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Saturn's moon Enceladus has an atmosphere greater than that of all others in its system besides Titan, as shown by the first ''Cassini'' flybys of Enceladus. The source of the atmosphere may be volcanism, geysers, or gases escaping from the surface or the interior. The atmosphere of Enceladus is composed of 91% water vapor, 4% nitrogen, 3.2% carbon dioxide, and 1.7% methane. ==Cryovolcanism== Following the ''Voyager'' encounters with Enceladus in the early 1980s, scientists postulated that the moon may be geologically active based on its young, reflective surface and location near the core of the E ring. Based on the connection between Enceladus and the E ring, scientists suspected that Enceladus was the source of material in the E ring, perhaps through venting of water vapor from Enceladus's interior. Thanks to data from a number of instruments on the ''Cassini'' spacecraft in 2005, cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, has been discovered on Enceladus. The first ''Cassini'' sighting of a plume of icy particles above Enceladus's south pole came from the Imaging Science Subsystem (ISS) images taken in January and February 2005, though the possibility of the plume being a camera artifact stalled an official announcement. Data from the magnetometer instrument during the February 17, 2005 encounter provided a hint that the feature might be real when it found evidence for an atmosphere at Enceladus. The magnetometer observed an increase in the power of ion cyclotron waves near Enceladus. These waves are produced by the interaction of ionized particles and magnetic fields, and the frequency of the waves can be used to identify the composition, in this case ionized water vapor. During the next two encounters, the magnetometer team determined that gases in Enceladus's atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower. The Ultraviolet Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations during the February 17 and July 14 encounters. Unlike the magnetometer, UVIS failed to detect an atmosphere above Enceladus during the February encounter when it looked for evidence for an atmosphere over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter. Fortuitously, ''Cassini'' flew through this gas cloud during the July 14 encounter, allowing instruments such as the ion and neutral mass spectrometer (INMS) and the cosmic dust analyzer (CDA) to directly sample the plume. INMS measured the composition of the gas cloud, detecting mostly water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide. The CDA "detected a large increase in the number of particles near Enceladus", confirming Enceladus as the primary source for the E ring. Analysis of the CDA and INMS data suggest that the gas cloud ''Cassini'' flew through during the July encounter, and observed from a distance with its magnetometer and UVIS, was actually a water-rich cryovolcanic plume, originating from vents near the south pole. Visual confirmation of venting came in November 2005, when ISS (Imaging Science Subsystem) imaged geyser-like jets of icy particles rising from the moon's south polar region. (As stated above, the plume was imaged before, in January and February 2005, but additional studies of the camera's response at high phase angles, when the Sun is almost behind Enceladus, and comparison with equivalent high-phase-angle images taken of other Saturnian satellites, were required before this could be confirmed.) The images taken in November 2005 showed the plume's fine structure, revealing numerous jets (perhaps issuing from numerous distinct vents) within a larger, faint component extending out nearly 500 km from the surface, thus making Enceladus the fourth body in the Solar System to have confirmed contemporary volcanic activity, along with Earth, Neptune's Triton, and Jupiter's Io.〔 ''Cassini's'' UVIS later observed gas jets coinciding with the dust jets seen by ISS during a non-targeted encounter with Enceladus in October 2007. Additional observations were acquired during a flyby on March 12, 2008. Data on this flyby revealed additional chemicals in the plume, including trace amounts of simple hydrocarbons such as methane, propane, acetylene and formaldehyde. The composition of Enceladus's plume as measured by the INMS instrument on ''Cassini'' is similar to that seen at most comets.〔 The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized subsurface chambers, similar to geysers on Earth. The intensity of the eruption of the south polar jets varies significantly as a function of the position of Enceladus in its orbit. The plumes are about four times brighter when Enceladus is at apoapsis (the point in its orbit most distant from Saturn) than when it is at periapsis. This is consistent with geophysical calculations that predict that the south polar fissures will be under compression near periapsis, pushing them shut, and under tension near apoapsis, pulling them open. Much of the south polar plume activity consists of broad curtain-like eruptions. Optical illusions from a combination of viewing direction and local fracture geometry previously made the plumes look like discrete jets. File:Fountains of Enceladus PIA07758.jpg|Plumes above the limb of Enceladus feeding the E ring File:False color Cassini image of jets in the southern hemisphere of Enceladus.jpg|A false-color ''Cassini'' image of the jets 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atmosphere of Enceladus」の詳細全文を読む スポンサード リンク
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