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Back to 25 Years of Voyager Main Page Grand Tour of the Outer Planets

Although Jupiter, as the closest of the giant outer planets to Earth, has been well studied for hundreds of years, the Voyager spacecraft were able to shed much new light on it, as well as on its satellites and magnetosphere.

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Voyager 1 took this photo of the planet Jupiter on Jan. 24, while still more than 25 million miles (40 million kilometers) away. As the spacecraft draws closer to the planet (about 1 million kilometers a day) more details are emerging in the turbulent clouds. The Great Red Spot shows prominently below center, surrounded by what scientists call a remarkably complex region of the giant planet's atmosphere. An elongated yellow cloud within the Great Red Spot is swirling around the spot's interior boundary in a counterclockwise direction with a period of a little less than six days, confirming the whirlpool-like circulation that astronomers have suspected from ground-based photographs. Ganymede, Jupiter's largest satellite, can be seen to the lower left of the planet. Ganymede is a planet-sized body larger than Mercury. This color photo was assembled at Jet Propulsion Laboratory's Image Processing Lab from three black and white images taken through filters.

Voyager discovered three new Jovian satellites. However, the most fascinating discovery occurred when Voyager 1 encountered Io, one of the four moons, first recorded by Galileo in 1610, that are now referred to collectively as the Galilean satellites. In March 1979, Voyager 1 observed nine erupting volcanoes on Io-the first evidence in the solar system of active volcanism on a body other than Earth! In June 1979 Voyager 2 was able to confirm that eight of these volcanoes were still active and that the other had erupted since Voyager 1's encounter. Researchers believe that there are probably many more volcanoes on Io.

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Left: This picture shows a special color reconstruction of one of the erupting volcanoes on Io discovered by Voyager 1 during its encounter with Jupiter on the 4th and 5th of March. The picture was taken March 4 about 5:00 p.m. from a range of about half a million kilometers showing an eruption region on the horizon. This method of color analysis allows scientists to combine data from four pictures, taken in ultraviolet, blue, green and orange light. In this picture one can see the strong change in color of the erupting plume. The region that is brighter in ultraviolet light (blue in this image) is much more extensive than the denser, bright yellow region near the center of the eruption. Scientists will use data of this type to study the amount of gas and dust in the eruption and the size of dust particles. Preliminary analysis suggests that the bright ultraviolet part of the cloud may be due to scattered light from very fine particles (the same effect which makes smoke appear bluish).
Right: Voyager 1 image of Io showing active plume of Loki on limb. Heart-shaped feature southeast of Loki consists of fallout deposits from active plume Pele. The images that make up this mosaic were taken from an average distance of approximately 490,000 kilometers (340,000 miles).

The Io volcanoes eject sulfur, oxygen, and sodium, at velocities up to 2,300 miles per hour, which is considerably faster than the 112 miles per hour rate at which Mount Etna spews volcanic material. At temperatures greater than 1800 degrees Kelvin, these volcanoes are the hottest planetary objects in the solar system. The plumes from the volcanoes rise more than 190 miles above the surface of Io, and are large enough to be seen by the Hubble Space Telescope.

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High resolution (left) and color (right) images of the volcano Ra Patera on Jupiter's moon seen by the Voyager 1 spacecraft during the 1979 flyby. The images reveal relatively bright lava flows emanating from Ra Patera, especially to the northeast.

Left: Voyager 2 took this picture of Io on the evening of July 9, 1979, from a range of 1.2 million kilometers (745,700 miles). On the limb of Io are two blue volcanic eruption plumes about 100 kilometers (62 miles) high. These two plumes were first seen by Voyager 1 in March 1979 and are designated Plume 5 (upper) and Plume 6 (lower). They apparently had been erupting for a period of at least four months and probably longer. A total of six plumes were seen by Voyager 2, all of which were first seen by Voyager 1. Plume 1, the largest volcano viewed by Voyager 1, was no longer erupting when Voyager 2 arrived. Plume 4 was not viewed on the edge of Io's disc by Voyager 2 and, therefore, it is not known whether it was still erupting.
Right: This pair of Hubble images of Jupiter's volcanic moon Io shows the surprising emergence of a 320-kilometer (200-mile) diameter, large yellowish-white feature near the center of the moon's disk (photo on the right). Scientists suggest the spot may be a new class of transient feature on the moon. For comparison, the photo on the left was taken in March 1994 before the spot emerged, and shows that Io's surface had undergone only subtle changes since it was last seen by the Voyager 2 probe in 1979. The new spot in the July 1995 Hubble image replaces a smaller whitish spot seen at about the same location in the March 1994 image. "The new spot surrounds the volcano Ra Patera, which was photographed by Voyager, and is probably composed of material, probably frozen gas, ejected from Ra Patera by a large volcanic explosion or fresh lava flows," according to John Spencer of Lowell Observatory in Flagstaff, Arizona. Credit: J. Spencer, Lowell Observatory/NASA

Voyager 1 confirmed the existence of the Io Plasma Torus, a donut-shaped cloud of plasma around Jupiter that results from the volcanic activity. The Voyager Plasma Sciences Experiment (PLS) and the Ultraviolet Spectrometer (UVS) were able to obtain detailed measurements of electron density, electron and ion temperature, and to estimate the composition of the torus.

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Animation of the Io plasma torus. Courtesy of Nick Thomas and Richard Lieu.

Voyager 1 took a photograph of the gigantic Jovian storm, the Great Red Spot, first observed by in the 17th Century. This storm, which has persisted for more than four hundred years, is more than twice the size of Earth. Voyager discovered that clouds associated with the Giant Red Spot move counter-clockwise. At the outer edge of the storm, clouds rotate in four to six days while, near the center, material moves almost randomly, with small motions. Other storms, that appear smaller and white, seem to interact with the Great Red Spot and with one another.

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This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on Feb. 25, 1979, when the spacecraft was 5.7 million miles (9.2 million kilometers) from Jupiter. Cloud details as small as 100 miles (160 kilometers) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex end variable wave motion.

In an unexpected discovery, on March 4, 1979, Voyager 1 obtained the first image of Jupiter's rings, using a system designed to search for faint rings. From Earth the ring had not been detected because it is so thin and appears transparent unless viewed straight on.

The discovery of Jupiter's ring system prompted the reprogramming of Voyager 2 so that it looked back from the shadow of Jupiter, toward the sun, and was able to obtain more detailed images of the giant planet.

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Left: Voyager 1's image of Jupiter's ring, seen as diagonal band across the center of the picture. The wavy white lines are star trails and the black dots are camera calibration points. Right: Image, taken by Voyager 2 on July 10, 1979, showing the rings from inside Jupiter's shadow.
Voyager 2 images revealed that Jupiter's main ring is over 4000 miles wide and includes the orbits of Jupiter's two small moons, Adrastea and Metis. Inside the main ring is the cloud-like halo, while outside the main ring, Voyager 2 observed an exceptionally faint gossamer ring.

The left image is a color composite, using orange and violet filters, of Jupiter's ring system-the two orange lines on the left, captured by Voyager 2. Jupiter's shadow is obscuring part of the lower ring. The right image, taken by Voyager 2 on July 10, 1979, shows the rings from inside Jupiter's shadow.

Voyager set the path for the Galileo spacecraft to conduct more detailed studies of Jupiter during its three orbits in 1996 and 1997. Galileo determined that Jupiter's gossamer ring actually consists of two rings, each of which is brighter along the edges than in the center.

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On November 9, 1996, NASA's Galileo spacecraft took these images of Jupiter's flat main ring and inner halo (left image) and the faint gossamer ring to the left of the main ring and halo (right image).

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