October 13, 1998

The Galileo imaging team released 12 new image products today.

The images can be found at:
www.jpl.nasa.gov/galileo (for a quick look)
www.jpl.nasa.gov/galileo/sepo (for a more detailed look)

In conjunction with the American Astonomical Society's (AAS) Division of Planetary Sciences (DPS) meeting in Madison, WI this week, the Galileo imaging team has prepared the following summaries of two of the main findings (Jovian lightning and Io Aurorae).

Galileo data shows Jupiter's lightning associated with low-pressure regions

MADISON, Wisconsin-Images of Jupiter's night side taken by the Galileo spacecraft reveal that the planet's lightning is controlled by the large-scale atmospheric circulation and is associated with low-pressure regions.

The new findings are being reported today (October 13, 1998) by Andrew Ingersoll at the 30th annual meeting of the American Astronomical Society's Division for Planetary Sciences.

"Lightning is an indicator of convection and precipitation," says Ingersoll, a Professor of Planetary Science at the California Institute of Technology and member of the Galileo Imaging Team. "These processes are the main sources of atmospheric energy, both on Earth and on Jupiter."

In a terrestrial hurricane, Ingersoll explains, the low pressure at the center draws air in along the ocean surface, where it picks up moisture. Energy is relased when the moisture condenses and falls out as rain.

On Jupiter, energy is transferred from the warm interior of the planet to the visible atmosphere in a similar process. The new findings show that lightning occurs in the low-pressure regions on Jupiter, too.

"On both planets, the air spins counterclockwise around a low in the northern hemisphere and clockwise around a low in the southern hemisphere," Ingersoll says. "The lows are called cyclones and the highs are called anticyclones.

On Jupiter the cyclones are amorphous, turbulent regions that are spread out in the east-west direction. In the Voyager movies they spawn rapidly-expanding bright clouds that look like huge thunderstorms. The Galileo lightning data confirm that convection is occurring there.

"We even caught one of these bright clouds on the day side and saw it flashing away on the night side less than two hours later," says Ingersoll.

In contrast, the Jovian anticyclones tend to be long-lived, stable, and oval-shaped. The Great Red Spot is the best example (it is three times the size of Earth and has been around for at least 100 years), but it has many smaller cousins. No lightning was seen coming from the anticyclones.

"That probably means that the anticyclones are not drawing energy from below by convection," says Ingersoll. "They are not acting like Jovian hurricanes"

Instead, the anticyclones maintain themselves by merging with the smaller structures that get spun out of the cyclones. "That's what we see in the Voyager movies, and the Galileo lightning data bear it out. "Whether the precipitation is rain or snow is uncertain, says Ingersoll.

"Models of terrestrial lightning suggest that to build up electrical charge, both liquid water and ice have to be present. Rain requires a relatively wet Jupiter, and that's a controversial subject.

"Water is hard to detect from the outside because it is hidden below the ammonia clouds. And the Galileo probe hit a dry spot where we didn't expect much water."

Fortunately the Galileo imaging system caught glimpses of a cloud so deep it has to be water, according to findings to be reported at the conference by Dr. Don Banfield of Cornell University and an imaging team affiliate. Banfield showed images of the water cloud near the convective centers in the cyclonic regions.

These results appear in the September issue of Icarus, the International Journal of Solar System Studies. "We know the water is there, and we know where it's raining," says Ingersoll. "This is a big step toward understanding how Jupiter's weather gets its energy."

Galileo Observes Details of a Giant Electrical Generator on Io

MADISON, Wisconsin- Jupiter's moon Io has long been suspected of being an enormous electrical generator, with a power output of more than a trillion watts - greater than the total produced by all the power stations in the U.S. combined. Voltages as high as 400,000 volts are generated across Io as Jupiter's rapidly rotating magnetic field sweeps past the satellite, acting much the same way as a generator in an automobile. Scientists first found evidence of Io's prodigious electrical energy production in radio emissions and later in bright spots close to the poles of nearby Jupiter, where much of the energy is dissipated. A new image of Io from NASA's Galileo spacecraft shows the moon glowing in the darkness of Jupiter's shadow and may shed light on the mechanism of Io's electrical power production and its interactions with Io's volcanically generated atmosphere. The new findings are being reported today (October 13, 1998) by Dr. Paul Geissler at the 30th annual meeting of the American Astronomical Society's Division for Planetary Sciences. Dr. Geissler is a Senior Research Associate at the University of Arizona's Lunar and Planetary Laboratory and an affiliate of the Galileo imaging team.

The new picture shows Io at visible wavelengths (red, green and violet) and represents the first visible color portrait of the moon while in eclipse. No surface features are apparent, but gases above the satellite's surface can be seen to glow in vivid red, green and blue hues never previously imaged. These colors are produced when Io's atmospheric gases are impacted by energetic charged particles trapped in Jupiter's magnetic field. The impacts cause the gases to become energized, and the resulting optical emissions form aurorae similar to those visible in the night skies of Earth. The different colors indicate distinct types of emissions, either due to variations in gas composition at different locations on Io or to differences in the way the gases are excited. The green glow near the middle of the disk indicates a concentration of gas (possibly oxygen) on the downstream side of Io, swept up in the wake of Jupiter's magnetic field. The bright red glow along the northern edge of Io may be also be due to oxygen (emitting at a different wavelength) or perhaps hydrogen or sodium; its identification will require more detailed spectral measurements. The red limb glow is much brighter in the north polar region of Io than in the south, and varies with the changing tilt of the external magnetic field.

"The most interesting of the aurorae are the bright blue glows seen at the sides of the satellite," said Dr. Geissler. These are the places where the "generator" is electrically connected to Jupiter. Large currents flow along magnetic field lines from these points to Jupiter's north and south polar regions, where they light up Jupiter's ionosphere in small spots called "footprints". Bright optical emissions at ultraviolet and visible wavelengths have been observed at the footprints by Galileo, the Hubble Space Telescope and ground-based observers, indicating that a large fraction of the energy generated on Io is spent in the footprints on Jupiter. The footprints are analogous to light bulbs, connected to a generator (Io) by wires (magnetic field lines). The bright blue glows seen on Io in eclipse are caused by the flow of this current through regions rich in sulfur dioxide gas associated with active volcanoes, such as Acala and Prometheus. The plumes that are lit up by the electrical activity are those located on the Jupiter-facing side of Io, and on the side of Io opposite to Jupiter. This extraordinary light show varies over time, both due to the wobble of Jupiter's external magnetic field and to changes in the volcanic activity on Io's surface.

Galileo has been orbiting Jupiter and its moons for 2 1/2 years, and is currently in the midst of a two-year extension, known as the Galileo Europa Mission. JPL manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.. JPL is a division of Caltech, Pasadena, CA.

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