PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov
Contact: Jane Platt
July 18, 1997
FOR IMMEDIATE RELEASE
GALILEO SPACECRAFT FINDS EUROPA HAS ATMOSPHERE
NASA's Galileo spacecraft has found an ionosphere on
Jupiter's moon Europa, an indication that the icy moon also has
an atmosphere, Galileo scientists reported today.
"While this discovery does not relate to the question of
possible life on Europa, it does show us there is a surface
process occurring there, and Europa is not just some dead hunk of
material," said lead investigator Dr. Arvydas Kliore of NASA's
Jet Propulsion Laboratory, Pasadena, CA. Kliore reports his
findings in the July 18 issue of Science magazine.
The ionosphere was detected through a series of six
occultation experiments performed during Galileo's encounters
with Europa in December 1996 and February 1997. During
occultation, Europa was positioned between the spacecraft and
Earth, causing interruption in the radio signal. Measurements of
the Galileo radio signal received at the Deep Space Network
stations in Goldstone, CA, and Canberra, Australia, showed that
the radio beam was refracted by a layer of electrons, or charged
particles, in Europa's ionosphere.
An ionosphere is a layer of charged particles (ions and
electrons) found in the upper levels of an atmosphere, created
when gas molecules in the atmosphere are ionized. On Europa,
this ionized layer can be caused either by the Sun's ultraviolet
radiation or by energetic particles trapped in Jupiter's magnetic
field, known as the magnetosphere. Europa and the other Jovian
satellites are immersed in this magnetosphere. "Most likely the
charged particles in Jupiter's magnetosphere are hitting Europa's
icy surface with great energy, knocking atoms of water molecules
off the moon's surface," Kliore said,
Europa's ionosphere has a maximum density of 10,000
electrons per cubic centimeter, which is significantly lower than
the average density of 20,000 to 250,000 electrons per cubic
centimeter found in Jupiter's ionosphere. This indicates that
Europa's ionosphere is tenuous; nonetheless, it is strong enough
for scientists to infer the presence of an atmosphere.
The latest Galileo findings follow last year's observations
by NASA's Hubble Space Telescope of oxygen emissions on Europa, a
strong hint that an atmosphere might exist on that moon.
The existence of an ionosphere and, by inference, an
atmosphere, on another Jovian moon, Io, was observed in 1973
during a radio occultation conducted by NASA's Pioneer 10
spacecraft and confirmed by recent Galileo occultations. Io is
believed to have an unusual atmosphere affected by sulfur dioxide
spewing from the moon's volcanic vents. Kliore and his colleagues
are currently studying two of Jupiter's other largest moons,
Ganymede and Callisto, to determine whether they also have
ionospheres and atmospheres.
"You could say an ionosphere or some kind of atmosphere has
been found on most solar system bodies studied so far," said
Kliore.
Participating with Kliore in the Europa radio occultation
experiments were Dr. David Hinson, professor at Stanford
University, Stanford, CA; Dr. Michael Flasar of NASA's Goddard
Space Flight Center, Greenbelt, MD; Dr. Andrew Nagy, professor at
the University of Michigan, Ann Arbor, MI, and Dr. Thomas
Cravens, professor at the University of Kansas, Lawrence, KS.
The Galileo mission is managed by JPL for NASA's Office of
Space Science, Washington, DC. The spacecraft entered the Jovian
system on December 7, 1995, and its primary mission will end in
November of this year. However, the mission has been extended
for two more years so the craft can conduct an intensive study of
Europa, with additional flybys of Callisto and Io, depending on
spacecraft health.
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7/17/97
JP