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JET PROPULSION LABORATORY
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Contact: Franklin O'Donnell

EMBARGOED FOR RELEASE 1:30 P.M. PACIFICFEBRUARY 4, 1999

GALILEO SPACECRAFT FINDS THIN ATMOSPHERE ON CALLISTO

       NASA's Galileo spacecraft has detected a thin carbon dioxide atmosphere on Jupiter's moon Callisto, and has confirmed the existence of carbon dioxide on Callisto's surface. The findings appear in the February 5 issue of the journal Science.

       This latest discovery means that all four of Jupiter's large Galilean moons -- Callisto, Europa, Io and Ganymede -- have some form of atmosphere.

       "Callisto's atmosphere is so tenuous that the carbon dioxide particles are literally drifting around without bumping into one another," said Dr. Robert Carlson of NASA's Jet Propulsion Laboratory, Pasadena, CA, principal investigator for Galileo's near-infrared mapping spectrometer instrument. "An atmosphere this thin is known as an exosphere."

       The instrument detected the carbon dioxide atmosphere during observations of Callisto made during the 10th orbit around Jupiter in September 1997. Carlson says he and other scientists were following up on discoveries made by Galileo upon its arrival at Jupiter's system in 1995. The spacecraft detected what appeared to be carbon dioxide on Callisto's surface. This latest finding confirms that the surface chemical was, in fact, carbon dioxide, and that the chemical also appears in the atmosphere above Callisto.

       "An atmosphere this thin is easily lost due to ultraviolet radiation from the Sun, which breaks the molecules into ions and electrons which are swept away by Jupiter's magnetic field," said Carlson. "For us to find such an atmosphere implies that there is a steady flux of carbon dioxide into the atmosphere. Venting of gas from the interior is one possibility, and Galileo images show surface erosion that suggests carbon dioxide outgassing."

       Previous findings indicated that two of Jupiter's moons, Europa and Ganymede, have a thin oxygen atmosphere, while Io's atmosphere contains sulfur dioxide.

       "We're anxious to look for other gases that may be contained in Callisto's atmosphere," said Carlson. Scientists will have that opportunity when Galileo observes Callisto during two of four flybys planned during the remainder of the current extended mission. Galileo will make observations of Callisto during encounters in May and June; it will be observing other targets during flybys of Callisto in August and September.

       Those encounters will take place before the two Io flybys that will wrap up the spacecraft's extended mission, known as Galileo Europa Mission. During the extended mission, Galileo has flown by Europa eight times. The spacecraft has been orbiting Jupiter and its four largest moons for more than three years.

       The Galileo mission is managed by JPL, a division of California Institute of Technology, Pasadena, CA.

       Additional information and images taken by the Galileo spacecraft are available on the Internet at http://www.jpl.nasa.gov/galileo.  Images are also available at http://photojournal.jpl.nasa.gov.

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