Auroral Lights on Io Reveal Secrets of Jovian Moon's Atmosphere

Aug. 5, 1999

Paul Geissler

Alfred McEwen

An astronaut landing on the Jovian moon Io would have a harsh environment to
deal with, but would be rewarded with the most dazzling auroral light show
in the solar system. A current study reveals new information about the
moon_s red, green and blue auroral lights and how they relate to Io_s
tenuous atmosphere.

Last October, a team of American and Taiwanese space scientists reported
their discovery in images taken by the Galileo spacecraft of colorful
auroral emissions from Io during eclipse by Jupiter. In tomorrow's issue
(Aug. 6) of Science, they publish results from an in-depth study of those
images .

The tenuous atmosphere of Jupiter's moon Io partially collapses in the
darkness of the giant planet's shadow, they now find. At the same time,
bright blue glows emanating from stealthy volcanic plumes grow even

"This is our first detailed look at visible aurorae on a solar system
satellite", said Paul Geissler of the University of Arizona, lead author of
the report. "The pictures help us to understand Io's atmosphere and the
processes that generate the emissions."

Co-authors of the Science article are Alfred S. McEwen, also with the
University of Arizona Lunar and Planetary Laboratory, Wing Ip of the Taiwan
National Central University, Michael J. Belton of National Optical Astronomy
Observatories, Torrence V. Johnson of the Jet Propulsion Laboratory in
Pasadena, William H. Smyth of Atmospheric and Environmental Research in
Cambridge, Mass., and Andy Ingersoll of the California Institute of

Io's aurorae, like those on Earth, are caused by the impact of electrons on
atmospheric gasses. Io is bathed by a swarm of charged particles that are
trapped by Jupiter's magnetic field, similar to the Van Allen radiation
belts surrounding our own planet.

In addition, a powerful electric current flows from Io to the poles of
Jupiter, caused by an enormous electrical potential some 400,000 volts
generated by the motion of the jovian magnetic field past Io. When these
electrons collide with the gasses in Io's atmosphere, they set off a
dazzling light show of red, green and blue emissions bright enough to be
visible to the naked eye. The red and green glows may be caused by neutral
oxygen and sodium atoms, respectively, Geissler said. The bright blue
emissions are probably due to sulfur dioxide vented from volcanoes on the
moon's surface. Some of these plumes are invisible in daylight, owing to a
lack of entrained dust particles, and can only be seen during eclipse, he
added. The currents cause the gasses to
light up, much the same as the glows from florescent lamps.

Io's eerie glow dims noticeably with time as the satellite lingers in
Jupiter's shadow. The likely explanation, concludes the international team
of scientists that analyzed the pictures, is a partial collapse of the
moon's atmosphere during eclipse. Some of Io's patchy atmosphere is derived
from sulfur dioxide ice on the surface of the satellite that is warmed by
the Sun and sublimes (evaporates). This component probably begins to
recondense in the absence of sunlight during eclipse. More surprisingly, the
blue glows associated with volcanic plumes appear to intensify while Io is
in darkness. This may indicate that some of the current flow between Io and
Jupiter is conducted through the interior of Io, particularly during periods
when the atmospheric conductivity is low.

The Galileo spacecraft has been orbiting Jupiter and its moons since
December 1995. Galileo is managed by the Jet Propulsion Laboratory,
Pasadena, Calif., for NASA.

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