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Jupiter Moon: Ganymede!

The solar system's largest moon, Ganymede, is captured here alongside the planet Jupiter in a color picture taken by NASA's Cassini spacecraft on Dec. 3, 2000. Cassini was 26.5 million kilometers from Ganymede when picture was taken. The images were taken on March 29, 1998 at a range of 918,000 kilometers by NASA's Galileo spacecraft.

The solar system's largest moon, Ganymede is larger than the planets Mercury and Pluto and Saturn's largest moon, Titan. If Ganymede orbited the Sun instead of Jupiter it could be classified as a planet. Both Ganymede and Titan have greater surface area than the entire Eurasian continent on our planet. Ganymede is viewable by a pair of good field glasses. Ganymede has a magnetic field, discovered by the Galileo spacecraft, which strongly suggests it has a metallic core. Ganymede has proven to be a fascinating world, the only moon known to have a magnetosphere, or magnetic environment, produced by a convecting metal core. The interaction of Ganymede's and Jupiter's magnetospheres may produce dazzling variations in the auroral glows in Ganymede's tenuous atmosphere of oxygen.

This color reconstruction of part of the northern hemisphere of Ganymede was made from pictures taken at a range of 313,000 kilometers. The scene is approximately 1,300 kilometers across.

The surface temperature of Ganymede is much colder than Earth, with daytime temperatures ranging from -297 to -171 degrees Fahrenheit. Compared to Earth's polar caps, Ganymede's polar terrain is relatively vast. The frost on Ganymede reaches latitudes as low as 40 degrees on average and 25 degrees at some locations. For comparison with Earth, Miami, Florida lies at 26 degrees north latitude, and Berlin, Germany is located at 52 degrees north. The surface of Ganymede shows a complex geological history that has given rise to two types of terrain. About 40% of the surface is covered by dark, highly cratered regions which are evidently the remains of the original crust. The other 60% is light, grooved landscape which forms intricate patterns. This younger terrain may be due to tensional faulting or the release of water from beneath the surface. Groove ridges as high as 700 meters (2,000 feet) have been observed stretching for thousands of kilometers.

This shows an entire hemisphere of Ganymede. The prominent dark region, called Galileo Regio, is about 3,200 km in diameter. The bright spots are relative recent impact craters.

Ganymede has no known atmosphere, but recently the Hubble Space Telescope detected ozone at its surface, the result of the interaction between trapped energy particles from Jupiter's magnetic field and the water particles on Ganymede. The amount of ozone is small as compared to Earth. This ozone hints that Ganymede probably has a thin tenuous oxygen atmosphere like that detected on Europa.

Ganymede was discovered in 1610 by Galileo Galilei.

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