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Agas torus is atoroidal cloud of gas orplasma that encircles a planet or moon. In theSolar System, gas tori tend to be produced by the interaction of a satellite'satmosphere with themagnetic field of a planet. The most famous example of this is theIo plasma torus, which is produced by the ionization of roughly 1 ton per second of oxygen and sulfur from the tenuous atmosphere ofJupiter's[1]volcanic moonIo. Before being ionized, these particles are part of a neutral torus, also centered on the orbit of Io. Energetic particle observations also suggest the presence of a neutral torus around the orbit of Jupiter's moonEuropa[2]although such a torus would be merged with the outer portions of an Io torus.
Other examples include the largely neutral torus of oxygen and hydrogen produced bySaturn's moonEnceladus. The Enceladus and Io tori differ in that particles in the Io torus are predominantly ionized while in the Enceladus torus, the neutral density is much greater than the ion density.
After theVoyager encounters, the possibility of a torus ofnitrogen produced by Saturn's moonTitan was proposed. Subsequent observations by theCassini spacecraft showed no clear evidence of such a torus. While neutral nitrogen could not be measured, the ions near the orbit of Titan were primarily hydrogen or water group (O+, OH+, H2O+ and H3O+) from the Enceladus torus. Trace amounts of nitrogen ions were detected but at levels consistent with an Enceladus source.
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