Illustration of Galileo at Jupiter

Scientists participating in NASA's Galileo mission have discovered that the Galileo spacecraft may have flown though a dense, high-altitude ionosphere during its encounter with Jupiter's volcanic moon Io last December. The discovery suggests that Io's atmosphere is time variable and is made of volcanic gas lofted to very high altitudes.

An ionosphere is a region of electrically charged gas that exists at the top of some planetary atmospheres. The surprising discovery is being reported by Galileo scientists this week at a meeting of the American Astronomical Society's Division of Planetary Sciences being held in Tuscon, AZ, along with other Galileo results, including remarkable new images of the planet and its moons.

Sensors on the spacecraft found a very dense region of ionized oxygen, sulfur and sulfur dioxide at 900 kilometers (550 miles) above Io that must be pumped into that region by Io's relentless volcanic activity, said Dr. Louis A. Frank of the University of Iowa and principal investigator on Galileo's plasma science experiment. Instead of being swept away by Jupiter's rotating magnetosphere as anticipated, the ionized gases surprisingly remain with Io, he said.

"Passage of the Galileo spacecraft through an ionosphere was not expected because images of the volcanic plumes previously taken with the Voyager spacecraft indicated that the plume heights extended only to a few hundred kilometers or less," said Frank. A radio occultation by NASA's Pioneer 10 spacecraft in 1973 indicated ionospheric heights only 50 to 100 kilometers (about 30 to 60 miles) above the surface, he added. "No one expected to see to see this at 900-kilometer altitude." The difference between what Pioneer saw and what Galileo has observed indicates that Io's atmosphere and ionosphere are variable and may grow and shrink with more or less volcanic activity, Frank said.

The results may lend credence to previous theories proposed by Galileo Project Scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory, Pasadena, CA, that invisible "stealth plumes" deliver volcanic gases to great heights above Io. Io's weak gravity field apparently permits the invisible gases emanating from the volcanoes to reach extraordinary heights far beyond the lower altitudes achieved by the dust and other volcanic ejecta that reflects sunlight and can be seen in images, Frank said.

Io Electron Beams

In a related finding, the energetic particle detector on the spacecraft measured intense, bi-directional electron beams that are aligned with Galileo's magnetic field in Io's vicinity. The beams are similar to those that impinge on Earth's atmosphere to produce aurora and, also, positive ions and electrons in Earth's atmosphere.

Dr. Donald J. Williams of the Applied Physics Laboratory of Johns Hopkins University and principal investigator on the energetic particle experiment, said the electron beams span the energy range of 15 kiloelectron volts to 190 kiloelectron volts and represent an energy deposition into Jupiter's atmosphere of up to 1 billion watts.

"This is sufficient energy input into the Jovian atmosphere to produce visible auroral emissions," Williams said. "These beams are a signature of remarkable particle acceleration processes that occur in the vicinity of Io  processes that are thought to be linked to Io's motion through Jupiter's plasma and magnetic field environment." Additional work is required to determine whether the beams play a role in producing some of the auroral emissions observed in Jupiter or if they are related to radio emissions that have been correlated with Io's orbital motion.

The electron beams must have a role in maintaining the Io torus, the doughnut-shaped cloud of ionized gases that flows between Jupiter and Io, Galileo scientists said. Auroral emissions in Io's atmosphere are one likely result of the electron beams, they reported, and the two-way electron highway that the beams produce between Jupiter and Io must contribute to some of the aurora observed in Jupiter's atmosphere as well.

Io Volcano Shifting?

Several images recently returned by Galileo show new details of surface features on the moon Ganymede and Io. One new image of the active volcano Prometheus on Io has been compared to one of the same features taken by NASA's Voyager spacecraft 17 years ago, and shows that the plume is now erupting from a position about 75 kilometers (about 46 miles) west from where the hot spot resided in 1979. It is not known if the plume source is the same or if the plume is now emanating from a new source. Overall, scientists studying Galileo images of Io are observing that a wide variety of surface changes have occurred in the nearly two decades since a spacecraft last visited Jupiter's system.

Frosted Rims on Ganymede

Bright white areas seen around the circular rims of high- latitude impact craters on Ganymede in new Galileo images of that moon are likely water-ice frosts, Galileo scientists reported. Even though the Sun is shining from the south, the north-facing walls of the ridges and craters are brighter than the walls facing the Sun. Images of regions elsewhere on Ganymede show more details of the remarkable juxtaposition of newer and older fractured and faulted terrain that characterizes so much of this big moon's surface. A stereoscopic view of Ganymede has also been produced with two images of the Galileo Reggae region (one was taken during the first Ganymede flyby in June and the second was acquired in the September flyby). The image, which was computer-reconstructed by imaging scientists at JPL, shows new topographic information about the moon.

Galileo science team members are reporting on numerous other new findings about Jupiter and its moons:

The photopolarimeter radiometer experiment produced heat maps of Jupiter's Great Red Spot, the day side of the moon Europa, the night side of Io, and both the day and night sides of Ganymede during the spacecraft's flyby of Ganymede in June. The images of the Great Red Spot show temperatures of the atmosphere at the 250 and 500 millibar pressure levels, much like terrestrial weather maps. The Great Red Spot is colder than its surroundings, consistent with earlier Voyager and Earth-based observations in which the spot is modeled as an anticyclonic vortex with central up-welling balanced by subsidence at its edges.

The radiometer also produced temperature data for Io indicating a nighttime temperature about 80 Kelvin to 85 K, or - 375 degrees Fahrenheit to -380 degrees F.

The first midday temperature for Europa, 128 Kelvin (-229 Fahrenheit), has allowed the radiometer instrument team to determine that that moon has a more porous or "fluffy" ice surface than the other moons. Researchers said that such porosity indicates Europa's surface is covered with finely powdered ice grains.

The near-infrared mapping spectrometer instrument and Galileo's solid state imaging camera measured hot regions on Io including erupting volcanoes and individual volcanoes, finding temperatures off 420 Kelvin to 620 K (296 degrees Fahrenheit to 656 F).

On Callisto and Ganymede, the near-infrared mapping spectrometer found surface features indicating the presence of hydrated materials, or possibly carbon dioxide frost.

The Jet Propulsion Laboratory, Pasadena, CA, manages the Galileo mission for NASA's Office of Space Science, Washington, DC. The Galileo mission home page is on the World Wide Web at Images from the mission are also posted on NASA's Planetary Photojournal at

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