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(īăp`ĭtəs), in astronomy, one of the named moons, or natural satellites, of SaturnSaturn,
in astronomy, 6th planet from the sun. Astronomical and Physical Characteristics of Saturn

Saturn's orbit lies between those of Jupiter and Uranus; its mean distance from the sun is c.886 million mi (1.
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. Also known as Saturn VIII (or S8), Iapetus is 907 mi (1460 km) in diameter, orbits Saturn at a mean distance of 2,212,885 mi (3,561,300 km), and has equal orbital and rotational periods of 79.33 earth days. Saturn's third largest moon, it was discovered in 1671 by the Italian-French astronomer Gian Domenico CassiniCassini
, name of a family of Italian-French astronomers, four generations of whom were directors of the Paris Observatory. Gian Domenico Cassini, 1625–1712, was born in Italy and distinguished himself while at Bologna by his studies of the sun and planets,
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. With a density of only 1.1, Iapetus is believed to consist almost completely of water ice. A notable surface feature is a ridge that encircles it along its equator and rises as high as 12 mi (20 km). The reflectivity and surface features of Iapetus's leading and trailing hemispheres are noticeably different. The leading hemisphere is remarkably dark, while the trailing hemisphere is light—the asymmetry is so marked that Cassini wrote that he could see Iapetus on one side of Saturn but not on the other. The difference in reflectivity is due in part to the accumulation of dust on the leading hemisphere; this dust is believed to come from the enormous but faint ring of Saturn discovered in 2009. Much of the difference in reflectivity, however, is believed to be due to residue left behind by the sublimation of ice. The trailing hemisphere is also heavily cratered, while the leading hemisphere is not. Unlike all the other moons but PhoebePhoebe
, in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn IX (or S9), Phoebe is 137 mi (220 km) in diameter, orbits Saturn at a mean distance of 8,047,985 mi (12,952,000 km), has an orbital period of 550.
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, Iapetus's orbit is inclined to the plane of Saturn's equator.


in Greek mythology, a Titan. By the nymph Clymene he fathered Atlas, Prometheus, Epimetheus, and Menoetius.
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(ÿ-ap -ĕ-tŭs) A satellite of Saturn, discovered in 1671 by Giovanni Domenico Cassini. The diameter of Iapetus is 1440 km so that in size it is the twin of the satellite Rhea but its density, 1.21 g cm–3, is much lower. It is a strange satellite with a dark leading hemisphere (albedo 0.04–0.05) compared with a bright trailing edge of albedo 0.6. The dark hemisphere carries the name Cassini Regio. It appears to be coated with some dark material. The demarcation between the dark and light regions is not abrupt and there is a transition region 200–300 km in width with a meandering boundary. The bright trailing edge is most heavily cratered; some of the craters have dark floors but it is not known if the dark material in them is the same as that covering Cassini Regio. The material on the leading edge is thought to be carbon-based, but it is unclear whether it was emitted from the interior of Iapetus or was debris from eruptions on other Saturnian satellites swept up by Iapetus from space.

Iapetus was photographed by Voyager 2 in 1981 and by Cassini–Huygens in 2004. The Cassini images revealed some new features not seen on the Voyager images, in particular a vast ancient impact basin over 400 km across, which has been heavily overprinted with more recent smaller craters, and a curious and extraordinary ridge that almost exactly coincides with Iapetus' equator. It appears as a 20-km-wide band, 1300 km long, that seems to girdle the whole satellite, extending above the surface of Iapetus to a height of up to 13 km. The equatorial ridge may be a mountain belt that has folded upward, or perhaps it was formed when material from inside Iapetus erupted onto the surface through a crack and accumulated locally.

See also Table 2, backmatter.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a satellite of Saturn, approximately 1,800 km in diameter and located at a mean distance of 3,563,000 km from the center of Saturn. It was discovered in 1671 by G. D. Cassini.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A satellite of Saturn that orbits at a mean distance of 2.207 × 106 miles (3.560×106 kilometers) and has a diameter of about 900 miles (1500 kilometers).
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The Iapetus Ocean was first opened and then closed, and other minor oceans, including the Rheic Ocean, developed and then disappeared.
The team focuses on dust coming from Phoebe, a dark and distant moon that circles Saturn in the opposite direction as Iapetus.
The Cambrian-Silurian tectonic evolution of the northern Appalachian and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus. In Lyell: the Past is the Key to the Present.
There has been much discussion about the Late Cambrian--Early Ordovician Orogeny (520 to 480 Ma) that introduced the Scandinavian Caledonides and supplied both Scandinavia and the Baltica with gigantic amounts of metalliferous anoxic black mud from Iapetus that was squeezed to death between Baltica and Laurentia.
The moon called Iapetus, for example, is two-toned: one side is gray; the other more reddish.
Iapetus has one side that is bright while its other face is strangely dark.
The ophiolite marks the suture zone of a proto-Atlantic ocean named Iapetus. Working from K-Ar age determinations on hornblende and amphibolites from Asbestos, Laurent and Vallerand (1974) proposed an age of 550 million years for the ophiolitic rocks.
Let's call this bristlecone pine Iapetus, who was the son of Gaia, father of Prometheus and, appropriately enough, the god of mortal life span.
Palaeomagnetic data place the Baltica Palaeocontinent at the temperate southern latitudes in the Early Ordovician, on the eastern side of the Iapetus Ocean.
From the sandstone-colored bands of the planet's fiercely windy upper atmosphere to the icy surface of its most distant moon, Iapetus, it is a spin through an alien place on a very distant shore.
Ice roads were also built to a second exploration location Conoco-Phillips is drilling this spring, the Iapetus No.
* Cassini is named after Jean-Dominique Cassini, an Italian scientist who discovered some of Saturn's other moons: Iapetus, Rhea, Tethys, and Dione.