Iapetus

Photomosaic of Cassini images taken Dec. 31, 2004, showing the dark Cassini Regio and its border with the bright Roncevaux Terra, several large craters, and the equatorial ridge
Discovery
Discovered by	G. D. Cassini
Discovery date	October 25, 1671
Orbital characteristics
Semi-major axis	3,560,820 km
Eccentricity	0. Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer Events 1147 - The Portuguese, under Afonso I, and Crusaders from England and Flanders conquer Lisbon after a In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape 0286125[1]
Orbital period	79. The orbital period is the time taken for a given object to make one complete Orbit about another object 3215 d
Inclination	17. A day (symbol d is a unit of Time equivalent to 24 Hours and the duration of a single Rotation of planet Earth with respect to the Inclination in general is the Angle between a Reference plane and another plane or axis of direction 28° (to the ecliptic) 15. 47° (to Saturn's equator) 7. 52° (to Laplace plane)
Satellite of	Saturn
Physical characteristics
Dimensions	1494. The Laplace plane, named after Pierre-Simon Laplace, is defined as the mean plane occupied by the orbit of a Satellite during a Precession cycle A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. 8×1424. 8 km[2]
Mean radius	735. 60 ± 3 km[2]
Surface area	6,700,000 km²
Mass	1. Equation A spheroid centered at the origin and rotated about the z axis is defined by the implicit equation \left(\frac{x}{a}\right^2+\left(\frac{y}{a}\right^2+\left(\frac{z}{b}\right^2 Square Kilometre ( US spelling square kilometer) symbol km2, is a decimal multiple of the SI unit of Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object 805635±0. 000375×1021 kg[3]
Mean density	1. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different 0830±0. 0066 g/cm³[3]
Equatorial surface gravity	0. For other uses of the words gram or gramme see Gram (disambiguation. A cubic centimetre or cubic centimeter (symbol cm3 —the abbreviation cc, though widely used is deprecated is a commonly used unit of Volume The surface gravity, g, of an astronomical or other object is the Gravitational acceleration experienced at its surface 223 m/s2
Escape velocity	0. In Physics, escape velocity is the speed where the Kinetic energy of an object is equal to the magnitude of its Gravitational potential energy 572 km/s
Rotation period	79. The rotation period of an astronomical object is the time it takes to complete one revolution around its Axis of rotation relative to the background stars 3215 d (synchronous)
Axial tilt	zero
Albedo	0. In Astronomy, synchronous rotation is a planetological term describing a body orbiting another where the orbiting body takes as long to rotate In Astronomy, axial tilt is the Inclination angle of a planet's rotational axis in relation to its orbital plane. The albedo of an object is the extent to which it diffusely reflects light from the sun 05-0. 5[4]
Apparent magnitude	10. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 2-11. 9[5]
Adjectives	Iapetian, Japetian

Iapetus (pronounced /aɪˈæpɨtəs/ eye-AP-ə-təs, or as Greek Ιαπετός), occasionally Japetus (/ˈdʒæpɨtəs/),^[6] is the third-largest moon of Saturn, and eleventh in the solar system, discovered by Giovanni Domenico Cassini in 1671. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer Iapetus is best known for its dramatic 'two-tone' coloration, but recent discoveries by the Cassini mission have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon. Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its

Discovery

Iapetus was discovered by Giovanni Domenico Cassini in October 1671 on the western side of Saturn. Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer Then Cassini tried unsuccessfully to observe it on the eastern side of the planet in early 1672. This pattern continued as Cassini observed Iapetus in December 1672 and February 1673, each time tracking it for a fortnight on the western side of Saturn, but he was unable to detect it during the intervening period, when it should have been on the eastern side. Note Please link only to revelant articles per "WPOVERLINK" Cassini finally observed Iapetus on the eastern side in 1705 with an improved telescope, finding it two magnitudes dimmer on that side. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value ^[7][8]

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn, so that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and the dark hemisphere on the other side. A separate article treats the phenomenon of Tidal resonance in Oceanography. The dark hemisphere was later named Cassini Regio in his honour. Cassini Regio is the enigmatic dark area that covers half of Saturn 's moon Iapetus.

Name

Iapetus is named after the Titan Iapetus from Greek mythology. In Greek mythology, the Titans ( Greek: Tītā́n; plural Tītânes) were a race of powerful Deities that ruled during the legendary In Greek mythology, Iapetus, also Iapetos or Japetus (Ἰαπετός was a Titan, the son of Uranus and Gaia, and father Greek mythology is the body of stories belonging to the ancient Greeks concerning their gods and Heroes the nature of the world and the origins and significance It is also designated Saturn VIII.

Iapetus was among four Saturnian moons labelled the Sidera Lodoicea by their discoverer Giovanni Cassini after King Louis XIV (the other three were Tethys, Dione and Rhea). Saturn has 60 confirmed moons. These include 22 regular satellites, which all have Prograde orbits that are not greatly inclined with respect Sidera Lodoicea (ˈsɪdərə ˌlɒdoʊˈɪʃ(iə is the name given by the astronomer Giovanni Domenico Cassini to the four moons of Saturn discovered by him in the Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer Early years Birth and ancestry Louis XIV was born in the Château de Saint-Germain-en-Laye on September 5 1638 and bore the Heir apparent TemplateInfobox Planet.--> Tethys (ˈtiːθɨs, /ˈtɛθɨs/, or TemplateInfobox Planet.--> Dione (daɪˈoʊni, or as in Greek TemplateInfobox Planet.--> Rhea (ˈriːə, or as in Greek However, astronomers fell into the habit of referring to them by numbers, with Iapetus as Saturn V. Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended and Iapetus became Saturn VII, and subsequently Saturn VIII after the discovery of Hyperion in 1848. TemplateInfobox Planet. --> Mimas (ˈmaɪməs, or as Greek TemplateInfobox Planet.--> This article is about the moon of TemplateInfobox Planet.--> Hyperion (haɪˈpɪəriən, or as in It is still known by that number today (see naming of natural satellites). In ancient times only the Sun and Moon, a few hundred Stars and the most easily visible Planets had names

The name Japetus (a spelling still in occasional use) was suggested by John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,^[6] in which he advocated naming the moons of Saturn after the Titans, sisters and brothers of the Titan Cronus (whom the Romans equated with their god Saturn). Sir John Frederick William Herschel 1st Baronet KH, FRS ( March 7, 1792 &ndash May 11, 1871)was an Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British Cronus or Kronos, ( Ancient Greek Κρόνος Krónos) was the leader and the youngest of the first generation of Titans, divine descendants Roman mythology, or more appropriately Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its Saturn ( Latin: Saturnus) was a major Roman God of agriculture and harvest The adjectival form is Iapetian (/ˌaɪəˈpiːtiən/) or Japetian (/dʒəˈpiːtiən/).

Geological features on Iapetus are named after characters and places from the French epic poem The Song of Roland (examples of names used include the craters Charlemagne and Baligant, and the bright region, Roncevaux Terra). The Song of Roland (La Chanson de Roland is the oldest remaining major work of French literature. Roncevaux Terra is the name given to the highly reflective side of Saturn 's moon Iapetus. The one exception is Cassini Regio, the dark region of the moon, named after the region's discoverer, Giovanni Cassini. Cassini Regio is the enigmatic dark area that covers half of Saturn 's moon Iapetus.

Physical characteristics

The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Ice is a Solid phase, usually crystalline, of a Non-metalic substance that is liquid or gas at Room temperature, such as Ammonia ^[9]

Cassini mosaic of Iapetus, showing the bright trailing hemisphere with part of the dark area appearing on the right-hand side

Unlike most moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles^[10]; also, its unique equatorial ridge (see below) is so high that it visibly distorts the moon's shape even when viewed from a distance. "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe An ellipsoid is a type of quadric surface that is a higher dimensional analogue of an Ellipse. These features often lead it to be characterized as walnut-shaped.

Iapetus is heavily cratered, and Cassini images have revealed large impact basins in the dark region, at least five of which are over 350 km wide. 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 larger body The largest has a diameter over 500 km; its rim is extremely steep and includes a scarp over 15 km high. Geometry, a diameter of a Circle is any straight Line segment that passes through the center of the circle and whose Endpoints are on the In Geomorphology, an escarpment is a transition zone between different physiogeographic provinces that involves a sharp steep Elevation differential characterized

A composite image map of Iapetus's surface

Two-tone coloration

In the 17th century, Giovanni Cassini observed that he could see Iapetus only on the west side of Saturn and never on the east. Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer He correctly deduced that Iapetus is locked in synchronous rotation about Saturn and that one side of Iapetus is darker than the other, a conclusion later confirmed by larger telescopes. In Astronomy, synchronous rotation is a planetological term describing a body orbiting another where the orbiting body takes as long to rotate

Cassini Regio

Roncevaux Terra

The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (albedo 0. The albedo of an object is the extent to which it diffusely reflects light from the sun 03–0. 05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0. Red is any of a number of similar Colors evoked by light consisting predominantly of the longest wavelengths of Light discernible by the human eye in the wavelength 5-0. 6, almost as bright as Europa). TemplateInfobox Planet.--> Europa (jʊˈroʊpə; or as Thus, the apparent magnitude of the trailing hemisphere is around 10. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 2, whereas that of the leading hemisphere is around 11. 9 — beyond the capacity of the best telescopes in the 17th century. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. The pattern of coloration is analogous to a spherical yin-yang symbol or the two sections of a tennis ball. In Chinese philosophy, the concept of yin and yang ( is used to describe how seemingly opposing forces are bound together intertwined and interdependent in the The dark region is named Cassini Regio, and the bright region Roncevaux Terra. Cassini Regio is the enigmatic dark area that covers half of Saturn 's moon Iapetus. Roncevaux Terra is the name given to the highly reflective side of Saturn 's moon Iapetus. The original dark material is believed to have come from outside Iapetus, but now it consists principally of lag from the sublimation of ice from the warmer areas of Iapetus's surface. It contains organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets; Earth-based observations have shown it to be carbonaceous, and it probably includes cyano-compounds such as frozen hydrogen cyanide polymers. Organic matter (or organic material) is Matter that has come from a once-living Organism; is capable of A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — Carbonaceous is the defining attribute of a substance rich in Carbon. Hydrogen cyanide is a Chemical compound with Chemical formula HCN A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds

On September 10, 2007, the Cassini orbiter passed within 1,640 kilometres (1,000 miles) of Iapetus and demonstrated that both hemispheres are heavily cratered. Events 506 - The Bishops of Visigothic Gaul meet in the Council of Agde. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. The color dichotomy of scattered patches of light and dark material in the transition zone between Cassini Regio and Roncevaux exists at very small scales, down to the imaging resolution of 30 meters. There is dark material filling in low-lying regions, and light material on the pole-facing slopes of craters, but no shades of grey. ^[11] The material is a very thin layer, only a few tens of centimeters (approx. one foot) thick at least in some areas,^[12] according to Cassini radar imaging and by the fact that very small meteor impacts have punched through to the ice underneath. ^[13]

Close-up of northern pole.

NASA scientists now believe that the dark material may be lag (residue) from the sublimation (evaporation) of water ice on the surface of Iapetus,^[13] possibly darkened further upon exposure to sunlight. Sublimation of an element or compound is a transition from the Solid to Gas phase with no intermediate liquid stage Because of its slow rotation of 79 days (equal to its revolution and the longest in the Saturnian system), Iapetus likely had the warmest daytime surface temperature and coldest nighttime temperature in the Saturnian system even before the development of the color contrast; near the equator, heat absorption by the dark material results in a daytime temperatures of 128 K in the dark Cassini Regio compared to 113 K in the bright Roncevaux Terra. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic ^[14] The difference in temperature means that ice preferentially sublimates from Cassini, and precipitates in Roncevaux and especially at the even colder poles. Over geologic time scales, this would further darken Cassini and brighten Roncevaux and the poles, with all exposed ice being lost from Cassini, creating a thermal positive feedback for ever greater contrast in albedo. Positive feedback, sometimes referred to as "cumulative causation" is a Feedback loop system in which the system responds to perturbation in the same direction It is estimated that, at current temperatures, over one thousand million years Cassini would lose about 20 meters of ice to sublimation, while Roncevaux would lose only 10 centimeters, not considering the ice transferred from the dark regions. ^[14] This model explains the distribution of light and dark areas, the absence of shades of grey, and the thinness of the dark material covering Cassini.

However, a separate process of color segregation would be required to get the thermal feedback started. The initial dark material is thought to have been debris blasted by meteors off small outer moons in retrograde orbits and swept up by the leading hemisphere of Iapetus. The core of this model is some 30 years old, and has been revived by the September flyby.

Light debris outside of Iapetus's orbit, either knocked free from the surface of a moon by micrometeoroid impacts or created in a collision, would spiral in as its orbit decays. A Micrometeoroid (also micrometeorite, micrometeor) is a tiny Meteoroid; a small particle of rock in space usually weighing less than a Gram It would have been darkened by exposure to sunlight. A portion of any such material that crossed Iapetus's orbit would have been swept up by its leading hemisphere, potentially coating it to create a contrast in albedo, and so a contrast in temperature, that could have been exaggerated by the thermal feedback described above.

Close-up of 10 km high mountains within the equatorial ridge in Iapetus's dark region

The largest reservoir of such material is Phoebe, the largest of the outer moons. Equatorial ridges are a feature of at least three of Saturn 's moons: the large moon Iapetus and the tiny moons Atlas and Pan. TemplateInfobox Planet.--> Phoebe (ˈfiːbi, or as in Greek Although Phoebe's composition is closer to that of the bright hemisphere of Iapetus than the dark one,^[15] dust from Phoebe would only be needed to establish a contrast in albedo, and presumably would have been largely obscured by later sublimation.

Overall shape

Current triaxial measurements of Iapetus give it dimensions of 747. 1 × 749 × 712. 6 km, with a mean radius of 736 ±2km. ^[2] However, these measurements may be inaccurate on the kilometer scale as Iapetus's entire surface has not yet been imaged in high enough resolution. The observed oblateness corresponds to a rotation period of 10 hours, not to the 79 days observed. A possible explanation for this is that the shape of the moon was frozen by formation of a thick crust shortly after its formation, while its rotation continued to slow afterwards due to tidal dissipation, until it became tidally locked. In Geology, a crust is the outermost solid shell of a planet or moon A separate article treats the phenomenon of Tidal resonance in Oceanography. ^[10]

Closeup of the equatorial ridge

Equatorial ridge

A further mystery of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about 1,300 km long, 20 km wide, 13 km high. Equatorial ridges are a feature of at least three of Saturn 's moons: the large moon Iapetus and the tiny moons Atlas and Pan. It was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004. Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia. "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " Parts of the ridge rise more than 20 km above the surrounding plains. The ridge forms a complex system including isolated peaks, segments of more than 200 km and sections with three near parallel ridges. ^[16] Within the bright Roncevaux Terra there is no ridge, but there are a series of isolated 10 km peaks along the equator. ^[17] The ridge system is heavily cratered, indicating that it is ancient. The prominent equatorial bulge gives the moon a walnut-like appearance. Walnuts (genus Juglans) are Plants in the family Juglandaceae.

It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are at least three current hypotheses, but none of them explains why the ridge is confined to Cassini Regio.

1. A team of scientists associated with the Cassini mission have argued that the ridge could be a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today. ^[18] The height of the ridge suggests a maximum rotational period of 17 hours. If Iapetus cooled fast enough to preserve the ridge but remain plastic long enough for the tides raised by Saturn to have slowed the rotation to its current tidally locked 79 days, Iapetus must have been heated by the radioactive decay of aluminium-26. Characteristics A tide is a repeated cycle of sea level changes in the following stages Over several hours the water rises or advances up a beach in the flood WikipediaNaming This isotope appears to have been abundant in the solar nebula from which Saturn formed, but has since all decayed. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides The formation and evolution of the Solar System is estimated to have begun The quantities of aluminium-26 needed to heat Iapetus to the required temperature give a tentative date to its formation relative to the rest of the Solar System: Iapetus must have come together earlier than expected, only two million years after the asteroids started to form. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but
2. The ridge could be icy material that welled up from beneath the surface and then solidified. If it had formed away from the then equator, this hypothesis requires that the rotational axis would have been driven to its current position by the ridge.
3. It has also been suggested that Iapetus could have had a ring system during its formation due to its large Hill sphere, and that the equatorial ridge was then produced by collisional accretion of this ring. A Hill sphere is roughly the volume around an Astronomical body (such as a Planet) where it dominates in attraction of Satellites to that body rather ^[19] However, the ridge appears too solid to be the result of a collapsed ring. Also, recent images show tectonic faults running through the ridge, apparently inconsistent with the collapsed ring hypothesis^[13].

Temperatures

Temperatures on the dark region's surface reach 130 K (-143. 2 °C or -226 °F) at the equator, as heating is made more effective by Iapetus's slow rotation. The Celsius Temperature scale was previously known as the centigrade scale. Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736 a German Physicist who proposed it in 1724 The brighter surfaces absorb less sunlight so temperatures there only reach about 100 K (-173. 2 °C or -280 °F). ^[20]

Orbit

Polar view of Iapetus's orbit (red) compared to the other large moons of Saturn

The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. TemplateInfobox Planet.--> Titan (ˈtaɪtən, or as It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. TemplateInfobox Planet.--> Phoebe (ˈfiːbi, or as in Greek The cause of this is unknown.

Because of this distant, inclined orbit, Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see. From Iapetus, Saturn would appear to be 1°56' in diameter (four times that of the Moon viewed from Earth). The angular diameter of an object as seen from a given position is the "visual diameter" of the object measured as an angle EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 ^[21]

Side view of Iapetus's orbit (red) compared to the other large moons, showing its unusually high inclination

Computer simulation of the appearance of Saturn from Iapetus when the moon is at the 'lowest' point in its inclined orbit. Saturn's rings are clearly visible (from the other large moons they can only be seen edge-on).

Exploration

Iapetus has been imaged multiple times from moderate distances by the Cassini orbiter. Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its However, its orbit makes close observation difficult. There has been one close targeted fly-by, at 1227 km on September 10, 2007; there are no plans for any others. Events 506 - The Bishops of Visigothic Gaul meet in the Council of Agde. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century.

Iapetus in fiction

See: Iapetus in fiction

Gallery

From the Voyager 2 spacecraft

Lit by light reflected off Saturn

Landslide on Iapetus

Cassini Regio

See also

• List of geological features on Iapetus

References

External links

• Cassini mission page - Iapetus
• Discussion of Iapetus dated October 2007
• Iapetus Profile by NASA's Solar System Exploration
• The Planetary Society: Iapetus
• Astronomy Picture of the Day article on Iapetus
• Mirror Objects in the Solar System? - refereed article discussing the speculative mirror matter, and Iapetus in this context
• A Moon with a View - Richard C. Hoagland's highly creative discussion of Iapetus's oddities
• New attempts to crack Saturn's 'walnut' moon - equatorial ridge formation theories