The Icy Moon
Europa is a unique moon of Jupiter that has fascinated scientists for hundreds of years. Its surface is among the brightest in the solar system, a consequence of sunlight reflecting off a relatively young icy crust. Its face is also among the smoothest, lacking the heavily cratered appearance characteristic of Callisto and Ganymede. Lines and cracks wrap the exterior as if a child had scribbled around it. Europa may be internally active, and its crust may have, or had in the past, liquid water which can harbor life.
Europa is named after the beautiful Phoenician princess who, according to Greek mythology, Zeus saw gathering flowers and immediately fell in love with. Zeus transformed himself into a white bull and carried Europa away to the island of Crete.
He then revealed his true identity and Europa became the first queen of Crete. By Zeus, she mothered Trojan war contemporaries Minos, Rhadamanthus, and Sarpedon. Zeus later re-created the shape of the white bull in the stars which is now known as the constellation Taurus.
The fascination with Europa began centuries ago in 1610 when Galileo Galilei discovered four Jovian satellites: Io, Callisto, Ganymede, and Europa. But only recently have we begun to learn more about the sphere. About forty years ago, modern astronomer Gerard Kuiper and others showed that Europa’s crust was composed of water and ice. In the 1970s, space exploration of Jupiter’s satellite system began with the Pioneer and Voyager fly-by missions which verified Kuiper’s analysis of Europa and discovered other characteristics. In 1995, the Galileo spacecraft began gathering more detailed images and measurements within the system, providing the information needed to piece together Europa’s past, present, and future.
Layers of Europa
It is estimated that Europa has an outer layer of water around 100 km (62 mi) thick; a part frozen as its crust, and a part as a liquid ocean underneath the ice. Recent magnetic-field data from the Galileo orbiter showed that Europa has an induced magnetic field through interaction with Jupiter’s, which suggests the presence of a subsurface conductive layer. This layer is likely a salty liquid-water ocean. Portions of the crust are estimated to have undergone a rotation of nearly 80°, nearly flipping over (see true polar wander), which would be unlikely if the ice were solidly attached to the mantle. Europa probably contains a metallic iron core.
Europa is the smoothest known object in the Solar System, lacking large-scale features such as mountains and craters. However; according to one theory, Europa’s equator may be covered in icy spikes called penitentes, which may be up to ten meters high, due to direct overhead sunlight on the equator, causing the ice to sublime forming vertical cracks. The prominent markings crisscrossing Europa appear to mainly be albedo features that emphasize low topography. There are few craters on Europa, because its surface is tectonically too active and therefore young. Europa’s icy crust has an albedo (light reflectivity) of 0.64, one of the highest of all moons. This indicates a young and active surface, based on estimates of the frequency of cometary bombardment that Europa likely experiences, the surface is about 20 to 180 million years old. There is currently no full scientific consensus among the sometimes contradictory explanations for the surface features of Europa.
Scientists’ consensus is that a layer of liquid water exists beneath Europa’s surface, and that heat from tidal flexing allows the subsurface ocean to remain liquid. Europa’s surface temperature averages about 110 K (−160 °C; −260 °F) at the equator and only 50 K (−220 °C; −370 °F) at the poles, keeping Europa’s icy crust as hard as granite. The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa’s slightly eccentric orbit and orbital resonance with the other Galilean moons). Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images. The most dramatic example is “chaos terrain”, a common feature on Europa’s surface that some interpret as a region where the subsurface ocean has melted through the icy crust. This interpretation is controversial. Most geologists who have studied Europa favor what is commonly called the “thick ice” model, in which the ocean has rarely, if ever, directly interacted with the present surface. The best evidence for the thick-ice model is a study of Europa’s large craters. The largest impact structures are surrounded by concentric rings and appear to be filled with relatively flat, fresh ice; based on this and on the calculated amount of heat generated by Europan tides, it is estimated that the outer crust of solid ice is approximately 10–30 km (6–19 mi) thick, including a ductile “warm ice” layer, which could mean that the liquid ocean underneath may be about 100 km (60 mi) deep. This leads to a volume of Europa’s oceans of 3 × 1018 m3, between two or three times the volume of Earth’s oceans.
The thin-ice model suggests that Europa’s ice shell may be only a few kilometers thick. However, most planetary scientists conclude that this model considers only those topmost layers of Europa’s crust that behave elastically when affected by Jupiter’s tides. One example is flexure analysis, in which Europa’s crust is modeled as a plane or sphere weighted and flexed by a heavy load. Models such as this suggest the outer elastic portion of the ice crust could be as thin as 200 metres (660 ft). If the ice shell of Europa is really only a few kilometers thick, this “thin ice” model would mean that regular contact of the liquid interior with the surface could occur through open ridges, causing the formation of areas of chaotic terrain.