Enceladus (moon)

Enceladus (en-sel'-a-dus, Greek Εγκέλαδος) is a moon of Saturn discovered in 1789 by William Herschel.

Name

Enceladus is named after the mythological Enceladus. It is also designated Saturn II.

The name "Enceladus" and the names of all seven satellites of Saturn then known were suggested by Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope ([3]).

Physical characteristics

Atmosphere

In March 2005 NASA announced that a magnetometer on the Cassini orbiter had detected a significant atmosphere, which might be ionized water vapour. The Cassini orbiter flew by Enceladus for the first time on February 17 2005, and performed an even closer flyby on March 9, 2005. Because Enceladus's gravity is too weak to retain an atmosphere, it must be replenished from some source, which NASA speculated to be icy volcanoes or geysers. [4] [5] [6] [7]. Although the atmosphere has been described by its discoverers as "substantial", this is only by comparison with most other icy moons; the atmosphere is a million million times thinner than Earth's, and is invisible to Cassini's imaging systems [8].

Surface

At least five different types of terrain have been identified on Enceladus. In addition to craters there are smooth plains and extensive linear cracks and ridges. At least some of the surface is relatively young, probably less than 100 million years. This means that Enceladus must have been active very recently with some sort of "water volcanism" or other process that renews the surface. The fresh, clean ice that dominates its surface gives Enceladus the highest albedo of any body in the solar system (Visual geometric albedo of 0.99). Because it reflects so much sunlight, the mean surface temperature is only -201°C.

Enceladus is much too small to be heated by the decay of radioactive material in its interior at present. Enceladus is locked in a 2:1 orbital resonance with Dione, similar to the situation between Io and Europa, and this may provide a tidal heating mechanism; however it is probably insufficient to melt water ice. Enceladus may therefore be composed of some low-melting point material instead of pure water, though no such material has yet been found by Cassini's VIMS visual and infrared spectrometer [9]. However, there are fissures, plains, corrugated terrain and other crustal deformations that indicates the interior of the moon may be liquid today, even though it should have frozen aeons ago. Furthermore, recent images from the Cassini orbiter show surface features remarkably similar to those of Europa, which may indicate that Enceladus also harbours a subsurface ocean.

Enceladus may be the source of the material in Saturn's tenuous E Ring, and since the material cannot persist in the ring for more than a few thousand years it may be due to very recent activity on Enceladus. Another possibility, though, is that the rings are maintained by high-velocity collisions between dust particles and the various moons.

Scientists officially recognise the following geological features on Enceladus:

• Craters
• Fossae (ditches)
• Planitia (plains)
• Sulci (long parallel grooves)

See also: List of geological features on Enceladus.

External links

• Images show Enceladus is a 'geologist's paradise' from New Scientist
• The Planetary Society: Enceladus information
• The Planetary Society: special page collecting Enceladus flyby images, with descriptions and context mosaics
• BBC article on atmosphere