Though Io and Europa are neighbors orbiting Jupiter, the two moons are extremely different. Io has volcanoes and mountains, and Europa has none. Europa has a significant ice fraction in its composition, while Io has none. Europa's surface is younger than Callisto's and

Ganymede's, and its icy surface makes it one of the brightest objects in the solar system: Its albedo is 0.64; in comparison, the Moon's albedo is only 0.07, though Neptune's moon Triton has an albedo of 0.7. Like Io, Europa also has a small magnetic field, about 10-7Tesla.

Europa is named after a lovely Phoenician princess from Greek mythology. Zeus saw Europa gathering flowers and immediately fell in love with her. Zeus transformed himself into a white bull and carried Europa away to the island of Crete. He then revealed his true identity and made Europa the first queen of Crete. Her sons fought in the Trojan War against Greece. Zeus later re-created the shape of the white bull in the stars, which is now known as the constellation Taurus.

Europa's surface is relatively flat, having no very high mountains or great basins, but it does display linear surface features, ridges, and troughs, both singly and in sets ("en echelon" in geologic nomenclature), along with craters and chaos terrain. Chaos terrains are circular or elliptical regions where the lineations of the icy crust have been erased and replaced with jumbled, broken ice. In places, early flat plains are still visible, but 22 percent of the surface has been resurfaced with chaos terrain, and 30 percent by other tectonic activity. (The early plains can be identified simply by the fact that the other tectonic features are clearly placed on top of and in these plains.) Fifty percent of Europa has been resurfaced in the last 50 to 100 Ma, which is a very short time geologically speaking. Like Io, therefore, Europa has a surface currently being resurfaced, though with ice in its case rather than molten rock.

Two examples of chaos terrain are shown in this book. In the first, shown on page 82, irregular blocks of water ice were formed by the shattering and movement of the existing crust.The blocks were shifted, rotated, tipped, and partially submerged within a mobile material that was either liquid water, warm mobile ice, or an ice and water slush. The presence of young fractures cutting through this region indicates that the surface froze again into solid, brittle ice. This first example of chaos terrain has blocks of ice with straight lines and the angles associated with brittle fracture, but this second feature shows round shapes and curves. In the second example, Thera and Thrace are two dark, reddish regions of enigmatic terrain that disrupt the older icy ridged plains on Europa (see the upper color insert on page C-7).Thera (left) is about 43 x 53 miles (70 x 85 km) and appears to

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