Enceladus is Saturn's sixth-largest moon. Its name is taken from one of the giant children of Gaia in Greek mythology, who was crippled in battle with Athena and buried beneath (and therefore caused) the Sicilian volcano Mt. Etna. All of Saturn's moons are named after Titans in Greek mythology, because the Roman god Saturn - in the Greek pantheon, known as Cronus - was said to be the first among the Titans. All features on the moon's surface, such as craters, plains, and ridges, are named after characters in the Arabian Nights.
- Orbit and Composition -
Enceladus's orbit is only about 240,000 kilometres (150,000 miles) from the planet, over one-half the distance from the Earth to our own Moon. At this distance, it is tidally locked, meaning that its period of rotation (one Enceladus "day) and period of orbit around Saturn last the same amount of time, about 1.3 Earth-days. The same process affects Earth's own Moon, and in each case means that an observer on the planet would always see the same "face" of the moon. Data from the Cassini mission indicates that dust escaping from Enceladus's surface fuels Saturn's most distant ring, the "E" ring. Dust escaping from the moon probably survives within the rings from a few thousand to a few million years, and is constantly replenished by new debris.
Enceladus is just over 300 miles (500 kilometres) across, making it substantially smaller than its sibling moons Rhea, Titan, and Iapetus, but still large enough to have compressed into a nearly spherical shape under the force of its own gravity. After the Voyager probes visited Saturn, astronomers were surprised to discover that Enceladus's surface was comparatively young at just a few hundred million years old. Since the moon itself is much older, the explanation - since confirmed - was that the surface is continually being worked and reworked not only by impact craters but by volcanic activity.
In the last several years, Cassini has turned up new information about Enceladus which make it more remarkable. Analysis indicates that the planet's interior is unusually hot thanks to tidal heating caused by Saturn as well as an unusually high early concentration of radioactive chemicals. This means that Enceladus could have not only the very common layer of internal water ice, but also an internal water ocean - a theory given further weight after Cassini spotted water geysers on the moon's surface in 2008. This puts it onto a very short list of outer solar system moons with underwater oceans which astronomers believe might be capable of supporting primitive life.
- Discovery and Exploration -
Enceladus was first discovered by William Herschel in 1789. Enceladus must be seen through a telescope from Earth - but, because its surface is icy and therefore highly reflective, it can be seen far more easily than a rocky moon of similar size. Both the Voyager 1 and Voyager 2 space probes were able to image Enceladus, establishing that the moon orbited within one of Saturn's massive and distinctive rings and probably even contributed much of the material to that ring, in the form of dust escaping from its surface.
In the past six years, much more detailed images of Enceladus have been taken by Cassini, the first (and so far only) space probe specifically sent to explore Saturn and its rings and moons. Cassini's close approaches turned up enough interesting clues about its warm interior and potential underground ocean to put Enceladus on the list of potential targets for the next Saturn space probe. However, for the time being, a space probe destined for Jupiter and its moon Europa (which has a much larger underground ocean) have taken priority over any potential future Saturn mission.