Aug 27, 1999

Galileo Takes a Closer Look at Io

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New pictures of Jupiter's exotic moon were captured on July 3, 1999, as Galileo passed within 81,000 miles of Io.


Jupiter's Moon Io -- credit JPL
August 27, 1999: Jupiter's exotic moon Io is often likened to a pepperoni pizza. Its mozzarella-colored surface is pockmarked by ruddy volcanic plumes that evoke images of sliced Italian sausage among imaginative scientists. The strange appearance of Io's surface is constantly changing as it is formed and reformed by lava flows of molten silicate rock. What drives this volcanic powerhouse? The most likely energy source is changing gravitational tides caused by nearby Jupiter.

Right: This image of Io, captured by Galileo on July 3, 1999 from a distance of 81,000 miles, is color balanced to show how that moon would look to the human eye. [more information]

On July 3, 1999 Galileo flew closer to Io than it has since entering orbit around Jupiter in 1995. During the encounter the spacecraft captured images with a resolution of only 1.3 km per picture element, giving scientists a detailed view of Io's surface.

The color image above was created by combining data from near-infrared, green and violet filters in the spacecraft's camera to approximate what the human eye would see when looking at Io. Comparison of this image to previous Galileo images reveals many changes due to the ongoing volcanic activity. A higher contrast, false color version (below) shows many small-scale volcanic features which were not recognized previously.




Above: This false color image shows small-scale features which were not recognized previously and which suggest that the lava and sulfurous deposits on Io are composed of complex mixtures (close-up A). Some of the bright, whitish, high-latitude (near the top and bottom) deposits have an ethereal quality like a transparent covering of frost (close-up B). Bright red areas were seen in previous images only as diffuse deposits. However, they now appear as both diffuse deposits and sharp linear features like fissures (close-up C). Some volcanic centers have bright and colorful flows, perhaps due to flows of sulfur (rather than silicate) lava (close-up D). In this region of Io, bright, white material can also be seen to emanate from linear rifts and cliffs.


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Beginning in October 1999, after a series of four orbit-changing encounters with Callisto spanning May to September 1999, Galileo is scheduled to make two daring close approaches to Io, possibly flying through a volcanic plume. The upcoming encounters are expected to yield images of unprecedented clarity and detail. The radiation belts of Jupiter are very intense at Io's distance from the huge planet, and this can disrupt spacecraft functioning. To insure that Galileo will be able to perform science observations from this unique vantage point, the Galileo flight team is preparing contingency plans to protect against radiation-related problems in the coming months.

Galileo has been orbiting Jupiter and its moons since December 1995. Its primary mission ended in December 1997, but the spacecraft is currently in the midst of a two-year extended mission.

More information about the Galileo mission is available at:

JPL manages Galileo for NASA' s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena, CA.


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Ice, Water and Fire the Galileo Europa Mission

Galileo home page at JPL, with the latest on Europa, Callisto and Io

Jet Propulsion Laboratory home page

Io from the SEDS Nine Planets web site

Callisto from the SEDS Nine Planets web site

Jupiter from the SEDS Nine Planets web site

Io: The Prometheus Plume Aug. 18, 1997 Astronomy Picture of the Day

Close-up of an Io volcano Aug. 4, 1995 Astronomy Picture of the Day

Sizzling Io July 6, 1998 Astronomy Picture of the Day

Related Stories:

Taking the Scenic Route to Io -- June 30, 1999. Galileo zoomed by Callisto this morning in the second of four encounters designed to bring the spacecraft closer to Jupiter's volcanic moon Io

Turn left at Callisto -- May 5, 1999. Galileo heads for a daring encounter with Io's volcanoes

Galileo buzzes Europa -- Feb. 2, 1999. Galileo executes a close flyby of Europa for the last time during the current mission.

The Frosty Plains of Europa -- Dec. 3, 1998. As Galileo returns new images of Europa, NASA scientists prepare to study samples from a potentially similar environment here on Earth.

Callisto makes a big splash -- Oct. 22, 1998. Scientists may have discovered a salty ocean and a possible ingredient for life on Jupiter's moon.

Galileo takes a close look at icy Europa -- Oct 2, 1998. The spacecraft flew within 2300 miles of the mysterious satellite last weekend.

Clues to possible life on Europa may lie buried in Antarctic ice -- Mar. 5, 1998. Exotic microbial forms turn up in ice above Antarctica's Lake Vostok.


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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: John M. Horack