Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                                
November 10, 1993
(Phone:  202/358-0883)                                         
Embargoed until Nov. 11, 6 p.m.

Michael Finneran
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-5565)


RELEASE:  93-208

IO'S "GHOST" DETECTED ON JUPITER


    A faint spot of infrared emission that moves across Jupiter in 
concert with the orbital motion of its moon Io has been discovered 
by researchers at NASA's Goddard Space Flight Center, Greenbelt, 
Md., and the University of Hawaii.
    
    "What we're looking at is a false image -- or ghost -- of Io 
that appears near the limb, or visible edge, of Jupiter when Io is 
in a favorable orbital position," said Goddard's Dr. John E.P. 
Connerney, lead author of a paper published in the Nov. 12 issue 
of the journal "Science."

    The image is caused by the electrical interaction between Io 
and Jupiter's magnetic field.  This interaction generates about 5 
million amperes of current and 2 million megawatts of power -- 
roughly three times the total electrical generation capacity of 
the United States, Connerney said.

    "In a sequence of many images, the spot appears and disappears 
like clockwork, as Io moves into and out of the appropriate 
orbital position," he said. 

    The image was detected with NASA's 3-meter infrared telescope 
atop the Mauna Kea volcano on the island of Hawaii.  The Infrared 
Telescope Facility is operated by the University of Hawaii under 
contract to NASA.

    Scientists have known for nearly 30 years that Io plays a 
significant role in Jupiter's magnetosphere, a vast region around 
the planet influenced by Jupiter's intense magnetic field.  In 
their 1979 fly-bys, the Voyager I and II spacecraft yielded 
important clues about this interaction, such as the discovery of 
active volcanoes on Io and the measurement of the electrical 
current generated by this moon. 

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    Io is the innermost of Jupiter's four principal moons and is 
263,000 miles (420,000 kilometers) from the planet.  Io is about 
the size of Earth's moon and roughly the same distance from 
Jupiter as Earth's moon is from Earth.

    Until now, researchers did not know if Io's electrical current 
penetrated into Jupiter's atmosphere or simply scattered into 
space.  The Voyager missions had led scientists to believe that 
the current was carried off the moon and did not reach the 
planet's atmosphere.  Researchers 25 years ago, however, had 
speculated what now has been definitively learned -- that this 
electrical current reaches Jupiter.

    Connerney said the recent findings should help scientists 
working on the Galileo mission to study the interaction between 
Jupiter and Io more closely.  The Galileo spacecraft is scheduled 
to arrive at Jupiter in 1995 and will orbit the planet for several 
years, conducting an extensive scientific investigation of the 
Jovian system.

    "They have several instruments onboard Galileo that could 
track this thing down," Connerney said, "and now we know exactly 
where to point the telescopes."

    Collaborating with Connerney on the "Science" paper were 
Richard Baron and Toby Owen of the Institute for Astronomy at the 
University of Hawaii and Takehiko Satoh of Hughes-STX in 
Greenbelt, Md.

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EDITORS' NOTE:  8 x 10 color prints are available to news media 
representatives by calling the Goddard Office of Public Affairs at 
301/286-5565.