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Enceladus (moon)

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This article is about the moon of Saturn; for the mythological giant, see Enceladus (mythology)
Enceladus (mythology)
In Greek mythology, Enceladus was one of the Gigantes, the enormous children of Gaia fertilized by the blood of castrated Uranus...

.

Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

. Until the two Voyager
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface. The Voyagers showed that the diameter of Enceladus is only 500 kilometres (310.7 mi), about a tenth of that of Saturn's largest moon, Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, and that it reflects almost all of the sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

 that strikes it. Voyager 1 found that Enceladus orbited in the densest part of Saturn's diffuse E ring, indicating a possible association between the two, while Voyager 2 revealed that despite the moon's small size, it had a wide range of terrains ranging from old, heavily cratered
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 surfaces to young, tectonically deformed terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...

, with some regions with surface ages as young as 100 million
Million
One million or one thousand thousand, is the natural number following 999,999 and preceding 1,000,001. The word is derived from the early Italian millione , from mille, "thousand", plus the augmentative suffix -one.In scientific notation, it is written as or just 106...

 years old.

In 2005 the Cassini spacecraft performed several close flybys of Enceladus, revealing the moon's surface and environment in greater detail. In particular, the probe discovered a water-rich plume venting from the moon's south polar region
Polar region
Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...

. This discovery, along with the presence of escaping internal heat
Internal heat
Internal heat is the heat source from the interior of celestial objects, such as planets, brown dwarfs, and stars, caused by gravity, nuclear fusion and decaying radioactive materials. The amount of internal heating depends on mass; the more massive the object, the more internal heat it has...

 and very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

s that lead to forced libration
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...

 or orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

; proximity to the planet can then lead to tidal heating
Tidal heating
Tidal heating occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of the moons and planets involved. Io, a moon of Jupiter, is the most volcanically active body in the solar system, with no impact craters surviving on its surface...

 of the satellite's interior, offering a possible explanation for the activity.

Enceladus is one of only three outer solar system bodies (along with Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

's moon Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

's moon Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

) where active eruptions have been observed. Analysis of the outgassing suggests that it originates from a body of sub-surface liquid water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology
Astrobiology
Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry,...

. The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E ring.

In May 2011 NASA scientists at an Enceladus Focus Group Conference reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".

Name


Enceladus is named after the Giant
Gigantes
In Greek mythology, the Giants were the children of Gaia, who was fertilized by the blood of Uranus, after Uranus was castrated by his son Cronus...

 Enceladus
Enceladus (mythology)
In Greek mythology, Enceladus was one of the Gigantes, the enormous children of Gaia fertilized by the blood of castrated Uranus...

 of Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. The name Enceladus — like the names of each of the first seven satellites of Saturn to be discovered — was suggested by William Herschel's son John Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

 in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope. He chose these names because Saturn
Saturn (mythology)
In ancient Roman religion and myth, Saturn was a major god presiding over agriculture and the harvest time. His reign was depicted as a Golden Age of abundance and peace by many Roman authors. In medieval times he was known as the Roman god of agriculture, justice and strength. He held a sickle in...

, known in Greek mythology as Cronus
Cronus
In Greek mythology, Cronus or Kronos was the leader and the youngest of the first generation of Titans, divine descendants of Gaia, the earth, and Uranus, the sky...

, was the leader of the Titans.

Features on Enceladus are named by the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 (IAU) after characters and places from the Arabian Nights
The Book of One Thousand and One Nights
One Thousand and One Nights is a collection of Middle Eastern and South Asian stories and folk tales compiled in Arabic during the Islamic Golden Age...

. Impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

s are named after characters, while other feature types, such as fossae
Fossa (geology)
Fossa is a term used in planetary geology to describe a long, narrow depression on the surface of an extraterrestrial body, such as a planet or moon...

 (long, narrow depressions), dorsa (ridges), planitia (plain
Plain
In geography, a plain is land with relatively low relief, that is flat or gently rolling. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or...

s), and sulci
Sulcus (geology)
Sulcus is, in astrogeology, a long parallel groove on a planet or a moon.For example, Uruk Sulcus is a bright region of grooved terrain adjacent to Galileo Regio on Jupiter's moon Ganymede....

 (long parallel grooves), are named after places. 57 features have been officially named by the IAU; 22 features were named in 1982 based on the results of the Voyager flybys, and 35 features were approved in November 2006 based on the results of Cassini's three flybys in 2005.

Exploration


Planned/previous Cassini encounters with Enceladus
Date
----
Distance (km)
----
February 17, 2005 1,264
March 9, 2005 500
March 29, 2005 64,000
May 21, 2005 93,000
July 14, 2005 175
October 12, 2005 49,000
December 24, 2005 94,000
January 17, 2006 146,000
September 9, 2006 40,000
November 9, 2006 95,000
June 28, 2007 90,000
September 30, 2007 98,000
March 12, 2008 52
June 30, 2008 84,000
August 11, 2008 54
October 9, 2008 25
October 31, 2008 200
November 8, 2008 52,804
November 2, 2009 103
November 21, 2009 1,607
April 28, 2010 103
May 18, 2010 201
August 13, 2010 2554
November 30, 2010 48
December 21, 2010 50
January 30, 2011 60000
February 20, 2011 68000
September 13, 2011 42000
October 1, 2011 99
October 19, 2011 1231
November 5, 2011 496
November 23, 2011 35000
December 11, 2011 20000

Enceladus was discovered by Fredrick William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

 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. Its faint apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 (+11.7m) and its proximity to much brighter Saturn and its rings make Enceladus difficult to observe from Earth, requiring a telescope with a mirror of in diameter, depending on atmospherical conditions and light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...

. Like many Saturnian satellites discovered prior to the Space Age
Space Age
The Space Age is a time period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events. The Space Age is generally considered to have begun with Sputnik...

, 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
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

 the view of Enceladus improved little from the dot first observed by Herschel. Only its orbital characteristics were known, with estimations of its mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

, density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 and albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

.

The two Voyager
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

 spacecraft obtained the first close-up images of Enceladus. Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

was the first to fly past Enceladus, at a distance of 202,000 km on November 12, 1980. Images acquired from this distance had very poor spatial resolution, but revealed a highly reflective surface devoid of impact craters, indicating a youthful surface. Voyager 1 also confirmed that Enceladus was embedded in the densest part of Saturn's diffuse E-ring. Combined with the apparent youthful appearance of the surface, Voyager scientists suggested that the E-ring consisted of particles vented from Enceladus's surface.
Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

passed closer to Enceladus (87,010 km) on August 26, 1981, allowing much higher resolution images of this satellite. These images revealed the youthful nature of much of its surface
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...

, as seen in Figure 1. They also revealed a surface with different regions with vastly different surface ages, with a heavily cratered mid- to high-northern latitude region, and a lightly cratered region closer to the equator. This geologic diversity contrasts with the ancient, heavily cratered surface of Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

, another moon of Saturn slightly smaller than Enceladus. The geologically youthful terrains came as a great surprise to the scientific community, because no theory was then able to predict that such a small (and cold, compared to Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

's highly active moon Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

) celestial body could bear signs of such activity. However, Voyager 2 failed to determine whether Enceladus was currently active or whether it was the source of the E-ring.
The answer to these and other mysteries would have to wait until the arrival of the Cassini spacecraft on July 1, 2004, when it went into orbit around Saturn. Given the results from the Voyager 2 images, Enceladus was considered a priority target by the Cassini mission planners, and several targeted flybys within 1,500 km of the surface were planned as well as numerous, "non-targeted" opportunities within 100,000 km of Enceladus. These encounters are listed at right. The flybys have yielded significant information concerning Enceladus's surface, as well as the discovery of water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 and complex hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

s venting from the geologically active South Polar Region. These discoveries have prompted the adjustment of Cassini's flight plan to allow closer flybys of Enceladus, including an encounter in March 2008 which took the probe to within 52 km of the moon's surface. The extended mission for Cassini included seven close flybys of Enceladus between July 2008 and July 2010, including two passes at only 50 km in the later half of 2008.

The discoveries Cassini has made at Enceladus have prompted several studies into follow-up missions. In 2007 NASA performed a concept study for a mission that would orbit Enceladus and would perform a detailed examination of the south polar plumes. The concept was not selected for further study. The European Space Agency also recently explored plans to send a probe to Enceladus in a mission to be combined with studies of Titan.

The Titan Saturn System Mission
Titan Saturn System Mission
Titan Saturn System Mission was a joint NASA/ESA proposal for an exploration of Saturn and its moons Titan and Enceladus, where many complex phenomena have been revealed by the recent Cassini–Huygens mission...

 (TSSM) is a joint NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

/ESA proposal for exploration of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's moons, including Enceladus. TSSM was competing against the Europa Jupiter System Mission
Europa Jupiter System Mission
The Europa Jupiter System Mission – Laplace was a proposed joint NASA/ESA unmanned space mission slated to launch around 2020 for the in-depth exploration of Jupiter's moons with a focus on Europa, Ganymede and Jupiter's magnetosphere...

 (EJSM) proposal for funding. In February 2009 it was announced that ESA/NASA had given the EJSM mission priority ahead of TSSM, although TSSM will continue to be studied for a later launch date.

Orbit



Enceladus is one of the major inner satellites of Saturn. It is the fourteenth satellite when ordered by distance from Saturn, and orbits within the densest part of the E Ring, the outermost of Saturn's rings
Rings of Saturn
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...

, an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

 and ending somewhere around the orbit of Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

.

Enceladus orbits Saturn at a distance of 238,000 km from the planet's center and 180,000 km from its cloudtops, between the orbits of Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

 and Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...

, requiring 32.9 hours to revolve once (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
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 with Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...

, 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.

Like most of the larger satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike the Earth's moon, Enceladus does not appear to librate
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...

 about its spin axis (more than 1.5°). 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, like the resonance with Dione, could have provided Enceladus with an additional heat source.

Interaction with E Ring


The E Ring is the widest and outermost ring of Saturn. It is an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

 and ending somewhere around the orbit of Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

, though some observations suggest that it extends beyond the orbit of Titan, making it 1,000,000 km wide. However, numerous mathematical models show that such a ring is unstable, with a lifespan between 10,000 and 1,000,000 years. Therefore, particles composing it must be constantly replenished. 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. This hypothesis was supported by Cassini's flyby.

There are actually two distinct mechanisms feeding the ring with particles. The first, and probably the most important, source of particles comes from the cryovolcanic
Cryovolcano
A cryovolcano is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form...

 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
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

 is only . 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.

Size and shape


Enceladus is a relatively small satellite, with a mean diameter of 505 kilometres (313.8 mi), only one-seventh the diameter of Earth's own Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

. In diameter Enceladus is small enough to fit within the length of the island of Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

. It could also fit comfortably within the states of Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...

 or Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...

, although as a spherical object its surface area is much greater, just over 800000 square kilometres (308,881.7 sq mi), almost the same as Mozambique
Mozambique
Mozambique, officially the Republic of Mozambique , is a country in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi and Zambia to the northwest, Zimbabwe to the west and Swaziland and South Africa to the southwest...

, or 15% larger than Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

.

Its mass and diameter make Enceladus the sixth most massive and largest satellite of Saturn, after Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

 , Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

 , Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

 , Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...

  and Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...

 . It is also one of the smallest of Saturn's spherical satellites, since all smaller satellites except Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

  have an irregular shape.

Enceladus has a shape of a oblate spheroid
Spheroid
A spheroid, or ellipsoid of revolution is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters....

; its dimensions, calculated from pictures taken by Cassini's ISS instrument, are of , with (a) corresponding to the diameter between sub- and anti-Saturnian poles, (b) to the diameter between the leading and trailing poles, and (c) to the distance between the north and south poles. This is the most stable orientation, with the moon's rotation along the short axis, and the long axis aligned radially away from Saturn.

Surface


Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

, in August 1981, was the first spacecraft to observe the surface in detail. Examination of the resulting highest resolution mosaic
Mosaic
Mosaic is the art of creating images with an assemblage of small pieces of colored glass, stone, or other materials. It may be a technique of decorative art, an aspect of interior decoration, or of cultural and spiritual significance as in a cathedral...

 reveals 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
Escarpment
An escarpment is a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations.-Description and variants:...

 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 probably the most reflective surface of any body in the solar system with a visual geometric albedo
Geometric albedo
The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle to that of an idealized flat, fully reflecting, diffusively scattering disk with the same cross-section....

 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).

Observations during three flybys by Cassini on February 17, March 9, and July 14 of 2005 revealed Enceladus's surface features in much greater detail than the Voyager 2 observations. For example, the smooth plains observed by Voyager 2 resolved into relatively crater-free regions filled with numerous small ridge
Ridge
A ridge is a geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. Ridges are usually termed hills or mountains as well, depending on size. There are several main types of ridges:...

s and scarps. In addition, numerous fractures were found within the older, cratered terrain, suggesting that the surface has been subjected to extensive deformation since the craters were formed. Finally, 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.

Impact craters



Impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

ing is a common occurrence on many solar system bodies. Much of Enceladus's surface is covered with craters at various densities and levels of degradation. From Voyager 2 observations, three different units
Geologic unit
A geological unit is a volume of rock or ice of identifiable origin and age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features that characterize it....

 of cratered topography were identified on the basis of their crater densities, from ct1 and ct2, both containing numerous 10–20 km-wide craters though differing in the degree of deformation, to cp consisting of lightly cratered plains. This subdivision of cratered terrains on the basis of crater density (and thus surface age) suggests that Enceladus has been resurfaced in multiple stages.

Recent Cassini observations have provided a much closer look at the ct2 and cp cratered units. These high-resolution observations, like Figure 6, reveal that many of Enceladus's craters are heavily deformed through viscous relaxation and fracturing
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...

. 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 dome
Dome
A dome is a structural element of architecture that resembles the hollow upper half of a sphere. Dome structures made of various materials have a long architectural lineage extending into prehistory....

d floors, or are recognized as craters only by a raised, circular rim (seen at center just below the terminator
Terminator (solar)
A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day side and the dark night side of a planetary body...

 in Figure 6). Dunyazad
Dunyazad (crater)
Dunyazad is a large crater on Saturn's moon Enceladus first discovered by the Voyager spacecraft. It is named after Dunyazad, the sister of Scheherazade in The Book of One Thousand and One Nights....

, the large crater seen in Figure 8 just left of top center, is a prime example of a viscously relaxed crater on Enceladus, with a prominent domed floor. In addition, many craters on Enceladus have been heavily modified by tectonic fractures. The 10-km-wide crater right of bottom center in Figure 8 is a prime example: thin fractures, several hundred meters to a kilometer wide, have heavily altered the crater's rim and floor. Nearly all craters on Enceladus thus far imaged by Cassini in the ct2 unit show signs of tectonic deformation. These two deformation styles—viscous relaxation and fracturing—demonstrate that, while cratered terrains are the oldest regions on Enceladus because of their high crater retention, nearly all craters on Enceladus are in some stage of degradation.

Tectonics


Voyager 2 found several types of tectonic features on Enceladus, including troughs
Trough (geology)
In geology, a trough generally refers to a linear structural depression that extends laterally over a distance, while being less steep than a trench.A trough can be a narrow basin or a geologic rift....

, scarps, and belts
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 of grooves and ridge
Ridge
A ridge is a geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. Ridges are usually termed hills or mountains as well, depending on size. There are several main types of ridges:...

s. Recent results from Cassini suggest that tectonism is the dominant deformation style on Enceladus. One of the more dramatic types of tectonic features found on Enceladus are rifts. These canyons can be up to 200 km long, 5–10 km wide, and one km deep. Figure 7 shows a typical large fracture on Enceladus cutting across older, tectonically deformed terrain. Another example can be seen running along the bottom of the frame in Figure 8. Such features appear relatively young, as they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.

Another example of tectonism on Enceladus is grooved terrain, consisting of lanes of curvilinear grooves and ridges. These bands, first discovered by Voyager 2, often separate smooth plains from cratered regions. An example of this terrain type can be seen in Figures 6 and 10 (in this case, a feature known as the Samarkand Sulci
Samarkand Sulci
Samarkand Sulci is a region of grooved terrain on the surface of Saturn's moon Enceladus. The feature is centered at 30.5° North Latitude, 326.8° West Longitude and is approximately 383 kilometers long. Samarkand Sulci consists of three parts. The southern and eastern extensions bound Sarandib...

). Grooved terrains such as the Samarkand Sulci are reminiscent of grooved terrain on Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

. However, unlike those seen on Ganymede, grooved topography on Enceladus is generally much more complex. Rather than parallel sets of grooves, these lanes can often appear as bands of crudely aligned, chevron-shaped features. In other areas, these bands appear to bow upwards with fractures and ridges running the length of the feature. Cassini observations of the Samarkand Sulci have revealed intriguing dark spots (125 and 750 m wide), which appear to run parallel to 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. Figure 9 shows sets of narrow fractures (still several hundred meters wide) that were first discovered by the Cassini spacecraft. Many of these fractures are found in bands cutting across cratered terrain. These fractures appear to propagate down only a few hundred meters into the crust. Many appear to have been influenced during their formation by the weakened regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

 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. Examples of linear grooves can be found in the lower left of the figure at top and Figure 10 (lower left), running from north to south from top center before turning to the southwest. These linear grooves can be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they appear to be among the youngest features on Enceladus. However, some linear grooves appear to be softened like the craters nearby, suggesting an older age. Ridges have also been observed on Enceladus, though not nearly to the extent as those seen on Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

. Several examples can be seen in the lower left corner of Figure 7. These ridges are relatively limited in extent and are up to one km tall. One-kilometer high domes have also been observed. Given the level of tectonic resurfacing found on Enceladus, it is clear that tectonism has been an important driver of geology on this small moon for much of its history.

Smooth plains


Two units of smooth plains were also observed by Voyager 2. These plains generally have low relief and have far fewer craters than in the cratered terrains and plains, indicating a relatively young surface age. In one of the smooth plain regions, Sarandib Planitia
Sarandib Planitia
Sarandib Planitia is a region of relatively un-cratered terrain on Saturn's moon Enceladus. It is located at 4.4° North Latitude, 298.0° West Longitude and is approximately 200 km across...

, no impact craters were visible down to the limit of resolution. Another region of smooth plains to the southwest of Sarandib, is criss-crossed by several troughs and scarps. Cassini has since viewed these smooth plains regions, like Sarandib Planitia and Diyar Planitia
Diyar Planitia
Diyar Planitia is a region of relatively un-cratered terrain on Saturn's moon Enceladus. It is located at 0.5° North Latitude, 239.7° West Longitude and is approximately 311 km across....

 at much higher resolution. Cassini images show smooth plain regions to be filled with low-relief ridges and fractures. These features are currently interpreted as being caused by shear deformation
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...

. The high resolution images of Sarandib Planitia have revealed a number of small impact craters, which allow for an estimate of the surface age, either 170 million years or 3.7 billion
1000000000 (number)
1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 109....

 years, depending on assumed impactor population.

The expanded surface coverage provided by Cassini has allowed for the identification of additional regions of smooth plains, particularly on Enceladus's leading hemisphere (the side of Enceladus that faces the direction of motion as the moon orbits Saturn). Rather than being covered in low relief ridges, this region is covered in numerous criss-crossing sets of troughs and ridges, similar to the deformation seen in the south polar region. This area is on the opposite side of the satellite from Sarandib and Diyar Planitiae, suggesting that the placement of these regions is influenced by Saturn's tides on Enceladus.

South polar region


Images taken by Cassini during the flyby on July 14, 2005 revealed a distinctive, tectonically deformed region surrounding Enceladus's south pole. This area, reaching as far north as 60° south latitude, is covered in tectonic fractures and ridges. The area has few sizable impact craters, suggesting that it is the youngest surface on Enceladus and on any of the mid-sized icy satellites; modeling of the cratering rate suggests that some regions of the SPT are possibly as young as 500,000 years, or younger. Near the center of this terrain are four fractures bounded on either side by ridges, unofficially called "tiger stripes
Tiger Stripes (Enceladus)
The tiger stripes of Enceladus consist of four sub-parallel, linear depressions in the south polar region of the Saturnian moon. First observed on May 20, 2005 by the Cassini spacecraft's Imaging Science Sub-system camera , the features are most notable in lower resolution images by their...

". These fractures appear to be the youngest features in this region and are surrounded by mint-green-colored (in false color, UV-green-near IR images), coarse-grained water ice, seen elsewhere on the surface within outcrops and fracture walls. Here the "blue" ice is on a flat surface, indicating that the region is young enough not to have been coated by fine-grained water ice from E ring. Results from the visual and infrared spectrometer (VIMS) instrument suggest that the green-colored material surrounding the tiger stripes is chemically distinct from the rest of the surface of Enceladus. VIMS detected crystalline water ice in the stripes, suggesting that they are quite young (likely less than 1000 years old) or the surface ice has been thermally altered in the recent past. VIMS also detected simple organic compounds in the tiger stripes, chemistry not found anywhere else on the satellite thus far.

One of these areas of “blue” ice in the south polar region was observed at very high resolution during the July 14 flyby, revealing an area of extreme tectonic deformation and blocky terrain, with some areas covered in boulders 10–100 m across.


The boundary of the south polar region is marked by a pattern of parallel, Y- and V-shaped ridges and valleys. The shape, orientation, and location of these features indicate that they are caused by changes in the overall shape of Enceladus. , there are two theories for what could cause such a shift in shape. First, the orbit of Enceladus may have migrated inward (from the article: "the lack of any plausible mechanism for increased flattening"), leading to an increase in Enceladus's rotation rate. Such a shift would have led to a flattening of Enceladus's rotation axis. Another theory suggests that a rising mass of warm, low density material in Enceladus's interior led to a shift in the position of the current south polar terrain from Enceladus's southern mid-latitudes to its south pole. Consequently, the ellipsoid shape of Enceladus would have adjusted to match the new orientation. One consequence of the axial flattening theory is that both polar regions should have similar tectonic deformation histories. However, the north polar region is densely cratered, and has a much older surface age than the south pole. Thickness variations in Enceladus's lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 is one explanation for this discrepancy. Variations in lithospheric thickness are supported by the correlation between the Y-shaped discontinuities and the V-shaped cusps along the south polar terrain margin and the relative surface age of the adjacent non-south polar terrain regions. The Y-shaped discontinuities, and the north-south trending tension fractures into which they lead, are correlated with younger terrain with presumably thinner lithospheres. The V-shaped cusps are adjacent to older, more heavily cratered terrains.

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.

Thanks to data from a number of instruments on the Cassini spacecraft in 2005, cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, has been discovered 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, though the possibility of the plume being a camera artifact stalled an official announcement. Data from the magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...

 instrument during the February 17, 2005 encounter provided a hint that the feature might be real when it found evidence for an atmosphere at Enceladus. The magnetometer observed an increase in the power of ion cyclotron waves
Electrostatic ion cyclotron wave
An electrostatic ion cyclotron wave is a longitudinal oscillation of the ions in a magnetized plasma, propagating nearly perpendicular to the magnetic field...

 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 ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

ized water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

. During the next two encounters, the magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...

 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
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

 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.

Fortuitously, Cassini flew through this gas cloud during the July 14 encounter, allowing instruments like the Ion and Neutral Mass Spectrometer
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

 (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
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

. 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.

Visual confirmation of venting came in November 2005, when ISS imaged geyser
Geyser
A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase . The word geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb geysa, "to gush", the verb...

-like jets
Plume (hydrodynamics)
In hydrodynamics, a plume is a column of one fluid or gas moving through another. Several effects control the motion of the fluid, including momentum, diffusion, and buoyancy...

 of icy particles rising from the moon's south polar region. (As stated above, the plume was imaged before, in January and February 2005, but additional studies of the camera's response at high phase angles, when the sun is almost behind Enceladus, and comparison with equivalent high phase images taken of other Saturnian satellites, were required before this could be confirmed.) 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
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, Neptune's Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

, and Jupiter's Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

. Cassini's UVIS later observed gas jets coinciding with the dust jets seen by ISS during a non-targeted encounter with Enceladus in October 2007.

Additional observations were acquired during a flyby on March 12, 2008. Data on this flyby revealed additional chemicals in the plume, including simple and complex hydrocarbons such as propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

, ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....

, and acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

. This finding further raises the potential for life beneath the surface of Enceladus. The composition of Enceladus's plume as measured by the INMS instrument on Cassini is similar to that seen at most comets.

Prior to the ammonia discovery


The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized sub-surface chambers, similar to geyser
Geyser
A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase . The word geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb geysa, "to gush", the verb...

s on Earth. Because no ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 was found in the vented material by INMS or UVIS, which could act as an anti-freeze, such a heated, pressurized chamber would consist of nearly pure liquid water with a temperature of at least 270 K (-3.1 °C), as illustrated in Figure 14. Pure water would require more energy to melt, either from tidal or radiogenic sources, than an ammonia-water mixture. Another possible method for generating a plume is sublimation
Sublimation (physics)
Sublimation is the process of transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase...

 of warm surface ice. During the July 14, 2005 flyby, the Composite Infrared Spectrometer (CIRS) found a warm region near the South Pole. Temperatures found in this region range from 85–90 K, to small areas with temperatures as high as 157 K (-116.2 °C), much too warm to be explained by solar heating, indicating that parts of the south polar region are heated from the interior of Enceladus. Ice at these temperatures is warm enough to sublimate at a much faster rate than the background surface, thus generating a plume. This hypothesis is attractive since the sub-surface layer heating the surface water ice could be an ammonia-water slurry at temperatures as low as 170 K (-103.2 °C), and thus not as much energy is required to produce the plume activity. However, the abundance of particles in the south polar plume favors the "cold geyser" model, as opposed to an ice sublimation model.

Alternatively, Kieffer et al. (2006) suggest that Enceladus's geysers originate from clathrate hydrates, where carbon dioxide, methane, and nitrogen are released when exposed to the vacuum of space by the active, tiger stripe fractures. This hypothesis would not require the amount of heat needed to melt water ice as required by the "Cold Geyser" model, and would explain the lack of ammonia.

Ammonia discovery


In July 2009 it was announced that ammonia had been discovered during flybys in July and October 2008.

Internal structure



Prior to the Cassini mission, relatively little was known about the interior of Enceladus. However, results from recent flybys of Enceladus by the Cassini spacecraft have provided much needed information for models of Enceladus's interior. These include a better determination of the mass and tri-axial ellipsoid shape, high-resolution observations of the surface, and new insights on Enceladus's geochemistry
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

.

Mass estimates from the Voyager program
Voyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...

 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
Gram
The gram is a metric system unit of mass....

/cm³. This density is higher than Saturn's other mid-sized icy satellites, indicating that Enceladus contains a greater percentage of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

s and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

. With additional material besides water ice, Enceladus's interior may have experienced comparatively more heating from the decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 of radioactive elements.

Castillo et al. 2005 suggested that Iapetus
Iapetus (moon)
Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

, and the other icy satellites of Saturn, formed relatively quickly after the formation of the Saturnian sub-nebula, 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 Enceladus's small size. Given Enceladus's relatively high rock-mass fraction, the proposed enhancement in 26Al and 60Fe would result in a differentiated
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

 body, with an icy mantle and a rocky core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...

. Subsequent radioactive and tidal
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

 heating would raise the temperature of the core to 1000 K, enough to melt the inner mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

. However, for Enceladus to still be active, part of the core must have melted too, forming magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 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 until the present, and would power the current geological activity.

In addition to its mass and modeled geochemistry
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

, researchers have also examined Enceladus's shape to test whether the satellite is differentiated or not. Porco et al. 2006 used limb measurements to determine that Enceladus's shape, assuming it is in hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

, 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).

Possible water ocean


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.
Candice Hansen, a scientist with NASA's Jet Propulsion Lab, headed up a research team on the plumes after they were found to be moving at ~2,189 kilometers per hour (1,360 miles per hour). Since that speed is difficult to attain unless liquids are involved, they decided to investigate the compositions of the plumes.

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
Tiger Stripes (Enceladus)
The tiger stripes of Enceladus consist of four sub-parallel, linear depressions in the south polar region of the Saturnian moon. First observed on May 20, 2005 by the Cassini spacecraft's Imaging Science Sub-system camera , the features are most notable in lower resolution images by their...

, 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. Alternatives such as the clathrate sublimation hypothesis can not explain how "salty" particles form. Additionally, Cassini found traces of organic compounds in some dust grains. Enceladus is therefore a candidate for harboring extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

.

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
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 and tidal heating
Tidal heating
Tidal heating occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of the moons and planets involved. Io, a moon of Jupiter, is the most volcanically active body in the solar system, with no impact craters surviving on its surface...

, as tidal heating alone is not enough to explain the heat. Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....

, another of Saturn's moons, is closer to the planet and has a much more eccentric orbit, meaning it should be exposed to far greater tidal forces than Enceladus, and yet its old and scarred surface implies that it is geologically dead.

See also


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