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Enceladus with blue imaged tiger stripes This image was taken during Cassini's close flyby of Enceladus on July 14, Blue Images are newest crystaline ice media/cassini html media/cassini html

Cycloids from Europa A volatile surface. The strange surface of Europa's ice shell, which overlies a subsurface ocean, as seen from the Galileo Jupiter orbiter in The cycloid shape of many of the ridges is best explained by the propagation of fractures in the thin ice shell, controlled by the daily variation of tidal stresses from Jupiter (7). The recent results from the Hubble Space Telescope, as reported by Roth et al., suggest that similar fractures elsewhere on Europa are actively jetting water vapor into space. The image is about 200 km across. ures-only ures-only

Geysers on Enceladus This photo of water geysers spouting from Saturn's moon Enceladus was taken by NASA's Cassini orbiter in October 2007 Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA View full size image "Enceladus is this very geologically active moon and there are these plumes of water vapor and ice crystals going off of its south polar region," Hedman, the author of a study appearing in the journal Nature this week, told SPACE.com. "What we found that was surprising was that depending on where Enceladus was in its orbit, those plumes, the amount of material escaping from beneath its surface actually varied quite a bit."Enceladus Hedman and his team analyzed 252 images from Cassini to characterize the dust shot out during the eruptions and track their occurrences. The brightness of the plumes increases by more than a factor of three when the moon is farthest from Saturn, Hedman said.images from Cassini Scientists chalk this difference up to tidal stresses that warp the natural satellite. Enceladus experiences tidal stress because of its relationship to Saturn's moon Dione, John Spencer, a scientist at the Southwest Research Institute in Boulder, Colo., said.Saturn's moon Dione

Cont. Dione is pulling in a rhythmic way on Enceladus and preventing its orbit from circularizing, which it would otherwise do," Spencer, who wrote a commentary in Nature about this research, said. "[Enceladus] is sometimes a bit closer to Saturn than at other times, and that means that the tidal stresses that Saturn imposes on Enceladus … are constantly varying, so Enceladus is continually being stretched and twisted by those forces, whereas if it were in a circular orbit, those forces would be constant and nothing would change." These tidal forces could be responsible for heating up the interior of the moon, Spencer said. Tidal heating — the distortion of the moon's shape that produces heat through friction — could be a reason for the moon's warm interior, but that only explains part of the heat production. The amount of heat observed coming from Enceladus is larger than what scientists expect to observe theoretically, Spencer said. geysers.html