Galileo Looks for Auroras on Ganymede

NASA's durable Galileo spacecraft flew above the solar system's largest moon this morning in search of extraterrestrial "Northern Lights"

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December 28, 2000 -- When NASA's Galileo spacecraft zipped past Ganymede this morning, the moon was deep inside Jupiter's shadow, giving scientists an excellent chance to examine faint glows that would be overwhelmed by sunlight at other times.

"By timing the encounter to happen while Ganymede was in eclipse, we put Galileo in the right place at the right time to see auroras," said Dr. Eilene Theilig, deputy project manager for Galileo at NASA's Jet Propulsion Laboratory. Pictures from today's flyby will be transmitted to Earth during the next three to five months.

Right: This frame from a JPL animation shows what auroras on Ganymede might look like as Galileo swings by the large moon. Credit: NASA TV.

In its 29th orbit around Jupiter since 1995, the durable spacecraft dipped 2,337 kilometers above the surface of the darkened moon. "It looks like a nice, calm flyby," said Jim Erickson, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "The team was prepared for problems, but we're happy without any. And we'll be even happier once we've passed this orbit's closest approach to Jupiter."

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Galileo last visited Ganymede in May, 2000, when it skimmed 800 kilometers over the moon's surface. During the encounter Galileo recorded data that hint at a liquid saltwater ocean hidden beneath Ganymede's icy crust.

Today's flyby was a special opportunity to study something almost as hard to see as an underground ocean. With direct sunlight blocked by Jupiter, scientists hoped to record faint shimmering auroras on Ganymede, comparable to Earth's Northern Lights.

"The auroral glows we plan to observe occur because Ganymede has a very tenuous atmosphere of gases," said Dr. Torrence Johnson, Galileo project scientist at JPL. "When these gases are hit by electrons from Jupiter's radiation belts, they glow. It's similar to what goes on in a fluorescent light bulb when you turn on the electricity."

Studying Ganymede's auroras could provide information about the chemical makeup of gases in Ganymede's atmosphere and also about Ganymede's unique magnetic field.

When Galileo flew past Ganymede in 1996, instruments on the spacecraft detected plasma waves -- a telltale sign of a magnetosphere around the moon. That discovery marked Ganymede as not only the largest moon in our solar system, but also the only one known with its own internally-generated magnetic field.

Above: A dynamic spectrogram of Ganymede's plasma waves. It shows intensity as a function of wave frequency (vertical axis) and time (horizontal axis). Scientists at the University of Iowa used the data to generate a audio signal shifted 9 times downward in frequency from the natural frequency range of the plasma waves (near 80 kHz). Click to listen: [3.9 MB Quicktime movie][1.3MB WAV audio] Credits: Galileo Plasma Wave Science Team led by Prof. Donald A. Gurnett, University of Iowa.

Mapping auroral glows around Ganymede might reveal even more: The paths of electrons approaching Ganymede from Jupiter's radiation belts are determined by lines of magnetic force, explained Johnson, so the location of the glows triggered by those electrons might delineate the shape of Ganymede's magnetic field.

Galileo's trajectory for today's Ganymede flyby once again exposed the orbiter to Jupiter's intense radiation belts, noted Erickson. With extensions to its original two-year mission, Galileo has survived three times the cumulative radiation dose it was designed to tolerate. Some of its 12 scientific instruments have been impaired by the radiation to varying degrees, but the spacecraft is still returning valuable scientific information. The effects of additional exposure next week cannot be predicted with certainty, Erickson said.

Above: Ganymede, which orbits Jupiter, is slightly larger than the planet Mercury and more than three-quarters the size of the planet Mars. If it orbited the Sun, Ganymede would surely be considered a planet. Shown here in their correct relative sizes are a Hubble Space Telescope image of Mars, a Galileo image of Ganymede, a Mariner 10 mosaic of Mercury (the smooth stripe represents an area of missing data), and a Galileo spacecraft picture of the Moon.

Galileo is collaborating with NASA's Cassini spacecraft on several studies of Jupiter and its surroundings this fall and winter, while Cassini passes Jupiter for a gravity boost toward its 2004 appointment with Saturn. JPL, a division of the California Institute of Technology in Pasadena, manages Galileo and Cassini for NASA's Office of Space Science, Washington, D.C. Cassini is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.

Evidence of a Magnetosphere at Ganymede from Galileo Plasma Wave Observations -- Galileo Plasma Wave Science Team led by Prof. Donald A. Gurnett, University of Iowa.

Galileo Mission Home Page -- from JPL

Jupiter Flyby Website -- learn more about joint Cassini-Galileo observations of Jupiter