A sensor on board the Cassini spacecraft built by scientists at the University of Maryland and one developed by the Applied Physics Laboratory of Johns Hopkins University detected ionized and non-ionized atoms of this cloud during Cassini's recent flyby of Jupiter.

The authors write that information from these sensors suggest that volcanic gases from Io - one of Jupiter's largest moons and the most volcanically active body in the solar system - escape Jupiter's magnetic field and populate the interplanetary environment around the planet.

"The University of Maryland-built CHEMS sensor detected ions of this cloud out to a distance of almost one astronomical unit [the distance between the Earth and the Sun]," said co-author Douglas C. Hamilton, a professor of physics at the University of Maryland and leader of the space physics team that designed and built the CHEMS (CHarge Energy Mass Spectrometer) sensor.

"CHEMS, which provides detailed measurements of the composition of ions, was also able to show that a significant portion of the particles in the cloud were sulfur and oxygen with sulfur dioxide likely present as well," Hamilton said. "Sulfur dioxide is the chief gas emitted by volcanos, indicating Io as the likely origin for much of the gas cloud that Cassini detected."

According to Hamilton the cloud is the result of various processes in the region around Jupiter that change some atoms derived from volcanic gas, first into ions, then back to atoms and, in some cases, back to ions again. Ions are atoms that have lost one or more electrons.

The first step of a volcanic gas atom's journey from Io to interplanetary space is when it becomes ionized and energized in Jupiter's magnetosphere. This ion now has the speed it needs to fly away from Jupiter, but because of its electrical charge, it remains held within the magnetosphere by the planet's magnetic field.

However, such energetic ions can pick up electrons from other atoms or molecules and once again become "normal" or electromagnetically neutral atoms. These energetic neutral atoms are no longer bound within Jupiter's magnetosphere and can zoom into interplanetary space.

Once in space some of these neutral particles are again converted into ions, this time through photoionization, a process by which electrons are knocked away from atoms by the impact of high energy ultraviolet photons from the sun.

The gas cloud extending far out beyond Jupiter's magnetosphere is thus a mixture of energetic atoms and ions that are flowing away from the planet.

On board Cassini, Maryland's CHEMS sensor detects ions, while atoms are imaged by the APL-developed INCA sensor.

INCA stands for Ion and Neutral Camera. During the flyby of Jupiter, INCA provided images showing some structural detail of Jupiter's magnetosphere, while also detecting atoms in the nebulous cloud that stretches away from the planet.

Detection of ions and determination of specific types of particles in the gas cloud were made by the CHEMS sensor.

On to Saturn Of course the primary target for Cassini is Saturn, which it will reach in 2004. Cassini, which is carrying the European Space Agency's Huygens Probe spacecraft, is the best-instrumented mission ever sent to another planet.

On Cassini, INCA and CHEMS are linked together by a central computer "brain" together with the LEMMS (Low Energy Magnetospheric Measurement System) sensor. The three sensors and their computer form Cassini's Magnetospheric Imaging Instrument, known as MIMI. MIMI is one of one of 12 science instruments on the main Cassini spacecraft and one of six instruments designed primarily to investigate the space environments around Saturn and its satellites. The Huygens probe will investigate Saturn's largest moon, Titan.

According to Maryland's Hamilton, the flyby of Jupiter was important scientifically because it has revealed new information about Jupiter's magnetosphere, its interaction with the solar wind and its surrounding nebula. However, it was also an important step on the road to Saturn. "This flyby has provided us an excellent test of the capabilities of MIMI and has allowed us to make important refinements to some of the software running on MIMI's micro-processing unit," he said.

"A nebula of gases from Io surrounding Jupiter" by Stamatios M Krimigis et al. (Applied Physics Lab, Johns Hopkins University), Douglas C Hamilton and George Gloeckler (Department of Physics, University of Maryland), Jannis Dandouras (CNES), Thomas P. Armstrong (Fundamental Technologies), Scott J Bolton (Jet Propulsion Laboratory), K C Hsieh (Department of Physics, University of Arizona), Norbert Krupp et al. (MPI fur Astronomie), Louis J Lanzerotti (Bell Labs), Nature vol. 415, no. 994-996.

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