Gas Tori of the Outer Planets Including the Pluto-Charon System

摘要

The global environments of the outer planets usually can be characterized by studying the extended structures of neutral gas clouds that originated from planetary moons and rings. For example, the neutral cloud of Io is the main supporter for the atomic sulfur and oxygen in the Jovian system. The other example is that in the Saturnian system the water-group neutral cloud is produced by the water plume of Enceladus. The dynamical and compositional effects of these neutral gas tori in the magnetospheres have been well studied. In this study, we pay attention to the other gas tori, which is less explored so far. Our cases include the oxygen gas cloud emitted from Callisto, the Titan originated hydrogen and methane gas clouds, and the time-variable gas halo emitted from Pluto. The Callisto torus is interesting because the corresponding pickup ions could be the sources of energetic magnetospheric particles. The content of Titan's methane cloud is closely related to the atmospheric escape rate of Titan. The detection or non-detection of the methane group ions can hence be used either to test the atmospheric escape models or to probe the dynamics of Saturn's outer magnetosphere. Finally, the gas cloud model of the Pluto-Charon system could be directly compared to the observations of the X-ray emission expected from the charge-transfer mechanism of the heavy solar wind ions.