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Enceladus is the sixth-largest moon of Saturn. It is distinctive for its albedo of 100%, meaning it reflects almost all incoming light and is nearly white in appearance, with blue "tiger stripe" formations. Some of the most fantastic space photography focuses on Enceladus among Saturn's rings. Enceladus is named after the Titan of the same name from Greek mythology.
Enceladus is small: 504 km in diameter. It is small enough that it varies in its sphericity by a factor of a few percent; it is a flattened elipsoid. Until the Voyager 1 and Voyager 2 spacecraft made fly-bys of Saturn, very little was known about Enceladus as it merely showed up as a dot on the most powerful telescopes. When they did fly by and take pictures and measurements, scientists gained a great deal of information about Enceladus as well as the other Saturnian moons. The most notable aspect was that some of Enceladus' plains had barely any craters.
Enceladus has a moon with surface geography of widely varying age, some areas as young as 100 million years old. Enceladus is geologically active, as discovered by the recent Cassini spacecraft which explored Saturn and moons in the early to mid-2000s. A plume of water was observed, heat emanating from the planet, and a near-complete lack of craters in the south polar region, showing geological activity. As for the source of cryovolcanism, it is thought that deep warm rocks created through tidal heating feed small underground water pockets, which in turn release their pressure to the surface through the path of least resistance.
Enceladus is likely the main source of particles for Saturn's outermost ring, the dusty, diffuse E ring. This dust is likely provided by cryovolcanic activity on Enceladus. Because its escape velocity is only 866 km/h, some percentage of the cryomagma escape's Enceladus' grasp during the most intense eruptions. This has been observed in probe-taken images of Enceladus.
In size, Enceladus is comparable to the states of Arizona or Colorado. It has numerous tectonic features originally discovered by Voyager 2, including scarps, ridges, troughs, and grooves. Some rifts on its surface are up to 200 km long, 5–10 km wide, and one km deep.
How could geyser activity that couldn't last more than 30 million years still be occurring on a moon supposedly 4.6 billion years old (age of our solar system)?