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Contents The Cassini-Huygens mission and the Enceladus flybys Discovery of Enceladus, Voyager observations, and Cassini's arrival Enceladus: Name and characteristics Cassini sheds new light on Enceladus Conclusion

Name. Saturn's moon Enceladus is named after the Greek mythological figure who was one of the Gigantes, the enormous children of Gaia (Earth). During the battle between the Gigantes and the Olympian gods, Enceladus was disabled by a spear thrown by the goddess Athena. He was buried on the island of Sicily, under Mount Etna, of which the volcanic fires were believed to be Enceladus' breath, and its tremors to be caused by him rolling his injured side beneath the mountain [7]. As it turned out, the name proved to be rather appropriate once the results of the Voyager and Cassini spacecraft observations became known.

Cassini shows Enceladus against a backdrop of Saturn and its rings. The E ring cannot be seen on this image as it is too faint. [NASA photo]

Satellite characteristics. Enceladus is one of Saturn's major inner satellites, and it is the 14th satellite when ordered by distance from Saturn. Enceladus orbits at a distance of 238,000 km from Saturn's center and 180,000 km from its surface, requiring 32.9 hours to revolve once around the planet. Like most of the larger satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn at all times, as in the case of the Moon in relation to Earth [1].

Enceladus orbits within the densest part of the E Ring, which is the widest and outermost ring of Saturn. Saturn's E Ring, an extremely wide but very diffuse disk of microscopic icy or dusty material [1], stretches nearly 200,000 miles from its inside edge to its outer bound and is so faint that scientists didn't discover it until about the mid 1970s. At that time, a curious thing was noticed: the ring was brightest around Enceladus, which, along with some of Saturn's other moons, wades through the E Ring's plane of debris while orbiting the planet [9]. This observation caused some scientists to suspect that Enceladus was somehow supplying material for the ring. Although this was a strange idea at the time, the fact that the E Ring existed at all was evidence for it [9]. Numerous mathematical models have shown Saturn's E ring to be unstable, with a lifespan between 10,000 and 1,000,000 years, and so the particles composing it need to be constantly replenished. Since Enceladus orbits inside the narrowest and highest density location of the E Ring, several theories have posited Enceladus as the main source of particles for the E Ring. Cassini's flyby has proven this hypothesis [1].

The evidence now seems to show that two sources from Enceladus feed Saturn's E Ring: (1) a cryovolcanic plume in Enceladus's South Polar Region and (2) the meteoritic bombardment of Enceladus, which raises dust particles from its surface. In cryovolcanism water and other volatiles are the materials erupting from a volcano, rather than silicate rock, which erupts from conventional [at least on earth] volcanoes [1].

	False-color view of Enceladus' surface, showing several tectonic and crater degradation styles.
	Taken by Cassini on 9 March 2005. [NASA photo]

Voyager 2 was the first spacecraft to observe Enceladus's surface in detail. Examinations resulting from the highest resolution imagery taken by the spacecraft revealed at least five different types of terrain, including several regions of cratered terrain, regions of smooth (young) terrain, and lanes of ridged terrain often bordering the smooth areas [4]. In addition, extensive linear cracks and scarps were observed. Given the relative lack of craters on the smooth plains, these regions are probably less than a few hundred million years old. Accordingly, Enceladus must have been recently active with "water volcanism" or other processes that renew the surface. The fresh clean ice that dominates its surface gives Enceladus the most reflective surface of any body in the solar system [6]. Because Enceladus reflects so much sunlight, the mean surface temperature at noontime only reaches -198 degrees Celsius (-198 C , or -324 F), which is somewhat colder than other Saturnian satellites [8].

Much of Enceladus's surface is covered with craters at various densities and levels of degradation, much like many other solar system bodies that have endured impact cratering. In addition, Voyager 2 found several types of tectonic features on Enceladus, including troughs, scarps, and belts of grooves and ridges [4]. Recent results from Cassini suggest that the dominant deformation style on Enceladus results from tectonics.

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List of Visuals

1. Enceladus against Saturn
   http://en.wikipedia.org/wiki/Image:Enceladus_against_Saturn.jpg
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2. EN004 Painting on the walls
   http://en.wikipedia.org/wiki/Image:EN004_Painting_on_the_walls.jpg
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