Europa was discovered by Galileo in 1610, along with the three other large satellites of Jupiter — lo, Ganymede, and Callisto. The four are now collectively called the Galilean satellites. Europa travels around Jupiter at an orbital distance of about 9.5 times the radius of Jupiter (9.5 RJ, or 670,900 km), which puts it deep within the strong jovian magnetic field and its associated radiation belts. With a radius of 1565 kilometers, Europa is 90% the radius of Earth's Moon. Its surface gravity is only some 13% that of Earth, and it has an escape velocity of about 2 km/s.
Europa's surface is highly reflective, and characteristic absorptions at certain wavelengths in the reflected sunlight, measured using ground-based telescopes beginning in the 1950s, indicate the presence of water ice. However, the mean density of Europa, about 3000 kg m-3, is substantially higher than that of ice (about 1000 kg m-3) and is lower than that of rock (about 3400 kg m-3, including the compression that occurs deep inside Europa's interior), implying that Europa consists of a mixture of water and rocky material.*
The Voyager spacecraft showed the surface to be relatively free of impact craters, suggesting that Europa's surface is younger than the surfaces of Ganymede and Callisto. In addition, Voyager data revealed that ice tectonics shapes Europa's surface geology and raised the possibility that liquid water might exist beneath its icy surface. In 1994, observations made by the Hubble Space Telescope revealed the presence of a tenuous oxygen-bearing atmosphere, probably formed as a result of the impact of energetic particles trapped by Jupiter's magnetic field onto the icy surface.
Since 1995 when the Galileo spacecraft began its mission in the jovian system, significant new discoveries about the properties of Europa's interior, surface, and atmosphere have been made. Galileo's images revealed the existence of ice blocks on the surface that appear to have drifted like icebergs from their original positions. Moreover, Galileo's magnetic and gravitational measurements provided additional indications, but not conclusive proof, of the likely presence of liquid water beneath a relatively thin layer of surface ice.**
While the results from telescopic observations, theoretical studies, and data from Voyager indicated that Europa was a fascinating body for additional study, it was the results from Galileo that raised the serious possibility that Europa is a potential abode of life.
subsurface ocean.5 Although multiple forms of metabolism are possible, it is likely that europan life forms, should they exist, would probably utilize chemical energy rather than photosynthesis to support metabolism. Thus, they might resemble terrestrial organisms found in environments that are considered hostile by human standards, such as hot springs and deep-sea thermal vents; these terrestrial organisms are often called "extremophiles." Certainly, a search for evidence of a liquid ocean and for the extent to which either prebiotic chemical activity or biological activity has progressed on Europa is warranted based on the currently available data, and the information obtained from such a search may help us to understand the chemical, prebiological, and biological evolution of our solar system.
In addition to the search for liquid water and the potential for the existence of either present or past life, the occurrence of relatively recent geologic processes on Europa makes it an appropriate and high-priority target for detailed exploration. Evidence for resurfacing and ice tectonics, dynamic interactions between the surface,