience interesting tidal forces, and that spectroscopy from telescopes revealed it to have a bright, water-ice-covered surface. In terms of the detailed nature of Europa’s surface, the slate was almost completely blank when the Voyager 1 and 2 probes made the first detailed reconnaissance of Jupiter’s system.

Voyager’s discoveries at Europa didn’t disappoint. The planet-size moon (only slightly smaller than Mercury) turned out to have one of the smoothest surfaces in the solar system. But that smooth surface is covered by a dense and intersecting network of cracks and low ridges that appear to divide the surface into icy plates that move relative to each other. This was one of the first clues that the surface is a relatively thin, tectonically active shell of ice that is floating on a thick layer of liquid water—an ocean—in the subsurface. Other clues that Europa is an ocean world come from the observation that there are very few impact craters on the surface, suggesting that it is geologically young and actively resurfacing itself. The NASA Galileo Jupiter Orbiter enabled much more detailed study of Europa during its mission from 1995 to 2003. More evidence for the presence of a deep ocean came from Galileo spectrometer observations of salty mineral deposits within some of the cracks and fractures—the kind of mineral deposits that would form from the evaporation of salty ocean water. Galileo magnetic field data also showed that the subsurface of Europa is electrically conductive, yet another indicator or the presence of salty ocean water beneath the icy crust.

The available data so far are consistent with Europa having a deep (60-mile [100-kilometer]) global ocean underneath a relatively thin (6- to 20-mile [10- to 30-kilometer]) icy shell, but direct proof of that ocean must await future missions that will orbit or land on (and possibly drill through) the Europan crust. In the meantime, astrobiologists have become excited about the prospects for Europa as a potential abode of life. With energy from tidal heating by Jupiter, organic molecules from a steady rain of comets and asteroids, and abundant liquid water, we may one day discover that Europa’s ocean is a habitable—or perhaps even inhabited—environment.

Voyager 2 photo of part of the flat, icy surface of Jupiter’s second moon, Europa. The surface is crisscrossed with countless cracks that look like plates of sea ice floating on liquid water in many places. The surface has very few impact craters, meaning it is geologically very young.