Europa

Europa, the Moon named after a Princess in Greek Mythology, is the smallest of the Galilean Satellites of Jupiter; it is only slightly smaller than Earth’s Moon and is thus a substantial world in its own right. It is in orbital resonance with both Io and Ganymede, such that for every four complete orbits by Io, Europa makes two orbits and Ganymede one. The resulting tidal heating makes Europa an active world, and it is a prime target for exploration for extraterrestrial life in a salty ocean inferred to exist below its thin icy shell. In contrast to the more strongly tidally heated (and ice-free) Io, however, no surface changes have been seen to occur on Europa during the era of close-up imaging by voyagers 1 and 2 (1979) and the Galileo orbiter (1995–2001).

Europa’s density, of 2.97 g/cm3, almost puts it in the terrestrial planet league, but its exterior is icy down to a depth of about 100 km. It is not known whether the ice is completely solid, or whether its lower part is liquid, which raises the fascinating possibility of a global ocean sandwiched between the solid ice and the underlying rock. Gravity data from the Galileo Orbiter show that, like Io, Europa has a dense, presumably iron-rich core (about 620 km in radius) below its rocky mantle. Europa has its own magnetic field, but it is unclear whether this is generated by convection within a liquid core or within a salty ocean beneath the ice.

Europa possesses a highly reflective surface with an albedo of about 0.7. Since the 1950s it has been known from spectroscopic studies that the surface is composed essentially of clean water-ice. More detailed recent observations by the Galileo Orbiter and the Hubble Space telescope have revealed regions where the ice appears to be salty; they have also revealed the presence of molecular oxygen (O2) and ozone (O3). The oxygen and ozone are thought to result from the breakdown of water molecules in the ice by exposure to solar ultraviolet radiation and charged particles from the solar wind. Europa’s surface is relatively smooth and younger than that of other icy satellites, to judge from the paucity of impact craters. This paucity demonstrates that Europa experiences a rapid rate of resurfacing, driven by tidal heating, though it is not so rapid as on Io.

Neither of the Voyager probes passed close to Europa, so even the best Voyager images revealed few features smaller than a few hundred meters across. They were adequate, however, to show a bright surface with low topographic relief, criss-crossed by a complex pattern of cracks filled by darker ice.

Although by far the most abundant constituent of Europa’s ice is water, it is likely to be contaminated by various salts (such as Sulphates, carbonates and chlorides of magnesium, sodium and potassium) and possibly by Sulphuric acid, resulting from chemical reactions between water and the underlying rock.

Although little is understood about the processes that have shaped Europa’s surface, it is clear that the satellite has had a complicated history. We cannot tell how old each region of the surface is, but there are abundant signs that there is, or at least has been, a liquid zone below the surface. A salty ocean below several kilometers of ice is not necessarily a hostile environment for life. Indeed, life could be richer and more complex than anything that is likely to have survived on Mars. The possibility of a life-bearing ocean below the ice has set the agenda for the future exploration of Europa, beginning with NASA’s proposed Europa orbiter mission.

Questions to Ponder

• Why do astronomers think they could be life on Europa?
• How are the earth and Europa alike?