WASHINGTON, March 2 (UPI) -- The solar system just grew a little stranger -- and perhaps lonelier -- as instruments aboard NASA's Cassini spacecraft have discovered a large and surprisingly dense magnetic gas cloud occupying the same orbit as Jupiter's icy moon Europa.
The discovery raises some doubts about whether the fourth-largest and second-closest Jovian moon is capable of sustaining life, as scientists have speculated.
Cassini - at present en route to a July 1, 2004, orbital rendezvous with Saturn -- focused its magnetospheric imaging instrument in the general direction of Europa during a recent flyby of Jupiter's neighborhood. In doing so, the spacecraft detected a cloud millions of miles in diameter in a torus, or donut, shape. The cloud is thought to be the result of Jupiter's severe bombardment of Europa with ion radiation -- radiation so strong it actually disturbs the moon's surface, kicking up and pulling apart water-ice molecules and dispersing them into space, NASA scientists explained.
Europa is one of only a few places in the solar system other than Earth where life might be possible. The ice-covered Europa, astrobiologists suspect, could harbor liquid oceans under its frozen surface. It has been the subject of preliminary studies by the Voyager spacecraft in the early 1980s and more recently by the Galileo spacecraft, which flew within 900 miles of the moon's surface in February 1999.
Although Europa's surface temperature is -260 degrees Fahrenheit, scientists think the gravitational effects of Jupiter and its neighboring moons could be keeping large parts of Europa's subsurface in a liquid state. The same tidal friction from Jupiter also causes the intense volcanic activity on the Jovian moon Io.
More detailed examination of the moon's surface should be possible with the Europa Orbiter mission, at present planned for launch in 2008. The orbiter is designed to conduct a 2-year investigation beginning with its orbital rendezvous in 2010. The craft will carry a radar-sounding device able to locate liquid beneath Europa's surface.
The orbiter also will carry other instruments to map Europa's surface in unprecedented detail, and to calculate where to drop a submarine-like probe that will melt through the ice and explore the oceans beneath.
After Cassini begins orbiting Saturn next year, it will release its piggy-backed Huygens probe about six months later for descent through the thick atmosphere of the Saturnian moon, Titan, other possible home for alien life.
According to Cassini's instruments, the mass of the torus cloud is about 60,000 tons, large enough to suggest Jupiter's intense radiation is causing significant disruptions on the Europan surface. In addition, the cloud seems to be exerting "considerable influence" on Jupiter's magnetic field, said Barry Mauk, head of the research team Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., which built the instruments.
"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. 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 cloud's discovery was made possible by 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 explained. "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.
(Editors: UPI photos WAX2003030201 and WAX2003030202 are available)