| Europa (moon
Discovered by: Galileo Galilei,
Europa is the smallest of Jupiter's four planet-sized moons, yet it is only slightly smaller than Earth's Moon. Europa is somewhat similar in bulk composition to the other terrestrial planets (primarily composed of silicate rock). Recent data from Galileo indicate that Europa has a layered internal structure perhaps with a small metallic core. However, Europa's surface is not at all like anything in the inner solar system. Its surface is exceedingly smooth with few features more than a few hundred meters high.
There are very few craters on Europa; only three craters larger than 5 km in diameter have been found. From the observations of water ice absorption bands, and due to the near absence of impact craters, we have inferred that the surface is ice rich and also very young and active, perhaps only 30 million years old. The precise age of Europa's surface is unknown. Voyager mapped only a fraction of the surface at high resolution. The images of Europa's surface strongly resemble images of sea ice on Earth. Scientists have postulated that a water-ice shell covers Europa and is more than 150 kilometers thick. It is possible that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat due to the pull of Jupiter and its other moons. If so, it would be the only place known in the Solar System besides Earth where liquid water exists in significant quantities.
Europa's most striking surface features are the series of dark streaks or cracks that crisscross the entire globe. The larger of these streaks or cracks are roughly 20 km across with diffuse outer edges and a central band of lighter material. These features indicate that the surface ice sheets of Europa are tectonically active. The latest theory for their origin is that they are produced by a series of volcanic eruptions or geysers. It is believed that these cracks are locations of eruptive sites from which liquid water has intermittently flowed out onto the surface and then frozen and thus erased the traces of impact craters. The Jupiter moon called Io has highly active volcanic systems that are driven by the pull of Jupiter. Similarly, although much less intense, heating may exist in the subsurface of Europa, accounting for the resurfacing processes on the planetary surface.
One the most compelling insights to emerge from 15 years of research on submarine volcanic-hydrothermal systems on Earth is the idea that volcanoes in the presence of liquid water can be sustained. Whether or not life can originate in these hydrothermal systems is controversial, but the evidence is unequivocal regarding the linkages between volcanic processes as we know them and abundant carbon-based life forms on and below the seafloor in the vicinity of active spreading centers.
Recent observations with the Hubble Space Telescope reveal that Europa has a very thin atmosphere (1x10-11 bar) composed primarily of oxygen. Of the 61 moons in the Solar System only four others (Io, Ganymede, Titan and Triton) are known to have atmospheres. Unlike the oxygen in Earth's atmosphere, Europa's is almost certainly not of biologic origin. It is most likely generated by sunlight and the subsequent splitting of water into hydrogen and oxygen. The hydrogen escapes leaving only oxygen.