Iapetus () is a
moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
of
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
. It is the 24th of Saturn’s 83 known moons. With an estimated diameter of 1,469 km, it is the third-largest moon of Saturn and the
eleventh-largest in the Solar System. Named after the
Titan Iapetus, the moon was discovered in 1671 by
Giovanni Domenico Cassini
Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 September 1712) was an Italian (naturalised French) mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the ...
.
A relatively low-density body made up mostly of
ice
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...
, Iapetus is home to several distinctive and unusual features, such as a striking difference in coloration between its leading hemisphere, which is dark, and its trailing hemisphere, which is bright, as well as a massive
equatorial ridge
Equatorial ridges are a feature of at least three of Saturn's moons: the large moon Iapetus and the tiny moons Atlas and Pan. They are ridges that closely follow the moons' equators. They appear to be unique to the Saturnian system, but it i ...
running three-quarters of the way around the moon.
History
Discovery
Iapetus was discovered by
Giovanni Domenico Cassini
Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 September 1712) was an Italian (naturalised French) mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the ...
, an Italian-born French astronomer, in October 1671. He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. This was also the case the following year, when he was again able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two
magnitudes dimmer on that side.
Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is
tidally locked
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked bo ...
, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side.
Etymology
Iapetus is named after the
Titan Iapetus from
Greek mythology
A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...
. The name was suggested by
John Herschel
Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wor ...
(son of
William Herschel
Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline ...
, discoverer of
Mimas and
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
) in his 1847 publication ''Results of Astronomical Observations made at the Cape of Good Hope'',
in which he advocated naming the moons of Saturn after the Titans, brothers and sisters of the Titan
Cronus (whom the
Romans
Roman or Romans most often refers to:
*Rome, the capital city of Italy
* Ancient Rome, Roman civilization from 8th century BC to 5th century AD
*Roman people, the people of ancient Rome
*''Epistle to the Romans'', shortened to ''Romans'', a lette ...
equated with their god
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
).
The name has a largely obsolete variant, Japetus
[ ,
with an adjectival form ''Japetian''.][George William Hill (1952) ''The Radiant Universe'', p. 280]
These occur because there was no distinction between the letters and in Latin, and authors rendered them differently.
When first discovered, Iapetus was among four Saturnian moons labelled the ''Sidera Lodoicea
Sidera Lodoicea is the name given by the astronomer Giovanni Domenico Cassini to the four moons of Saturn discovered by him in the years 1671, 1672, and 1684 and published in his ''Découverte de deux nouvelles planètes autour de Saturne'' in 16 ...
'' by their discoverer Giovanni Cassini
Giovanni Domenico Cassini, also known as Jean-Dominique Cassini (8 June 1625 – 14 September 1712) was an Italian (naturalised French) mathematician, astronomer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the ...
after King Louis XIV (the other three were Tethys, Dione and Rhea). However, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being Saturn V. Once Mimas and Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
were discovered in 1789, the numbering scheme was extended and Iapetus became Saturn VII. With the discovery of Hyperion in 1848, Iapetus became ''Saturn VIII'', which it is still its Roman numerical designation today.
Geological features on Iapetus are named after characters and places from the French epic poem ''The Song of Roland
''The Song of Roland'' (french: La Chanson de Roland) is an 11th-century '' chanson de geste'' based on the Frankish military leader Roland at the Battle of Roncevaux Pass in 778 AD, during the reign of the Carolingian king Charlemagne. It i ...
''. Examples of names used include the craters Charlemagne and Baligant, and the northern bright region Roncevaux Terra. The one exception is Cassini Regio
Cassini Regio (adjective ''Cassinian'' ) is the enigmatic dark area that covers the leading half of Saturn's moon Iapetus. It is named after Giovanni Cassini, the discoverer of Iapetus; ' Regio' is a term used in planetary geology for a large a ...
, the dark region of Iapetus, which is named after the moon's discoverer.
Orbit
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. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. 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 cause of this highly inclined orbit is unknown; however, the moon is not likely to have been captured. One suggestion for the cause of Iapetus' orbital inclination is an encounter between Saturn and another planet.
Formation
The moons of Saturn are typically thought to have formed through co-accretion, a similar process to that believed to have formed the planets in the Solar System. As the young gas giants formed, they were surrounded by discs of material that gradually coalesced into moons. However, a proposed model on the formation of Titan suggests that Titan was instead formed in a series of giant impacts between pre-existing moons. Iapetus and Rhea are thought to have formed from part of the debris of these collisions. More-recent studies, however, suggest that all of Saturn's moons inward of Titan are no more than 100 million years old; thus, Iapetus is unlikely to have formed in the same series of collisions as Rhea and all the other moons inward of Titan, and—along with Titan—may be a primordial satellite.
Physical characteristics
The low density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of Iapetus indicates that it is mostly composed of ice
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...
, with only a small (~20%) amount of rocky materials.
Unlike most of the large moons, its overall shape is neither spherical
A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...
nor ellipsoid, but has a bulging waistline and squashed poles.[Cowen, R. (2007). Idiosyncratic Iapetus, ''Science News'' vol. 172, pp. 104–106]
references
/ref> Its unique equatorial ridge (see below) is so high that it visibly distorts Iapetus's shape even when viewed from a distance. These features often lead it to be characterized as walnut
A walnut is the edible seed of a drupe of any tree of the genus ''Juglans'' (family Juglandaceae), particularly the Persian or English walnut, '' Juglans regia''.
Although culinarily considered a "nut" and used as such, it is not a true ...
-shaped.
Iapetus is heavily cratered, and ''Cassini'' images have revealed large impact basins, at least five of which are over wide. The largest, Turgis, has a diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...
of ; its rim is extremely steep and includes a scarp about high.
Iapetus is known to support long-runout landslides or sturzstroms, possibly supported by ice sliding.
Two-tone coloration
The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (albedo
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
0.03–0.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5–0.6, almost as bright as Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
). Thus, the apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...
of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9—beyond the capacity of the best telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
s in the 17th century. The dark region is named Cassini Regio
Cassini Regio (adjective ''Cassinian'' ) is the enigmatic dark area that covers the leading half of Saturn's moon Iapetus. It is named after Giovanni Cassini, the discoverer of Iapetus; ' Regio' is a term used in planetary geology for a large a ...
, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it. The original dark material is believed to have come from outside Iapetus, but now it consists principally of lag
Lag, or similar, may refer to:
Lag
* Łąg, Poland
* Lag (company), a French guitar maker
* Lag (cue sports), a brief pre-game competition to determine which player will go first
* Latency (engineering), a slower response time in computing, commu ...
from the sublimation (evaporation) of ice from the warmer areas of the moon's surface, further darkened by exposure to sunlight. It contains organic compounds
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
similar to the substances found in primitive meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
s or on the surfaces of comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...
s; Earth-based observations have shown it to be carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
aceous, and it probably includes cyano-compounds such as frozen hydrogen cyanide
Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on an ...
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s.
Images from the ''Cassini'' orbiter passed within show that both Cassini Regio and the Terra's are heavily cratered. The color dichotomy of scattered patches of light and dark material in the transition zone between Cassini Regio and the bright areas exists at very small scales, down to the imaging resolution of . There is dark material filling in low-lying regions, and light material on the weakly illuminated pole-facing slopes of craters, but no shades of grey. The dark material is a very thin layer, only a few tens of centimeters (approx. one foot) thick at least in some areas, according to Cassini radar imaging and the fact that very small meteor
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as mi ...
impacts have punched through to the ice underneath.
Because of its slow rotation of 79 days (equal to its revolution and the longest in the Saturnian system), Iapetus would have 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 in the dark Cassini Regio compared to in the bright regions. The difference in temperature means that ice preferentially sublimates from Cassini Regio, and deposits
A deposit account is a bank account maintained by a financial institution in which a customer can deposit and withdraw money. Deposit accounts can be savings accounts, current accounts or any of several other types of accounts explained below.
...
in the bright areas and especially at the even colder poles. Over geologic time scales, this would further darken Cassini Regio and brighten the rest of Iapetus, creating a positive feedback
Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the ...
thermal runaway
Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...
process of ever greater contrast in albedo, ending with all exposed ice being lost from Cassini Regio. It is estimated that over a period of one billion years at current temperatures, dark areas of Iapetus would lose about of ice to sublimation, while the bright regions would lose only , not considering the ice transferred from the dark regions. 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 Regio. The redistribution of ice is facilitated by Iapetus's weak gravity, which means that at ambient temperatures a water molecule can migrate from one hemisphere to the other in just a few hops.
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 was revived by the September 2007 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. 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, coating it; once this process created a modest contrast in albedo, and so a contrast in temperature, the thermal feedback described above would have come into play and exaggerated the contrast. In support of the hypothesis, simple numerical models of the exogenic deposition and thermal water redistribution processes can closely predict the two-toned appearance of Iapetus. A subtle color dichotomy between Iapetus's leading and trailing hemispheres, with the former being more reddish, can in fact be observed in comparisons between both bright and dark areas of the two hemispheres. In contrast to the elliptical shape of Cassini Regio, the color contrast closely follows the hemisphere boundaries; the gradation between the differently colored regions is gradual, on a scale of hundreds of kilometers. The next moon inward from Iapetus, chaotically rotating Hyperion, also has an unusual reddish color.
The largest reservoir of such infalling material is Phoebe, the largest of the outer moons. Although Phoebe's composition is closer to that of the bright hemisphere of Iapetus than the dark one, dust from Phoebe would only be needed to establish a contrast in albedo, and presumably would have been largely obscured by later sublimation. The discovery of a tenuous disk of material in the plane of and just inside Phoebe's orbit was announced on 6 October 2009, supporting the model. The disk extends from 128 to 207 times the radius of Saturn, while Phoebe orbits at an average distance of 215 Saturn radii. It was detected with the Spitzer Space Telescope.
Overall shape
Current triaxial measurements of Iapetus give it radial dimensions of , with a mean radius of . 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 would be consistent with hydrostatic equilibrium if Iapetus had a rotational period of approximately 16 hours, but it does not; its current rotation period is 79 days. A possible explanation for this is that the shape of Iapetus 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
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked bo ...
.
Equatorial ridge
A further mystery of Iapetus is the equatorial ridge
Equatorial ridges are a feature of at least three of Saturn's moons: the large moon Iapetus and the tiny moons Atlas and Pan. They are ridges that closely follow the moons' equators. They appear to be unique to the Saturnian system, but it i ...
that runs along the center of Cassini Regio, about long, wide, and high. It was discovered when the ''Cassini'' spacecraft imaged Iapetus on December 31, 2004. Peaks in the ridge rise more than above the surrounding plains, making them some of the tallest mountains in the Solar System. The ridge forms a complex system including isolated peaks, segments of more than and sections with three near parallel ridges. Within the bright regions there is no ridge, but there are a series of isolated peaks along the equator. The ridge system is heavily cratered, indicating that it is ancient. The prominent equatorial bulge
An equatorial bulge is a difference between the equatorial and polar diameters of a planet, due to the centrifugal force exerted by the rotation about the body's axis. A rotating body tends to form an oblate spheroid rather than a sphere.
On E ...
gives Iapetus a walnut
A walnut is the edible seed of a drupe of any tree of the genus ''Juglans'' (family Juglandaceae), particularly the Persian or English walnut, '' Juglans regia''.
Although culinarily considered a "nut" and used as such, it is not a true ...
-like appearance.
It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are at least numerous current hypotheses, but none of them explains why the ridge is confined to Cassini Regio. Theories include that the ridge is a remnant of Iapetus' oblate shape during its early life, that it was created by the collapse of a ring system, that it was formed by icy material welling from Iapetus' interior, or that it is a result of convective overturn.
Exploration
Iapetus has been imaged multiple times from moderate distances by the ''Cassini'' orbiter. However, its great distance from Saturn makes close observation difficult. ''Cassini'' made one targeted close flyby, at a minimum range of , on September 10, 2007.
In popular culture
The monolith featured during the climax of Arthur C. Clarke's 1968 novel '' 2001: A Space Odyssey'' is located on Iapetus.
A team of scientists explore Iapetus in ''The Saturn Game
"The Saturn Game" is a science fiction novella by American writer Poul Anderson, originally published in ''Analog Science Fiction and Fact'' in February 1981.
Plot summary
Imaginative roleplaying provides relief for some of the crew on the long, ...
'', a science-fiction novella by Poul Anderson
Poul William Anderson (November 25, 1926 – July 31, 2001) was an American fantasy and science fiction author who was active from the 1940s until the 21st century. Anderson wrote also historical novels. His awards include seven Hugo Awards and ...
(1981).
Iapetus is also the setting for Czech writer Julie Nováková's story "The Long Iapetan Night", published in the November 2020 edition of Asimov's Science Fiction Magazine.
See also
* Former classification of planets
* Iapetus in fiction
* List of tallest mountains in the Solar System
This is a list of the tallest mountains in the Solar System. This list includes peaks on all celestial bodies where significant mountains have been detected. For some celestial bodies, different peaks are given across different types of measure ...
* Lists of astronomical objects
This is a list of lists, grouped by type of astronomical object.
Solar System
* List of Solar System objects
* List of gravitationally rounded objects of the Solar System
* List of Solar System objects most distant from the Sun
* List of So ...
* Subsatellite
Notes
References
External links
''Cassini'' mission page – Iapetus
Discussion of Iapetus dated October 2007
Iapetus Profile
a
NASA's Solar System Exploration site
Google Iapetus 3D
interactive map of the moon
*
refereed article discussing the speculative mirror matter
In physics, mirror matter, also called shadow matter or Alice matter, is a hypothetical counterpart to ordinary matter.
Overview
Modern physics deals with three basic types of spatial symmetry: reflection, rotation, and translation. The known el ...
, and Iapetus in this context
A Moon with a View
Richard C. Hoagland
Richard Charles Hoagland (born April 25, 1945), is an American author and a proponent of various conspiracy theories about NASA, lost alien civilizations on the Moon and on Mars and other related topics. Hoagland has been documented to misappropr ...
's imaginative discussion of Iapetus's oddities
New attempts to crack Saturn's 'walnut' moon
equatorial ridge formation theories
''Cassini'' images of Iapetus
Images of Iapetus at JPL's Planetary Photojournal
* Movie o
Iapetus's rotation
at the National Oceanic and Atmospheric Administration site
Iapetus basemap (May 2008) from ''Cassini'' images
Iapetus atlas (October 2008) from ''Cassini'' images
Iapetus nomenclature
an
Iapetus map with feature names
from th
USGS planetary nomenclature page
Flight over Iapetus
(video)
{{Authority control
16711025
Discoveries by Giovanni Domenico Cassini
Moons with a prograde orbit