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

This image of Jupiter's space environment, or magnetosphere, was taken with the innovative energetic neutral atom (ENA) imaging technique by the Magnetospheric Imaging Instrument on NASA's Cassini spacecraft as it flew past Jupiter in early 2001 at a distance of about 6 million miles. The central object represents ENA emissions from Jupiter itself, a consequence of the impingement of energetic ions from Jupiter's magnetosphere onto the upper atmosphere. The two outer objects represent ENA emissions from a massive donut-shaped gas cloud ‹ or torus ‹ that shares an orbit with Jupiter's moon Europa, at a distance from Jupiter of more than 400,000 miles. Credit: NASA/JPL/JHUAPL

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.

This illustration of Jupiter's space environment showing magnetically trapped radiation ions (in red), the neutral gas torus of the volcanic moon Io (green) and the newly discovered neutral gas torus of the moon Europa (blue). The white lines represent magnetic field lines. Energetic neutral atoms (ENAs) are emitted from the Europa torus regions because of the interaction between the trapped ions and the neutral gases. Credit: NASA/JPL/JHUAPL

"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.