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Missions to Jupiter
   -  Pioneer 10
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Jupiter Moons
   -  Metis
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   -  Europa
   -  Ganymede
   -  Callisto
   -  Themisto
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Jupiter Summary


Jupiter Moon: Io!

Galileo spacecraft acquired its highest resolution images of Jupiter's moon Io on July 3, 1999 during its closest pass to Io since orbit insertion in late 1995. (false color) The Galilean satellite Io floats above the cloudtops of Jupiter in this image captured on the dawn of the new millennium, January 1, 2001, two days after Cassini's closest approach.

The densest of Jupiter's Galilean satellites, Io is similar in size and density to our own Moon. The internal heat generated by the continual gravitational tug-of-war with Jupiter, Europa, and Ganymede, makes Io the most volcanically active body in the Solar System. It has high reflectivity caused by the formation of a crystalline layer that originated from the many volcanoes on the surface. Observations on Earth detected sodium vapor emissions that formed a sodium vapor cloud extending 16,000 kilometers from the surface.

North is to the top of the picture, and Jupiter is towards the right. The images were taken on May 31, 1998 at a range of 1.3 million kilometers by Galileo.

Above is an image acquired by NASA's Galileo spacecraft while the moon was in Jupiter's shadow. Gases above the satellite's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). The green and red emissions are probably produced by mechanisms similar to those in Earth's polar regions that produce the aurora, or northern and southern lights. Bright blue glows mark the sites of dense plumes of volcanic vapor, and may be places where Io is electrically connected to Jupiter.

The surface temperature of Io is about -143 C (-230 F) with varying temperatures around the active volcanic regions. One "hot spot" on Io's surface has a temperature of about 17 C (60 F), probably a non-molten lava lake.

The volcanoes, the most interesting feature of Io, are caused by a complex gravitational interaction between Europa, Ganymede and Jupiter's tidal forces. The results can often be seen by large bulging, as great as 100 meters (330 feet) on the surface. When the Voyager spacecraft flew past they recorded a total of 11 active volcanoes, the largest of which, Pele, ejected a plume of material 300 km (190 miles) high.

This pair of images taken by NASA's Galileo spacecraft captures adynamic eruption at Tvashtar Catena, a chain of volcanic bowls on Jupiter's moon Io. They show a change in the location of hot lava overa period of a few months in 1999 and early 2000.

Because the surface is continually being relaid, there are no visible impact craters. The surface colors of mottled red, orange, yellow, brown, and white are due to sulfur and frozen sulfur dioxide. The terrain consists mostly of flat plains rising to less than 1 km with some mountain ranges up to 9 km high. A torus of sodium gas and sulfur ions is spread out along Io's orbit. Io was discovered in 1610 by Galileo Galilei.

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