March 16, 2002

NEW YORK (AP) -- Astronomers probing
the vast northern lights that ring Jupiter's
north pole have found a mysterious X-ray
"hot spot" that flares up like a beacon
every 45 minutes.

Scientists said it could take years to
explain this pulsating region, which its
discoverers speculate may be related to
bursts of radio waves that emanate from
the giant planet at a similar interval.

"It came as a complete surprise, but
scientists live for that.  Sometimes the
things that are most unexpected are the
most important," said Christopher
Russell, a University of California,
Los Angeles professor of geophysics.  Russell was not involved in the research.

The discovery of the hot spot is one of
several surprises that have emerged from a unique opportunity scientists had last
year to study Jupiter and the enormous
magnetic cocoon that surrounds it.

The occasion was a space science first -
the first time two spacecraft had visited
Jupiter, or any outer planet,
at the same time.

In early January 2001, the Galileo orbiter
that has been circling Jupiter since 1995
and the Cassini probe, which swung past
Jupiter on its way to Saturn, passed
through Jupiter's magnetosphere - a
zone of magnetically-charged particles
trapped within its magnetic field.

Images taken during the same period by
the Hubble space telescope and the
Chandra X-Ray Observatory,
both in orbit around the Earth,
complemented the spacecraft's observations.

Seven papers that arose from that data
were highlighted in the February 28 issue
of the journal Nature, describing various
aspects of Jupiter's magnetosphere and its
interaction with the planet's upper atmosphere.

The data transmitted back showed, as
scientists had predicted, that Jupiter's
magnetosphere changes shape as it is
buffeted by interplanetary shock waves
created by the solar wind - the stream
of particles thrown off by the sun.
Earth's magnetosphere acts similarly.

But Jupiter's is more complex and far
larger.  At about 100 times as wide as
the planet, it's so large that if it were
visible to the naked eye, it would appear
larger than the full moon to an observer
on Earth.  And its cometlike tail
extends past the orbit of Saturn.

Randy Gladstone, a planetary scientist
at the Southwest Research Institute in
San Antonio who was lead author on the
X-ray hot spot paper, said his team is
still in the early stages of trying to figure
out what is causing the pulsating spot.

He said the disturbance accounts for most
of the X-ray emissions that are seen in
Jupiter's northern auroras.  Scientists had
thought those emissions were produced by
sulfur and oxygen ions blasted into space
by Jupiter's volcanic moon, Io, and were
then energized by circulating through
the Jovian magnetic field.

Now, it's not clear what is behind those
emissions, Gladstone said.  The evidence
points to an origin much farther away
from Jupiter, near the edge of its
magnetosphere, where it meets the
solar wind.

"Something is causing these emissions -
from X-rays to ultraviolet to radio (waves).
There's something connecting all of these
emissions to have them happen over
all these wavelengths," Gladstone said.

John Clarke, a professor of astronomy
at Boston University, and colleagues
authored another paper that reported
the discovery of a ghostly glowing trail
etched by Io into Jupiter's ionosphere,
the region high in its atmosphere
where auroras form.

His team also found that two of
Jupiter's three other large moons -
Ganymede and Europa - etch smaller,
oval "magnetic footprints."

Scientists already knew Io, the most
volcanically active body known in the solar
system, was producing a similar footprint.

But Clarke said that since Europa and
Ganymede do not have volcanoes but still
produce footprints in Jupiter's ionosphere,
it appears some unknown mechanism is
causing all three moons to leave their
mark there.

Jupiter's fourth large moon, Callisto,
may cast such a spot, too.  But whether
it does may remain unanswered for years
to come because no new missions to
Jupiter are planned, and the Galileo
spacecraft will end with a fiery plunge
into Jupiter next year (2003).