Enceladus, is the sixth largest moon of Saturn, and 17th largest moon in the Solar System.
Enceladus is one of only three outer solar system bodies (along with Jupiter's moon Io and Neptune's moon Triton) where active eruptions have been observed.
In May 2011 NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".
Discovery
Enceladus was discovered by Fredrick William Herschel on August 28, 1789, during the first use of his new 1.2 m telescope, then the largest in the world. Herschel first observed Enceladus in 1787, but in his smaller, 16.5 cm telescope, the moon was not recognized.
Naming
Enceladus is named after the Giant Enceladus of Greek mythology.
In Greek mythology, Enceladus, was one of the Gigantes, the enormous children of Gaia (Earth) fertilized by the blood of castrated Uranus.
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. The volcanic fires of Etna were said to be the breath of Enceladus, and its tremors to be caused by him rolling his injured side beneath the mountain.
Stats
Diameter: 504 km
Semi-major axis: 237,950 km
Orbital Period: 1.37 days
Orbit
Enceladus is one of the major inner satellites of Saturn. Enceladus orbits within the densest part of the E Ring, the outermost of Saturn's rings, an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea. As Enceladus is orbiting inside this ring, in a place where it is narrowest but present in its highest density. Therefore, several theories suspected Enceladus to be the main source of particles for the E Ring.
Enceladus is currently in a 2:1 mean motion orbital resonance with Dione, completing two orbits of Saturn for every one orbit completed by Dione. This resonance helps maintain Enceladus's orbital eccentricity (0.0047) and provides a heating source for Enceladus's geologic activity.
Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin-orbit libration. This libration, like the resonance with Dione, could have provided Enceladus with an additional heat source.
Interaction with E Ring
There are two distinct mechanisms feeding the ring with particles.
The first, and probably the most important, source of particles comes from the cryovolcanic plume in the South polar region of Enceladus. While a majority of particles fall back to the surface, some of them escape Enceladus's gravity and enter orbit around Saturn, since Enceladus's escape velocity is only 866 km/h.
The second mechanism comes from meteoric bombardment of Enceladus, raising dust particles from the surface. This mechanism is not unique to Enceladus, but is valid for all Saturn's moons orbiting inside the E Ring.
Physical characteristics
The fresh, clean ice that dominates its surface gives Enceladus probably the most reflective surface of any body in the solar system with a visual geometric albedo of 1.38. Because it reflects so much sunlight, the mean surface temperature at noon only reaches −198 °C (somewhat colder than other Saturnian satellites).
Cryovolcanism
Following the Voyager encounters with Enceladus in the early 1980s, scientists postulated that the moon may be geologically active based on its young, reflective surface and location near the core of the E ring.
Based on the connection between Enceladus and the E ring, it was thought that Enceladus was the source of material in the E ring, perhaps through venting of water vapor from Enceladus's interior. However, the Voyagers failed to provide conclusive evidence that Enceladus is active today.
Instruments on the Cassini spacecraft in 2005 discovered cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, on Enceladus.
The first Cassini sighting of a plume of icy particles above Enceladus's south pole came from the Imaging Science Subsystem (ISS) images taken in January and February 2005.
Visual confirmation of venting came in November 2005, when ISS imaged geyser-like jets of icy particles rising from the moon's south polar region. The images taken in November 2005 showed the plume's fine structure, revealing numerous jets (perhaps issuing from numerous distinct vents) within a larger, faint component extending out nearly 500 km from the surface, thus making Enceladus the fourth body in the solar system to have confirmed volcanic activity, along with Earth, Neptune's Triton, and Jupiter's Io.
Atmosphere
Data from the magnetometer instrument during the February 17, 2005 encounter found evidence for an atmosphere at Enceladus. The magnetometer observed an increase in the power of ion cyclotron waves near Enceladus. These waves are produced by the interaction of ionized particles and magnetic fields, and the frequency of the waves can be used to identify the composition, in this case ionized water vapor.
During the next two encounters, the magnetometer team determined that gases in Enceladus's atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower.
The Ultraviolet Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations during the February 17 and July 14 encounters. Unlike the magnetometer, UVIS failed to detect an atmosphere above Enceladus during the February encounter when it looked for evidence for an atmosphere over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter.
Cassini flew through this gas cloud during the July 14 encounter, allowing instruments like the Ion and Neutral Mass Spectrometer (INMS) and the cosmic dust analyzer (CDA) to directly sample the plume. INMS measured the composition of the gas cloud, detecting mostly water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide.
CDA "detected a large increase in the number of particles near Enceladus," confirming the satellite as the primary source for the E ring. Analysis of the CDA and INMS data suggest that the gas cloud Cassini flew through during the July encounter, and observed from a distance with its magnetometer and UVIS, was actually a water-rich cryovolcanic plume, originating from vents near the south pole.
Cassini's flyby on March 12, 2008 revealed additional chemicals in the plume, including simple and complex hydrocarbons such as propane, ethane, and acetylene.
In July 2009 it was announced that ammonia had been discovered during flybys in July and October 2008.
Possible Water Ocean? Life?
In late 2008, scientists observed water vapor spewing from Enceladus's surface. This could indicate the presence of liquid water, which might also make it possible for Enceladus to support life.
Eventually it was discovered that in the E-ring about 6% of particles contain 0.5–2% of sodium salts by mass, which is a significant amount. In the parts of the plume close to Enceladus the fraction of "salty" particles increases to 70% by number and >99% by mass. Such particles presumably are frozen spray from the salty underground ocean.
On the other hand, the small salt-poor particles form by homogenous nucleation directly from the gas phase. The sources of salty particles are uniformly distributed along the tiger stripes, whereas sources of "fresh" particles are closely related to the high-speed gas jets. The "salty" particles move slowly and mostly fall back onto the surface, while the fast "fresh" particles escape to the E-ring, explaining its salt-poor composition.
The "salty" composition of the plume strongly suggests that its source is a subsurface salty ocean or subsurface caverns filled with salty water. The presence of liquid water under the crust means there has to be an internal heat source. Scientists now believe it is a combination of radioactive decay and tidal heating, as tidal heating alone is not enough to explain the heat.
Additionally, Cassini found traces of organic compounds in some dust grains. Enceladus is therefore a candidate for harboring extraterrestrial life.
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