Iapetus

Click image for description
Discovery
Discovered by	Giovanni Domenico Cassini
Discovered in	October 25, 1671
Orbital characteristics
Semimajor axis	3,560,820 km
Eccentricity	0.0286125 [1]
Revolution period	79.3215 d
Inclination	15.47° (to Saturn's equator)
Is a satellite of	Saturn
Physical characteristics
Mean diameter	1472 km
Surface area	6,700,000 km2
Mass	1.9739×1021 kg
Mean density	1.27 g/cm3
Surface gravity	0.2553 m/s2
Escape velocity	0.61 km/s
Rotation period	79.3215 d (synchronous)
Axial tilt	zero
Albedo	0.04-0.5
Atmosphere	none

Iapetus (eye-ap'-ə-təs, IPA /aɪˈæpətəs/, Greek Ιαπετός) is the third-largest moon of Saturn, discovered by Giovanni Domenico Cassini in 1671. Iapetus is best known for its dramatic 'two-tone' coloration, but recent discoveries by the Cassini mission have revealed several other unusual physical characteristics. These mysteries are currently under investigation by scientists and new information about Iapetus is accumulating continuously. Wikipedia does not have an article with this exact name. ... Giovanni Domenico (Jean-Dominique) Cassini Portrait Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian astronomer and engineer. ... October 25 is the 298th day of the year (299th in leap years) in the Gregorian Calendar, with 67 days remaining. ... Events May 9 - Thomas Blood, disguised as a clergyman, attempts to steal the Crown Jewels from the Tower of London. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... In geometry, the semi-major axis (also semimajor axis) a applies to ellipses and hyperbolas. ... A kilometer (Commonwealth spelling: kilometre), symbol: km is a unit of length in the metric system equal to 1,000 metres (from the Greek words χίλια (khilia) = thousand and μέτρο (metro) = count/measure). ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... Inclination in general is the angle between a reference plane and another plane or axis of direction. ... Moons of solar system scaled to Earths Moon A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Diameter is an AAA (authentication, authorization and accounting) protocol for applications such as network access or IP mobility. ... Area is a physical quantity expressing the size of a part of a surface. ... Square kilometre (US spelling: Square kilometer), symbol km², is an SI unit of surface area. ... Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... Gravity is a force of attraction that acts between bodies that have mass. ... Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the slope of the tangent to that point In physics or physical science, acceleration (symbol: a) is defined as the rate of change (or derivative with respect to... In physics, for a given gravitational field and a given position, the escape velocity is the minimum speed an object without propulsion, at that position, needs to have to move away indefinitely from the source of the field, as opposed to falling back or staying in an orbit within a... On a prograde planet like the Earth, the sidereal day is shorter than the solar day. ... In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting. ... Axial tilt is an astronomical term regarding the inclination angle of a planets rotational axis in relation to its orbital plane. ... Albedo is the measure of reflectivity of a surface or body. ... Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. ... IPA may refer to: The International Phonetic Alphabet or India Pale Ale ... Moons of solar system scaled to Earths Moon A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Giovanni Domenico (Jean-Dominique) Cassini Portrait Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian astronomer and engineer. ... Events May 9 - Thomas Blood, disguised as a clergyman, attempts to steal the Crown Jewels from the Tower of London. ... Cassini-Huygens is a joint NASA/ESA/ASI unmanned space mission intended to study Saturn and its moons. ...

Name

Iapetus is named after the mythological Iapetus. It is also designated Saturn VIII. In Greek mythology Iapetus, or Iapetos, was a Titan, the son of Uranus and Gaia, and father (by an Oceanid named Clymene or Asia) of Atlas, Prometheus, Epimetheus, and Menoetius and through Prometheus and Epimetheus and Atlas an ancestor of the human race. ...

Giovanni Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Lodoicea Sidera ("the stars of Louis") to honour king Louis XIV. However, astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V. Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended to Saturn VII. Giovanni Domenico (Jean-Dominique) Cassini Portrait Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian astronomer and engineer. ... Atmosphere none Tethys (tee-thÉ™s or teth-É™s, IPA , Greek Τηθύς) is a moon of Saturn that was discovered by Giovanni Domenico Cassini in 1684. ... Atmosphere none Dione (dye-oe-nee, Greek Διώνη) is a moon of Saturn discovered by Giovanni Cassini in 1684. ... Atmosphere none Rhea (ree-a, Greek ‘Ρέα) is the second largest moon of Saturn and was discovered in 1672 by Giovanni Domenico Cassini. ... Louis XIV (Louis-Dieudonné) (September 5, 1638 – September 1, 1715), reigned as King of France and of Navarre from May 14, 1643 until his death at the age of 77. ... Atmospheric characteristics Pressure 146. ... Atmosphere none Mimas (mye-mÉ™s, IPA , Greek Μίμᾱς, rarely Μίμανς) is a moon of Saturn that was discovered in 1789 by William Herschel. ... [5] Atmospheric characteristics Pressure trace, significant spatial variability [6], [7] Water vapour 91% [8] Carbon dioxide 3. ... 1789 was a common year starting on Thursday (see link for calendar). ...

The names of all seven satellites of Saturn then known come from 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 ([2]), wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used. John Herschel Sir John Frederick William Herschel (7 March 1792 – 11 May 1871) was an English mathematician and astronomer. ... William Herschel Sir Wilhelm Friedrich Herschel, FRS (November 15, 1738 – August 25, 1822) was a German-born British astronomer and composer who became famous for discovering the planet Uranus. ... 1847 was a common year starting on Friday (see link for calendar). ... This page is a candidate for speedy deletion, because: it is patent nonsense. ...

Orbit

The orbit of Iapetus is somewhat unusual. Although it is one of Saturn's largest moons, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. The cause of this is unknown. Atmospheric characteristics Pressure 146. ... For other meanings see Phoebe. ...

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. The rings of Saturn are a series of planetary rings that orbit the planet Saturn. ...

Physical characteristics

Recent Cassini image of the night hemisphere of Iapetus, illuminated by reflected light from Saturn

The low density of Iapetus indicates that it is primarily composed of ice, with only a small amount of rocky materials. Iapetus, a moon of Saturn, with its dark side illuminated by reflected light from Saturn Original caption released with image: New details on Iapetus are illuminated by reflected light from Saturn in this revealing Cassini image. ... Iapetus, a moon of Saturn, with its dark side illuminated by reflected light from Saturn Original caption released with image: New details on Iapetus are illuminated by reflected light from Saturn in this revealing Cassini image. ... Saturns moon Iapetus lit by Saturnshine. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... The acronym ICE can refer to: InterCity Express, a German high-speed train InterCity Express (CityTrain), an interurban train used by QR CityTrain in South East Queensland, Australia Internal combustion engine, a fuel engine In-circuit emulator, a computer hardware device In case of emergency, emergency number in mobile phones...

Furthermore, the overall shape of Iapetus is neither spherical nor ellipsoid—unusual for a large moon; parts of its globe appear to be squashed flat, and its unique equatorial ridge (see below) is so high that it visibly distorts the moon's shape even when viewed from a distance. Scientists are currently unable to describe Iapetus's shape perfectly as the Cassini probe has not yet imaged its entire surface. [3] Current triaxial measurements of Iapetus give it dimensions of 747.1 × 749 × 712.6 km, with a mean radius of 736 km.[4]

Iapetus is a heavily cratered body, and Cassini images have revealed large impact basins in the dark region, at least three of which are over 350 km wide. The largest has a diameter over 500 km; its rim is extremely steep and includes a scarp over 15 km high.

Two-tone coloration

In the seventeenth century, Giovanni Cassini observed that he could see Iapetus only on one side of Saturn and not on the other. He drew the conclusion that one side of Iapetus was darker than the other, a conclusion confirmed by images from the Voyager and Cassini spacecraft. (16th century - 17th century - 18th century - more centuries) As a means of recording the passage of time, the 17th century was that century which lasted from 1601-1700. ... Giovanni Domenico (Jean-Dominique) Cassini Portrait Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian astronomer and engineer. ... The Voyager spacecraft Launch of Voyager 2 Voyager is also the name of a planned series of unmanned probes to Mars, cancelled in 1968. ...

The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere is dark (albedo .03–.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles is bright (albedo .5-.6, almost as bright as Europa). The pattern of coloration is analogous to a spherical yin-yang symbol. The dark region is named Cassini Regio, and the bright region Roncevaux Terra. Albedo is the measure of reflectivity of a surface or body. ... Red is any of a number of similar colors at the lowest frequencies of light discernible by the human eye. ... Atmospheric characteristics Atmospheric pressure 1 µPa Oxygen 100% Europa redirects here. ... Taoists Taijitu The concept of Yin Yang originates in ancient Chinese philosophy, most likely from the observations of day turning into night and night into day. ... Photomosaic of Cassini images taken Dec. ... Roncevaux Terra is the name given to the highly reflective side of Saturns moon Iapetus. ...

The origin of this dark material is not currently known, though several theories have been proposed (see below). Its thickness is also unknown; there are no bright craters present on the dark hemisphere, so if the dark material is thin it must either be extremely recent, or constantly renewed, as otherwise a meteor impact would have punched through the layer to reveal brighter underlying material. Tycho crater on Earths moon. ...

When NASA's Voyager 2 flew past Iapetus on August 22, 1981 at a relatively distant 966,000 km (600,000 mi), the spacecraft's cameras could make out few details in the area of dark material, but revealed the bright side to be icy and heavily cratered. On December 31, 2004, the Cassini spacecraft passed within 123,000 km (77,000 mi) of Iapetus and photographed Cassini Regio at far a higher resolution than Voyager was able, but the mystery surrounding its origin has only deepened. Voyager 2 is an unmanned interplanetary spacecraft. ... August 22 is the 234th day of the year in the Gregorian calendar (235th in leap years), with 131 days remaining. ... 1981 (MCMLXXXI) was a common year starting on Thursday of the Gregorian calendar. ... December 31 is the 365th day of the year (366th in leap years) in the Gregorian Calendar. ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ...

Cassini is scheduled for a much closer approach on September 10, 2007 — 1,200 km (800 mi).

Sources from space

Close-up of northern pole.

The dark material might be formed of 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. Download high resolution version (1000x850, 251 KB) Wikipedia does not have an article with this exact name. ... Download high resolution version (1000x850, 251 KB) Wikipedia does not have an article with this exact name. ... A meteorite is an extraterrestrial body that survives its impact with the Earths surface without being destroyed. ... Comet Hale-Bopp For other uses, see Comet (disambiguation). ... Flash point −17. ... Polymer is a term used to describe a very long molecule consisting of structural units and repeating units connected by covalent chemical bonds. ...

There have also been suggestions that the dark material originated from other Saturnian moons. For example, it was long suspected that the material may have spiralled in from Phoebe, having been knocked free from the smaller moon's surface by micrometeor impacts and then swept up by Iapetus' leading hemisphere. However, despite being widely cited, this theory is no longer tenable: observations have shown Phoebe's surface to have a different color to that of the dark material of Iapetus (indeed in 2005 it was announced that Phoebe's composition is closer to the bright Iapetian hemisphere than the dark one). For other meanings see Phoebe. ... Worlds second largest Meteorite in Culiacan, Mexico A meteorite is a relatively small extra-terrestrial body that reaches the Earths surface. ...

Another suggestion has been than Iapetus became coated by dark material created in the aftermath of the destruction of the object which went on to become Hyperion, whose shape is consistent with its formation in a violent impact. However, there remain doubts as to whether or not such an event can produce a stream of debris able to produce the distribution of dark material seen on Iapetus. Hyperion (hye-peer-ee-ən, IPA , Greek Ὑπερίων) is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. ...

Internal sources

It is possible that the dark material may have originated from some internal source, perhaps brought to the surface by a combination of meteor impact and cryovolcanism. This theory is supported by the apparent concentration of the material on crater floors. It has been suggested that since Iapetus is far from Saturn and would have avoided much of the heating its other moons received during the formation of the Solar system, Iapetus may have retained methane or ammonia ice in its interior that later erupted to the surface as cryovolcanic lava and was then blackened by solar radiation, charged particles, and cosmic rays. A dark ring of material about 100 kilometers in diameter straddling the border between the leading and trailing hemispheres of Iapetus is suggestive of such vulcanism, resembling structures that have formed on the Moon and on Mars as a result of volcanic material flowing into impact craters with a central peak. Photo of a burst of meteors with extended exposure time A meteor is the visible path of a meteoroid that enters the Earths (or another bodys) atmosphere, commonly called a shooting star or falling star. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Cryovolcanoes are pseudo-volcanoes believed to be present on Titan, the largest moon of Saturn. ... The simplest hydrocarbon, methane, is a gas with a chemical formula of CH4. ... Ammonia is a compound of nitrogen and hydrogen with the formula NH3. ... Solar irradiance spectrum at top of atmosphere. ... In physics, a charged particle is a particle with an electric charge. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... Bulk composition of the moons mantle and crust estimated, weight percent Oxygen 42. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ...

Photomosaic of Cassini images taken Dec. 31, 2004, showing the dark Cassini Regio, large craters, and the newly discovered equatorial ridge

An alternative internal source may be the evaporation of water ice. Due to its slow rotation, Iapetus has the warmest surface in the Saturnian system (130 K in the dark region) allowing the sublimation of water ice on the surface. After sublimation, the water then freezes back to the surface and re-heats until it reaches a location where it is no longer able to sublimate. The dark areas may be the result of such a process, since the material there lacks water. However, this hypothesis fails to explain why only one hemisphere is dark. Download high resolution version (1663x1558, 335 KB) Wikipedia does not have an article with this exact name. ... Download high resolution version (1663x1558, 335 KB) Wikipedia does not have an article with this exact name. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... Sublimation of an element or substance is a conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. ...

The equatorial ridge

A further mystery was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004, and revealed an equatorial ridge about 20 km wide and 13 km high extending 1300 km through the center of Cassini Regio [5]. Parts of the ridge rise more than 20 km over the surrounding plains. The ridge follows the moon's equator almost perfectly and is confined to Cassini Regio. Some bright mountains near the boundary of Cassini Regio that apparently belong to this ridge were seen in Voyager photos; however, the Voyagers were unable to make out any details in the dark region itself, so the extent of the ridge is only now apparent. The ridge system is heavily cratered indicating that it is ancient.[6] December 31 is the 365th day of the year (366th in leap years) in the Gregorian Calendar. ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... The ridge on Iapetus The ridge on Atlas Equatorial ridges are a feature of at least two of Saturns moons. ...

The images are currently being analyzed by scientists and no firm conclusions have yet been announced about the ridge's origin. At least three hypotheses are in circulation.

One possibility is that the ridge is a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today.[7] The height of the ridge suggests a maximum rotational period of 17 hours. In order for Iapetus to have cooled quickly 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 could only have been heated by the radioactive decay of aluminium-26. This isotope appears to have been abundant in the solar nebula from which Saturn formed, but has since all decayed. The quantities of Al₂₆ 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. The tide is the cyclic rising and falling of Earths ocean surface caused by the tidal forces of the Moon and the Sun acting on the Earth. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... Isotopes are forms of an element, therefore their nuclei have the same atomic number — the number of protons in the nucleus — but different mass numbers because they contain different numbers of neutrons. ... ã ... An asteroid is a predominantly rocky body that orbits around its star. ...

Another possibility is that the ridge is icy material that has welled up from beneath the surface and then solidified. However, this does not explain why the ridge follows the equator.

A third possibility has been suggested by Paulo C.C. Freire of Arecibo Observatory, who proposes that the ridge and Cassini Regio were created when Iapetus grazed the outer edges of Saturn's rings in the distant past. However, Freire's theory requires Iapetus to have been later ejected to its current, distant orbit around Saturn. [8] The Arecibo Observatory is located in Arecibo, Puerto Rico on the north coast of the island. ...

See also

• List of geological features on Iapetus

This is a list of named geological features on Iapetus. ...

Speculation that Iapetus is artificial

The oddness of Iapetus has occasionally led to speculation that it may be an artificial construction by extraterrestrials. In 1980, Donald Goldsmith and Tobias Owen suggested that the striking bicoloration of Iapetus might be the result of alien modification of a natural object. The existence of extraterrestrial life remains hypothetical though human beings continue to search Extraterrestrial life is life that may exist and originate outside the planet Earth, the only place in the Universe known to support life. ... 1980 (MCMLXXX) was a leap year starting on Tuesday. ...

In 2005, Richard C. Hoagland speculated that Iapetus might be a fully or partially artificially constructed world by an ancient (and likely long-gone) extraterrestrial civilization. His thesis relies on the moon's angular shape (unusual in a moon of Iapetus's size, which ought to be compressed into an approximately spherical or ellipsoid form under pressures generated by its own gravity), the equatorial ridge, and a close examination of surface features. [9] Richard C. Hoagland (born April 25, 1945) is a propounder of theories on astronomical topics that are considered offbeat by many mainstream astronomers. ...

The scientific mainstream considers such ideas fanciful at best.

Iapetus in fiction

• In Arthur C. Clarke's novel 2001: A Space Odyssey (1968), astronaut Dave Bowman finds an enigmatic alien monolith waiting for him on the surface of Iapetus. A vast white ellipse is on the moon's surface, with the monolith appearing as a black dot in the center. When the Voyager space probes arrived at Iapetus thirteen years later, they discovered that there was indeed a black region within the moon's brighter hemisphere. Clarke reports that Carl Sagan, who was on the Voyager imaging team, sent him a photo, with the note 'Thinking of you...' (Due to budget considerations, the film version of 2001 relocated the monolith to an orbit around Jupiter).
• In Kim Stanley Robinson's futuristic novel The Memory of Whiteness, Iapetus is populated by the descendants of Soviet colonists who retain a Communist political system.
• In David Weber's novel The Armageddon Inheritance, genocidal aliens (the Achuultani) attempt to use Iapetus as a kinetic energy weapon to destroy all life on Earth.

Sir Arthur C. Clarke Sir Arthur Charles Clarke (born December 16, 1917) is a British author and inventor, most famous for his science-fiction novel 2001: A Space Odyssey, and for collaborating with director Stanley Kubrick on the film of the same name. ... 1968 (MCMLXVIII) was a leap year starting on Monday (the link is to a full 1968 calendar). ... Dr. Carl Edward Sagan (November 9, 1934 – December 20, 1996) was an American astronomer, astrobiologist, and highly successful science popularizer. ... Kim Stanley Robinson at the 63rd World Science Fiction Convention in Glasgow, August 2005 Kim Stanley Robinson (born March 23, 1952) is an American science fiction writer, probably best known for his award-winning Mars trilogy. ... The Memory of Whiteness is a science fiction novel written by Kim Stanley Robinson in 1985. ... State motto (Russian): Пролетарии всех стран, соединяйтесь! (Transliterated: Proletarii vsekh stran, soedinyaytes!) (Translated: Workers of the world, unite!) Capital Moscow Official language None; Russian (de facto) Government Federation of Soviet republics Area  - Total  - % water 1st before collapse 22,402,200 km² Approx. ... This article is about communism as a form of society and as a political movement. ... Honor Harrington from Honor Among Ememies cover, by David Mattingly. ... The Armageddon Inheritance is a science fiction novel written by David Weber in two books containing a total of 27 chapters. ...

External links

Wikimedia Commons has media related to:

Iapetus (moon)

• Cassini mission page - Iapetus
• The Planetary Society: Iapetus
• Astronomy Picture of the Day article on Iapetus
• Did Iapetus Consume One of Saturn's Rings?
• Hoagland's extremely long, detailed, thorough and artificial world theory
• Is Iapetus made of mirror matter? Z.K. Silagadze's article from Acta Physica Polonica B

Image File history File links Commons-logo. ... Wikimedia Commons logo by Reid Beels The Wikimedia Commons (also called Commons or Wikicommons) is a repository of free content images, sound and other multimedia files. ... In physics, mirror matter, also called shadow matter or Alice matter, is a hypothetical counterpart to regular matter suggested by Tsung Dao Lee and Chen Ning Yang [1] in 1956, when it was discovered that nature violates P-symmetry. ... Hyperion (hye-peer-ee-ən, IPA , Greek Ὑπερίων) is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. ... For other meanings see Phoebe. ...