MEDIA RELATIONS OFFICE
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
"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
Images are also available at