Galileo spies high-altitude ionosphere on Io

17 Jun 2002

NASA's Galileo spacecraft may have flown though a dense, high-altitude ionosphere during its encounter with Jupiter's volcanic moon Io last December, scientists reported this week. 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 was reported by Galileo scientists this week at a meeting of the American Astronomical Society's Division of Planetary Sciences in Tucson, AZ, along with other Galileo results, including remarkable new images of the planet and its moons.

The results may lend credence to previous theories proposed by Galileo project scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory (JPL), 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 reflect sunlight and can be seen in images.

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.