Enceladus is the sixth-largest moon of Saturn and the 18th largest in the Solar System. It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. It is covered by clean, freshly deposited snow hundreds of meters thick, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon reaches only , far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide variety of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrain.
Enceladus was discovered on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecraft, Voyager 1 and Voyager 2, flew by Saturn in 1980 and 1981. In 2005, the spacecraft Cassini started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapour, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totalling about per second. More than 100 geysers have been identified. Some of the water vapour falls back as snow, now several hundred metres thick; the rest escapes and supplies most of the material making up Saturn's E ring. According to NASA scientists, the plumes are similar in composition to comets. In 2014, NASA reported that Cassini had found evidence for a large south polar subsurface ocean of liquid water with a thickness of around . The existence of Enceladus's subsurface ocean has since been mathematically modelled and replicated.
These observations of active cryoeruptions, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is currently geologically active. Like many other satellites in the extensive systems of the giant planets, Enceladus participates in an orbital resonance. Its resonance with Dione excites its orbital eccentricity, which is damped by tidal forces, tidally heating its interior and driving the geological activity.
Cassini performed chemical analysis of Enceladus's plumes, finding evidence for hydrothermal activity, possibly driving complex chemistry. Still-ongoing research suggests that Enceladus's hydrothermal environment could be habitable to some of Earth's hydrothermal vent microorganisms, and that plume-found methane could be produced by such organisms.
History
Discovery
Enceladus was discovered by William Herschel on August 28, 1789, during the first use of his new 40-foot telescope, then the largest in the world, at Observatory House in Slough, England. (reported by ) Its faint apparent magnitude (HV = +11.7) and its proximity to the much brighter Saturn and Saturn's rings make Enceladus difficult to observe from Earth with smaller telescopes. Like many satellites of Saturn discovered prior to the Space Age, Enceladus was first observed during a Saturnian equinox, when Earth is within the ring plane. At such times, the reduction in glare from the rings makes the moons easier to observe. Prior to the Voyager missions the view of Enceladus improved little from the dot first observed by Herschel. Only its orbital characteristics were known, with estimations of its mass, density and albedo.
Naming
Enceladus is named after the giant Enceladus of Greek mythology. The name, like the names of each of the first seven satellites of Saturn to be discovered, was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.As reported by He chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans.
Geological features on Enceladus are named by the International Astronomical Union (IAU) after characters and places from Richard Francis Burton's 1885 translation of The Book of One Thousand and One Nights. Impact craters are named after characters, whereas other feature types, such as fossae (long, narrow depressions), dorsa (ridges), planitiae (plains), sulci (long parallel grooves), and rupes (cliffs) are named after places. The IAU has officially named 85 features on Enceladus, most recently Samaria Rupes, formerly called Samaria Fossa.
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek epsilon (the initial of Enceladus) combined with the crook of the Saturn symbol as the symbol of Enceladus (). This symbol is not widely used.
Orbit and rotation
Enceladus is the second major moon from Saturn. It orbits at from Saturn's center and from its cloud tops, between the orbits of Mimas and Tethys. It orbits Saturn every 32.9 hours, fast enough for its motion to be observed over a single night of observation.
Enceladus is currently in a 2:1 mean-motion orbital resonance with Dione, completing two orbits around Saturn for every one orbit completed by Dione. This resonance maintains Enceladus's orbital eccentricity (0.0047), which is known as a forced eccentricity. This non-zero eccentricity results in tidal deformation of Enceladus. The dissipated heat resulting from this deformation is the main heating source for Enceladus's geologic activity.
Enceladus orbits within the densest part of Saturn's E ring, the outermost of its major rings, and is the main source of the ring's material composition.
Like most of Saturn's larger satellites, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike Earth's Moon, Enceladus does not appear to librate more than 1.5° about its spin axis. However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin–orbit libration. This libration could have provided Enceladus with an additional heat source.
Source of the E ring
Plumes from Enceladus, which are similar in composition to comets, have been shown to be the source of the material in Saturn's E ring. The E ring is the widest and outermost ring of Saturn (except for the tenuous Phoebe ring). It is an extremely wide but diffuse disk of microscopic icy or dusty material distributed between the orbits of Mimas and Titan.
Mathematical models show that the E ring is unstable, with a lifespan between 10,000 and 1 million years; therefore, particles composing it must be constantly replenished. Enceladus is orbiting inside the ring, at its narrowest but highest density point. In the 1980s, some astronomers suspected that Enceladus is the main source of particles for the ring. This hypothesis was confirmed by Cassini first two close flybys in 2005.
The Cosmic Dust Analyzer (CDA) "detected a large increase in the number of particles near Enceladus", confirming it 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.
Visual confirmation of venting came in November 2005, when Cassini imaged geyser-like jets of icy particles rising from Enceladus's south polar region. (Although the plume was imaged before, in January and February 2005, additional studies of the camera's response at high phase angles, when the Sun is almost behind Enceladus, and comparison with equivalent high-phase-angle images taken of other Saturnian satellites, were required before this could be confirmed.)
Physical characteristics Shape and size
Enceladus is a relatively small satellite composed of ice and rock. It is a scalene ellipsoid in shape; its diameters, calculated from images taken by Cassini ISS (Imaging Science Subsystem) instrument, are between the sub- and anti-Saturnian poles, between the leading and trailing hemispheres, and between the north and south poles.
Enceladus is only one-seventh the diameter of Earth's Moon. It ranks sixth in both mass and size among the satellites of Saturn, after Titan (), Rhea (), Iapetus (), Dione () and Tethys ().
Internal structure
Before the Cassini mission, little was known about the interior of Enceladus. However, flybys by Cassini provided information for models of Enceladus's interior, including a better determination of the mass and shape, high-resolution observations of the surface, and new insights on the interior.
Initial mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice. However, based on the effects of Enceladus's gravity on Cassini, its mass was determined to be much higher than previously thought, yielding a density of 1.61 g/cm3. This density is higher than those of Saturn's other mid-sized icy satellites, indicating that Enceladus contains a greater percentage of silicates and iron.
suggested that Iapetus and the other icy satellites of Saturn formed relatively quickly after the formation of the Saturnian subnebula, and thus were rich in short-lived radionuclides. These radionuclides, like aluminium-26 and iron-60, have short half-lives and would produce interior heating relatively quickly. Without the short-lived variety, Enceladus's complement of long-lived radionuclides would not have been enough to prevent rapid freezing of the interior, even with Enceladus's comparatively high rock–mass fraction, given its small size.
Given Enceladus's relatively high rock–mass fraction, the proposed enhancement in 26Al and 60Fe would result in a differentiated body, with an icy mantle and a rocky core. Subsequent radioactive and tidal heating would raise the temperature of the core to 1,000 K, enough to melt the inner mantle. For Enceladus to still be active, part of the core must have also melted, forming magma chambers that would flex under the strain of Saturn's tides. Tidal heating, such as from the resonance with Dione or from libration, would then have sustained these hot spots in the core and would power the current geological activity.
In addition to its mass and modelled geochemistry, researchers have also examined Enceladus's shape to determine if it is differentiated. used limb measurements to determine that its shape, assuming hydrostatic equilibrium, is consistent with an undifferentiated interior, in contradiction to the geological and geochemical evidence. However, the current shape also supports the possibility that Enceladus is not in hydrostatic equilibrium, and may have rotated faster at some point in the recent past (with a differentiated interior). Gravity measurements by Cassini show that the density of the core is low, indicating that the core contains water in addition to silicates.
Subsurface ocean
Evidence of liquid water on Enceladus began to accumulate in 2005, when scientists observed plumes containing water vapour spewing from its south polar surface, with jets moving 250 kg of water vapour every second at up to into space. Soon after, in 2006 it was determined that Enceladus's plumes are the source of Saturn's E Ring. 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 are heavier and mostly fall back to the surface, whereas the fast "fresh" particles escape to the E ring, explaining its salt-poor composition of 0.5–2% of sodium salts by mass.
Gravimetric data from Cassini December 2010 flybys showed that Enceladus likely has a liquid water ocean beneath its frozen surface, but at the time it was thought the subsurface ocean was limited to the south pole. The top of the ocean probably lies beneath a ice shelf. The ocean may be deep at the south pole.