* Astronomy

Enceladus continues to exhale water ice into Saturn orbit, keeping the E ring topped off with tiny particles.


Credit NASA/JPL

Enceladus (505 kilometres across) is a source of much interest for planetary scientists, being nearly seven times smaller than Earth's own moon, yet having active geology that appears to involve near-surface liquid water.
The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on Aug. 11, 2006 at a distance of approximately 2.2 million kilometres from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 164 degrees. Image scale is 13 kilometres per pixel.

Two slim crescents smile toward the Cassini spacecraft following an occultation event.
Taken only five minutes after Enceladus (505 kilometres across) first approached the limb of Rhea (1,528 kilometres across), this view shows the bright little moon emerging from behind the larger moon's crescent. (See Enceladus Approaches for the earlier view.)


Credit NASA/JPL

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 4, 2006 at a distance of approximately 1.4 million kilometres from Rhea and 1.9 million kilometres from Enceladus. The view was obtained at a Sun-moon-spacecraft, or phase, angle of about 142 degrees relative to both moons. Image scale is 8 kilometres per pixel on Rhea and 11 kilometres on Enceladus.

This image of Enceladus was taken by the Cassini space probe on June 13, 2006, when it was approximately 4,059,328 kilometres away.



The image was taken using the CL1 and CL2 filters.

NASA-Funded Study Says Saturn's Moon Enceladus Rolled Over

Saturn's moon Enceladus - an active, icy world with an unusually warm south pole - may have performed an unusual trick for a planetary body. New research shows Enceladus rolled over, literally, explaining why the moon's hottest spot is at the south pole.

Enceladus recently grabbed scientists' attention when the Cassini spacecraft observed icy jets and plumes indicating active geysers spewing from the tiny moon's south polar region.

"The mystery we set out to explain was how the hot spot could end up at the pole if it didn't start there" - Francis Nimmo, assistant professor of Earth sciences, University of California, Santa Cruz, US.

The researchers propose the reorientation of the moon was driven by warm, low-density material rising to the surface from within Enceladus. A similar process may have happened on Uranus' moon Miranda. Their findings are in this week's journal Nature.

"It's astounding that Cassini found a region of current geological activity on an icy moon that we would expect to be frigidly cold, especially down at this moon's equivalent of Antarctica. We think the moon rolled over to put a deeply seated warm, active area there" - Robert Pappalardo, co-author and planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, US.

Pappalardo worked on the study while at the University of Colorado.
Rotating bodies, including planets and moons, are stable if more of their mass is close to the equator.

"Any redistribution of mass within the object can cause instability with respect to the axis of rotation. A reorientation will tend to position excess mass at the equator and areas of low density at the poles" - - Francis Nimmo.

This is precisely what happened to Enceladus.
Nimmo and Pappalardo calculated the effects of a low-density blob beneath the surface of Enceladus and showed it could cause the moon to roll over by up to 30-degrees and put the blob at the pole.
Pappalardo used an analogy to explain the Enceladus rollover.

"A spinning bowling ball will tend to roll over to put its holes -- the axis with the least mass -- vertically along the spin axis. Similarly, Enceladus apparently rolled over to place the portion of the moon with the least mass along its vertical spin axis" - Robert Pappalardo.

The rising blob (called a "diapir") may be within either the icy shell or the underlying rocky core of Enceladus. In either case, as the material heats up it expands and becomes less dense, then rises toward the surface. This rising of warm, low-density material could also help explain the high heat and striking surface features, including the geysers and "tiger-stripe" region suggesting fault lines caused by tectonic stress.
Internal heating of Enceladus probably results from its eccentric orbit around Saturn.

"Enceladus gets squeezed and stretched by tidal forces as it orbits Saturn, and that mechanical energy is transformed into heat energy in the moon's interior" - Francis Nimmo.

Future Cassini observations of Enceladus may support this model. Meanwhile, scientists await the next Enceladus flyby in 2008 for more clues.
This research was supported by grants from NASA. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of Caltech, manages the mission for NASA's Science Mission Directorate. The Cassini orbiter was designed, developed and assembled at JPL.

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