Iapetus (moon)
 Click image for description | |
|---|---|
| Discovery | |
| Discovered by | Giovanni Cassini |
| Discovered in | 1671 |
| Orbital characteristics | |
| Semimajor axis | 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 | km2 |
| Mass | 21 kilogram>kg |
| Mean density | 1.27 g/cm3 |
| Surface gravity | m/s2 |
| Rotation period | (Synchronous rotation>synchronous) |
| Axial tilt | 14.84° |
| Albedo | 0.04-0.5 |
| Atmosphere | none |
The name "Iapetus" and the names of all seven satellites of Saturn then known were suggested by John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope. [1]
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. Though Phoebe also has a low albedo, 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