Using a sensitive new imaging instrument on NASA’s
Cassini spacecraft, researchers have discovered a large and
surprisingly dense gas cloud, called sharing an orbit with
Jupiter’s icy moon Europa.

Stretching millions of miles around Jupiter, the donut-
shaped cloud, known as a “torus,” is believed to result from
the uncommonly severe bombardment of ion radiation the
jovian giant sends toward Europa. That radiation damages
Europa’s surface, kicking up and pulling apart water-ice
molecules and dispersing them along the moon’s orbit into a
neutral-gas torus with a mass of about 60,000 tons.

The cloud’s mass indicates the intense radiation Europa
faces has more severe consequences than scientists thought,
says Dr. Barry Mauk of Johns Hopkins University Applied
Physics Laboratory (APL), Laurel, Md. Mauk heads the
laboratory’s research team whose findings appear in the Feb.
27 issue of the journal Nature. The mass also shows that
Europa, in an orbit some 671,000 kilometers (416,000 miles)
from Jupiter, wields considerable influence on the magnetic
configuration around the giant planet.

“Surprisingly, Europa’s gas cloud compares to that generated
by the volcanically active satellite Io,” Mauk said. “But
where Io’s volcanoes are constantly spewing materials,
mostly sulfur and oxygen, Europa is a comparatively quiet
moon, and the gas we see is a direct consequence of its icy
surface being bombarded so intensely,” he said.

“By acting as both a source and a sink of charged radiation
particles, the dense gas torus gives Europa much greater
influence than was previously thought on the structure of,
and energy flow within, Jupiter’s huge space environment,
its magnetosphere,” Mauk said.

The APL team studied images of Jupiter taken in late 2000
and early 2001 with the laboratory’s Ion and Neutral Camera
on NASA’s Cassini spacecraft, now in route to Saturn. Mauk
says this is the first substantial discovery made at an
extraterrestrial planet using an innovative technique known
as energetic neutral atom imaging.

“Planetary magnetospheres glow with energetic neutral atoms,
much like a red-hot piece of iron glows with photons of
light, and such neutral-atom glows can be remotely imaged,”
Mauk said. “To this point, no instrument has imaged that
activity beyond Earth’s magnetosphere. Energetic neutral
atom imaging makes visible the three-dimensional structure
of planetary space environments, which, until recently, were
invisible to remote imaging techniques,” he said.

Research team members at the APL and co-authors on the
Nature paper, “Energetic neutral atoms from a trans-Europa
gas torus at Jupiter,” include Dr. Donald Mitchell, Dr.
Stamatios Krimigis, Dr. Edmond Roelof and Dr. Christopher
Paranicas. Krimigis, head of the Space Department at the
laboratory, is principal investigator for Cassini’s
Magnetospheric Imaging Instrument, which includes the Ion
and Neutral Camera.

The Magnetospheric Imaging Instrument built by the APL is
one of 12 science instruments on the main spacecraft and one
of six instruments designed to investigate the space
environments around Saturn and its moons. Cassini will begin
orbiting Saturn on July 1, 2004, and release its piggybacked
Huygens probe about six months later for descent through the
thick atmosphere of the moon Titan. Cassini-Huygens is a
cooperative mission of NASA, the European Space Agency and
the Italian Space Agency. NASA’s Jet Propulsion Laboratory
(JPL), Pasadena, Calif., manages the mission for NASA’s
Office of Space Science, Washington. JPL is a division of
the California Institute of Technology in Pasadena, Calif.

An energetic neutral atom image showing the donut-shaped
cloud around Jupiter is available at:
http://www.jhuapl.edu/newscenter/pressreleases/2003/030227.htm

For more information about Cassini, on the Internet visit:
http://saturn.jpl.nasa.gov

For more information about the Magnetospheric Imaging
Instrument and its science mission, on the Internet, visit:
http://sd-www.jhuapl.edu/CASSINI