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In ancient Greek religion, Io was a princess beloved of Zeus, who turned her into a white heifer to disguise her from Hera, his goddess wife. Quite unlike the passive heifer, Io the moon is one of the most interesting and exciting objects in the solar system. Io, shown in the upper color insert on page C-5, is the most volcanically active body in the solar system, and aside from Earth, the only other body on which man has actually witnessed volcanic activity.There are 150 to 300 volcanic hot spots on Io, along with lava lakes and caldera-like depressions called paterae (patera is a planetary geology term for a shallow, irregular crater; see the sidebar "Fossa, Sulci, and Other Terms for Planetary Landforms" on page 146). Io has huge volcanic plumes and visible lava lakes, as well as mountains 52,000 feet (16 km) high, twice as high as Mount Everest. On Io, the mountains may actually be formed in ways similar to the ways mountains are formed on Earth: There is photographic evidence for plate tectonics of a sort, with subduction and thrusting of crustal plates forming mountains. Io's mountain Mongibello Mons is higher than any mountain in North America.

Io has valleys as much as 2.5 miles (4 km) deep, 200 dark fresh lava flows, and 100 active volcanic hot spots. Io is so volcanically active that it is constantly sending lighter elements into its atmosphere, where they are stripped away by Jupiter's magnetic field.This process may be what has made Io the densest body in the outer solar system. Though Io is just a little bit larger than Earth's Moon, it has a metallic core that may make up 50 percent of its radius, and its heat output is even greater than the Earth's. Some of Io's surprising physical characteristics are listed in the table here, including its high density and commensurately high gravity and its magnetic field strength.

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