Iapetus (moon)

Iapetus

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Discovery
Discovered by	Giovanni Domenico Cassini
Discovered in	1671
Orbital characteristics
Semimajor axis	3,561,300 km
Eccentricity	0.0283
Revolution period	79d 7h 55m
Inclination	7.52°
Is a satellite of	Saturn
Physical characteristics
Mean diameter	1436 km
Surface area	6,700,000 km2
Mass	1.9739×1021 kg
Mean density	1.27 g/cm3
Surface gravity	0.2553 m/s2
Rotation period	79d 7h 55m (synchronous)
Axial tilt	14.84°
Albedo	0.04-0.5
Atmosphere	none

Iapetus ("eye AP uh tuss") is the third-largest moon of Saturn, discovered by Giovanni Domenico Cassini in 1671. It is named after the mythological Iapetus. It is also designated Saturn VIII.

Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Lodicea Sidera ("the stars of Louis") to honour king Louis XIV. Astronomers fell into the habit of referring to them and Titan as Saturn 1 through Saturn 5. Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn 7.

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, wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.

Physical characteristics

The low density of Iapetus indicates that it is primarily composed of ice, with only a small amount of rocky materials.

The surface of Iapetus' has a distinctive two-tone pattern of colouration. The leading hemisphere is dark (albedo 0.03–0.05) with a slight reddish color, while its trailing hemisphere is bright (albedo 0.5, almost as bright as Europa). This difference is so striking that Cassini noted that he could see Iapetus only on one side of Saturn and not on the other; the dark region is named Cassini Regio after him, with the bright region named Roncevaux Terra. NASA's Voyager 2 flew past Iapetus on August 22, 1981. Passing at a relatively distant 966,000 km (600,000 miles), the spacecraft's cameras could make out few details in the area of dark material. The images revealed the bright side to be icy and heavily cratered. The moon's poles are also free of dark material.

The dark material might be organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets; it has been shown to be carbonaceous by Earth-based observation and probably includes cyano compounds such as frozen hydrogen cyanide polymers. The origin of this dark material is not currently known, though several theories have been proposed. The thickness of the layer is also not clear; there are no bright craters present on the dark hemisphere, so if the dark material is thin it must be constantly renewed since otherwise a meteor impact would punch through the layer to reveal brighter underlying material.

It is possible that the dark material may have originated from some internal source, perhaps brought to the surface by some combination of meteor impact and volcanism. 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 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.

An alternate theory is that the dark material may have originated from Phoebe, knocked free from the smaller moon's surface by micrometeor impacts and then swept up by Iapetus' leading hemisphere. However, Phoebe's surface has a slightly different color from that of the dark material of Iapetus.

Iapetus is one of only two major Saturnian moons to have a significantly inclined orbital plane (the other is Phoebe).

See also: List of geological features on Iapetus

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 Iapetus. A vast black circle has been painted on the moon's surface, with the monolith occupying a smaller white circle at the centre. Remarkably, when the Voyager space probes arrived at Iapetus eighteen years later, they did indeed photograph an enormous, roughly circular black region with a whiter region within it. Clarke reports that a member of the imaging team sent him a photo, with the note 'Thinking of you...'
• In Kim Stanley Robinson's futuristic novel The Memory of Whiteness, Iapetus is populated by the descendents of Soviet colonists who retain a Communist political system.