Enceladus is the sixth-largest moon of the planet Saturn. It is about 500 kilometers (310 miles) in diameter, or about a tenth of that of Saturn's largest moon Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon only reaches around -198 °C (-324 °F), far colder than a light-absorbing body would be. In spite of its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains.
Enceladus was discovered by William Herschel on August 28, 1789. Little was known about the Moon until the two Voyager spacecrafts, Voyager 1 and Voyager 2, passed near the moon in the early 1980s. In 2005, the Cassini spacecraft began multiple close flybys of Enceladus, showing its surface and environment in greater detail. Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling about 200 kg (440 lb) per second. Over 100 geysers have been identified. Some of the water vapor falls back as "snow" while 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 found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mi).
These geyser observations, 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 is trapped 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.
On June 27, 2018, scientists reported the detection of complex macromolecular organics on Enceladus' jet plumes, as sampled by the Cassini orbiter.
History[]
Discovery[]
Voyager 2 view of Enceladus in 1981.
Enceladus was discovered by William Herschel on August 28, 1789, during the first use of his new 1.2 (47 in) telescope, which was the largest in the world. 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.
Name[]
Enceladus is named after the giant Enceladus of Greek Mythology. The name, like the names of each of the first seven moons of Saturn that were discovered, was suggested by William Herschel's son John Herschel in his 11847 publication Results of Astronomical Observations made at the Cape of Good Hope. He chose these names because Saturn, known in gree kmythology as "Cronus", was the leader of the Titans.
Orbit and rotation[]
Enceladus is one of the major inner satellites of Saturn along with Dione, Tethys, and Mimas. It orbits at 238,000 km from Saturn's center and 180,000 km 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.
Geology[]
Surface features[]
Voyager 2 was the first spacecraft to observe Enceladus's surface in detail, in August 1981. Examination of the resulting highest-resolution imagery revealed at least five different types of terrain, including several regions of cratered terrain, regions of smooth (young) terrain, and lanes of ridged terrain often bordering the smooth areas. In addition, extensive linear cracks and scarps were observed. Given the relative lack of craters on the smooth plains, these regions are probably less than a few hundred million years old. Accordingly, Enceladus must have been recently active with "water volcanism" or other processes that renew the surface. The fresh, clean ice that dominates its surface gives Enceladus the most reflective surface of any body in the Solar System, with a visual geometric albedo of 1.38 and bolometric Bond albedo of 0.81 ± 0.04.
Observations during three flybys by Cassini on February 17, March 9, and July 14, 2005, revealed Enceladus' surface features in much greater detail than the Voyager 2 observations. The smooth plains, which Voyager 2 had observed, resolved into relatively crater-free regions filled with numerous small ridges and scarps. Numerous fractures were found within the older, cratered terrain, suggesting that the surface has been subjected to extensive deformation since the craters were formed. Some areas contain no craters, indicating major resurfacing events in the geologically recent past. There are fissures, plains, corrugated terrain and other crustal deformations. Several additional regions of young terrain were discovered in areas not well-imaged by either Voyager spacecraft, such as the bizarre terrain near the south pole. All of this indicates that Enceladus's interior may be liquid today, even though it should have been frozen long ago.
Impact craters[]
Impact cratering is a common occurrence on many Solar System bodies. Much of Enceladus' surface is covered with craters at various densities and levels of degradation. This subdivision of cratered terrains on the basis of crater density (and thus surface age) suggests that Enceladus has been resurfaced in multiple stages.
Cassini observations provided a much closer look at the crater distribution and size, showing that many of Enceladus' craters are heavily degraded through viscous relaxation and fracturing. Viscous relaxation allows gravity, over geologic time scales, to deform craters and other topographic features formed in water ice, reducing the amount of topography over time. The rate at which this occurs is dependent on the temperature of the ice: warmer ice is easier to deform than colder, stiffer ice. Viscously relaxed craters tend to have domed floors, or are recognized as craters only by a raised, circular rim. Dunyazad crater is a prime example of a viscously relaxed crater on Enceladus, with a prominent domed floor.
Tectonic features[]
Voyager 2 found several types of tectonic features on Enceladus, including troughs, scarps, and belts of grooves and ridges. Results from Cassini suggest that tectonics is the dominant mode of deformation on Enceladus, including rifts, one of the more dramatic types of tectonic features that were noted. These canyons can be up to 200 km long, 5–10 km wide, and 1 km deep. Such features are geologically young, because they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.
Evidence of tectonics on Enceladus is also derived from grooved terrain, consisting of lanes of curvilinear grooves and ridges. These bands, first discovered by Voyager 2, often separate smooth plains from cratered regions. Grooved terrains such as the Samarkand Sulci are reminiscent of grooved terrain on Ganymede. However, unlike those seen on Ganymede, grooved topography on Enceladus is generally more complex. Rather than parallel sets of grooves, these lanes often appear as bands of crudely aligned, chevron-shaped features. In other areas, these bands bow upwards with fractures and ridges running the length of the feature. Cassini observations of the Samarkand Sulci have revealed dark spots (125 and 750 m wide) located parallel to the narrow fractures. Currently, these spots are interpreted as collapse pits within these ridged plain belts.
In addition to deep fractures and grooved lanes, Enceladus has several other types of tectonic terrain. Many of these fractures are found in bands cutting across cratered terrain. These fractures probably propagate down only a few hundred meters into the crust. Many have probably been influenced during their formation by the weakened regolith produced by impact craters, often changing the strike of the propagating fracture. Another example of tectonic features on Enceladus are the linear grooves first found by Voyager 2 and seen at a much higher resolution by Cassini. These linear grooves can be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they are among the youngest features on Enceladus. However, some linear grooves have been softened like the craters nearby, suggesting that they are older. Ridges have also been observed on Enceladus, though not nearly to the extent as those seen on Europa. These ridges are relatively limited in extent and are up to one kilometer tall. One-kilometer high domes have also been observed. Given the level of resurfacing found on Enceladus, it is clear that tectonic movement has been an important driver of geology for much of its history.