Voyager 2 image of Iapetus.

IAPETUS - MOON OF SATURN

Iapetus was the son of Uranus and Gaia. With Clymene he fathered Atlas, Menoetius, Prometheus and Epimetheus. Features on Iapetus are named after characters and locations from the medieval French epic, "Chanson de Roland" (Roland's Song). Iapetus was discovered by Cassini in 1671.

Orbit

Iapetus orbits about 3,560,000 kilometres from Saturn. It has a fairly circular orbit, but one which is inclined to the plane of Saturn's equator by nearly 15o. Iapetus, of the moons which move about Saturn in the same direction as its rotation, is the most highly inclined. It takes Iapetus just over 79 days to complete one orbit of Saturn, the same time it takes to rotate on its axis - Iapetus therefore exhibits synchronous rotation.

The orbits of Titan, Hyperion, Iapetus, and Phoebe.

Physical properties

Iapetus is the third largest of Saturn's moons with a diameter of 1460 kilometres, only Rhea and Titan are larger. Iapetus has a density of 1020 kg m-3, which is about the same as water.

Comparison between Iapetus and Earth's moon.

Interior

The low density of Iapetus suggests it is made of mostly water ice, some light compounds like ammonia and methane, and only a small amount of rock. Some of Saturn's moons have a density of less than 1000 kg m-3 probably because there are empty spaces or voids in the rock and ice.

Magnetic field

No magnetic field has been detected.

Atmosphere

No atmosphere has been detected.

Surface

Fairly good images of Iapetus' surface were obtained by Voyager 2 which made a flyby at 909,000 kilometres. Voyager images reveal two distinct zones. While the leading hemisphere has a low albedo (0.05), reflecting very little light, the trailing hemisphere is moderately bright (0.5), reflecting 50% of the light which reaches its surface. The bright trailing edge is craggy and cratered. The Voyager 1 spacecraft's nearest approach was about 3.2 million kilometres, but it was able to image the Saturn-facing hemisphere.

It has been suggested that Iapetus' leading hemisphere is coated with rocky dust and particles which have been ejected in micrometeorite impacts upon Phoebe, Saturn's outermost moon. Though the surfaces of the two bodies are dark, closer analysis shows that the two have a different colour. If material did originate on Phoebe, it has been changed somehow. Another problem with the Phoebe-dust theory is that the boundary between light and dark zones is very sharp. If Iapetus was moving through a zone of debris and dust particles, the coating it would receive might be expected to have diffuse or fuzzy edges.

Voyager 2 image of Iapetus' from 910,000 kilometres.

A more satisfactory alternative is that the dark zones are a result of internal activity. The Voyager 1 picture is less detailed but shows a large dark ring about 200 kilometres across, located on the equator. The dark material appears to occupy the floor of a crater. It could have been flooded with a mixture of water, ammonia, methane and dirty rocky materials from Iapetus' interior.

It is highly likely that the dark materials which cover the leading hemisphere are also volcanic. In the high resolution Voyager images, bright areas within the dark leading edge can be seen. These islands appear to be elevated regions which have survived inundation by volcanic flows. This helps to explain why the dark regions have a well defined boundary.

Even in the high resolution Voyager images, no detail at all can be seen in the dark areas. If there were raised surface details like crater rims, ridges or hills, they would have illuminated edges and cast a shadow. The dark materials are probably quite thick and more easily attributable to volcanic outpourings than a superficial coating of dust.