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E nceladus H ot plasma flow Saturn V apor cloud


covered with fresh-fallen snow, placing it among the most reflective objects in the solar system. Although Enceladus was previously thought to be a cold and dead rock mass, data from the Cassini spacecraft indicate evidence of ice volcanism, which might explain its smooth surface features. In July 2005 , Cassini’s instruments detected a


cloud of water vapor over the moon’s southern pole and warm fractures where evaporating ice probably supplies the vapor cloud.4 4


So far,


> Shifting m agnetic  elds. This artist’s conception show s the detection of a dy nam ic atm osphere on Saturn’ s icy m oon Enceladus. The Cassini m agnetom eter is designed to m easure the m agnitude and direction of the


m agnetic  elds of Saturn and its m oons. During Cassini’ s three close  y b y s of Enceladus on Feb uary 17 , March 9 , and J uly 14 , 2005 , the instrum ent detected a bendi ng of the m agnetic el d around Enceladus thought to be caused b y electric currents generated b y the interaction of atm ospheric particles and the m agnetosphere of Saturn. The graphic show s the m agnetic


 eld ob served b y Cassini, as w ell as the predicted vapor cloud b eing vented from the south pole of Enceladus. Cassini’ s m agnetom eter ob served b ending of the m agnetic  eld consistent w ith its draping around a conducting ob j ect. ( Im age courtesy of NASA/ J PL. )


A B Lam b da Scorpius Starlight Starlight Bellatrix J uly Feb ruary


> Indications of an atm osphere. On J uly 11, 2005 , the Cassini ultraviolet im aging spectrograph ob served the star Bellatrix as it passed b ehind Enceladus, as seen from the spacecraft. The starlight


w as ob served to dim w hen it got close to Enceladus, indicating the presence of an atm osphere isolated to the southern pole ( A) . The ultraviolet im aging spectrograph indicated that the atm osphere


w as w ater vapor, b ased on ab sorption features in the spectrum of the star. The colors show the undim m ed star signal ( b lue) versus the dim m ed star signal ( red) . As Bellatrix reem erged from b ehind Enceladus, there w as no ob served dim m ing of the starlight. In another occultation ( B) of the star Lam b da Scorpius, no sign of an atm osphere w as detected, im ply ing that the atm osphere is localized tow ard the southern pole. ( Im age courtesy of NASA/ J PL/ Space Science Institute. )


Enceladus is the smallest body found that displays evidence of active volcanism. Scientists theorize that warm spots in the moon’s icy and cracked surface are probably the result of heat from tidal energy like the volcanoes on Jupiter’s moon Io. Its geologically young surface of water- base ice, softened by heat from below, resembles areas on Jupiter’s moons, Europa and Ganymede. Cassini flew within 175 km [ 109 miles] of Enceladus on July 14, 2005 . Data collected during that flyby confirm an extended and dynamic atmosphere. This atmosphere was first detected by Cassini’s magnetometer during a distant flyby earlier in 2005 (above left). Cassini’s magnetometer detected distur- bances in the magnetic field caused by small currents of ionized gas from the atmosphere around this moon. These could be detected by the instrument long before imaging instruments could be applied to confirm this finding. As Cassini approached this small body, imaging instruments were able to make measurements that showed gas composition, further confirming the presence of an atmosphere. The ion and natural mass spectrometers and the UVIS showed that the southern atmosphere contains water vapor (left). The mass spectrometer found that water vapor comprises about 65 % of the atmosphere, with molecular hydrogen at about 20% . The rest is mostly carbon dioxide and some combination of molecular nitrogen and carbon monoxide. The variation of water-vapor density with altitude suggests that the water vapor may come from a localized source comparable to a geothermal hot spot. The ultraviolet results strongly suggest a local vapor cloud. The fact that the atmosphere persists on this low-gravity world, instead of instantly escaping into space, suggests that the moon is geologically active enough to replenish the water vapor at a slow, continuous rate. High-resolution images show that the south pole has an even younger and more fractured appearance than the rest of Enceladus, complete


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