Iapetus (moon)

Iapetus is the third largest moon of Saturn, and eleventh in the solar system, discovered by Giovanni Cassini in 1671. Iapetus is best known for its dramatic 'two-tone' coloration, but recent discoveries by the Cassini mission have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon.

Discovery

Iapetus was discovered by Giovanni Domenico Cassini in October 1671 on the western side of Saturn. Then Cassini tried unsuccessfully to observe it on the eastern side of the planet in early 1672. This pattern continued as Cassini observed Iapetus in December 1672 and February 1673, each time tracking it for a fortnight on the western side of Saturn, but he was unable to detect it during the intervening period, when it should have been on the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with an improved telescope, finding it two magnitudes dimmer on that side.

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn, so that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and the dark hemisphere on the other side. The dark hemisphere was later named Cassini Regio in his honour.

Name

Iapetus is named after the Titan Iapetus from Greek mythology. It is also designated Saturn VIII.

Iapetus was among four moons of Saturn labelled the Sidera Lodoicea by their discoverer Giovanni Cassini after King Louis XIV (the other three were Tethys, Dione and Rhea). However, astronomers fell into the habit of referring to them by numbers, with Iapetus as Saturn V. Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended and Iapetus became Saturn VII, and subsequently Saturn VIII after the discovery of Hyperion in 1848. It is still known by that number today.

Physical characteristics

The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials.

Unlike most moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles; also, its unique equatorial ridge (see below) is so high that it visibly distorts the moon's shape even when viewed from a distance. These features often lead it to be characterized as walnut-shaped.

Iapetus is heavily cratered, and Cassini images have revealed large impact basins in the dark region, at least five of which are over 350 km wide. The largest has a diameter over 500 km; its rim is extremely steep and includes a scarp over 15 km high.

Two-tone coloration

In the 17th century, Giovanni Cassini observed that he could see Iapetus only on the west side of Saturn and never on the east. He correctly deduced that Iapetus is locked in synchronous rotation about Saturn and that one side of Iapetus is darker than the other, a conclusion later confirmed by larger telescopes.

The difference in coloring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright. Thus, the apparent magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9 — beyond the capacity of the best telescopes in the 17th century. The pattern of coloration is analogous to a spherical yin-yang symbol or the two sections of a tennis ball. The dark region is named Cassini Regio, and the bright region Roncevaux Terra.