11/10/93: IO'S "GHOST" DETECTED ON JUPITER

Paula Cleggett-Haleim
Headquarters, Washington, D.C.                November 10, 1993

Michael Finneran
Goddard Space Flight Center, Greenbelt, Md.

RELEASE:  93-208

    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.

    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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