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Donald Savage
Headquarters,Washington        Feb. 27, 2003
(Phone: 202/358-1547)

Michael Buckley 
Johns Hopkins University Applied Physics Laboratory, Laurel, 
Md.
(Phone: 240/228-7536 or 443/778-7536)

Guy Webster 
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 818/354-6278)

RELEASE: 03-086

NASA'S NEW SPACECRAFT TOOL REVEALS MASSIVE JUPITER GAS CLOUD

     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.h tm

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 

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