Iapetus

Iapetus

, in astronomy

Iapetus (īăpˈĭtəs), in astronomy, one of the named moons, or natural satellites, of Saturn. 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 Cassini. 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 Phoebe, Iapetus's orbit is inclined to the plane of Saturn's equator.

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Iapetus

, in Greek mythology

Iapetus, in Greek mythology, a Titan. By the nymph Clymene he fathered Atlas, Prometheus, Epimetheus, and Menoetius.

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Iapetus

(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.

Iapetus

 

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.

Iapetus

[‚yap·əd·əs]

(astronomy)

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.