Iapetus (Saturn VIII)

 

   

Natural-color image of Iapetus. The view was obtained by the Cassini spacecraft narrow-angle camera on March 27, 2015 at a distance of approximately 1 million kilometers from Iapetus. Image scale on Iapetus is about 6 kilometers. Images taken using red, green and blue spectral filters were combined to create this natural-color view. The moon's brightness has been enhanced in order to make the dark terrain visible. The image also was enlarged by a factor of two compared to the original data. The large basin at lower right, within the dark terrain, is named Turgis. The slightly smaller crater at the nine o'clock position is Falsaron. The two prominent craters just above image center are Roland and Turpin. At the limb around the three o'clock position is the darkened rim of the crater Naimon. 

Credit: NASA/JPL-Caltech/Space Science Institute

Iapetus (Saturn VIII), a moon of Saturn, was discovered by Giovanni Domenico Cassini (1625–1712) on October 25, 1671. He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. This was also the case the following year, when he was again able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of 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. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side.

   Iapetus is the third-largest moon of Saturn. It is 1469 km in diameter, and orbits at a distance of 3,561,000 km from Saturn, with an orbital period of 79.3 days. Iapetus is unique in the Solar System in its appearance: the leading hemisphere — facing the direction of orbital motion — is dark, with an albedo of around 0.03–0.05, while the trailing hemisphere is bright, with an albedo of 0.5–0.6. Its magnitude thus changes in the course of its orbit, as Cassini himself noticed, and he guessed correctly at the reason. 

 

   Iapetus is heavily cratered, and the Cassini spacecraft images have revealed large impact basins, at least five of which are over 350 km wide. The largest, Turgis, has a diameter of 580 km. Iapetus has on its surface a massive equatorial ridge — a chain of 10-km high mountains — about 1,300 km long, running three-quarters of the way around the moon. There are two theories on how the ridge formed. Some scientists think the ridge was formed at an earlier time when Iapetus rotated much faster than it does today; others think the ridge is made of material left from the collapse of a ring.

 

Cassini false-color mosaic of the bright trailing hemisphere of Iapetus, with part of the dark area appearing on the right (the equatorial ridge is in profile on the right limb). The large crater Engelier is near the bottom; to its lower right can be seen the rim of a partly obliterated, slightly smaller older crater, Gerin. The view was acquired with the Cassini spacecraft narrow-angle camera on September 10, 2007, at a distance of about 73,000 kilometers from Iapetus.
Credit: NASA/JPL/Space Science Institute

   Scientists have long wondered why one hemisphere of Iapetus is so dark in comparison to its other hemisphere, and in comparison to other surfaces in the Saturn system. Iapetus may be sweeping up particles from the more-distant dark moon, Phoebe. If that is the darkening mechanism, it should be steadily renewing the dark surface because very few fresh bright craters are detected within the dark terrain. An alternate theory is that there might be ice volcanism distributing darker material to the surface. Volcano-like eruptions of hydrocarbons might form the dark surfaces, particularly after chemical reactions caused by solar radiation. 

 

   The September 2007 Cassini flyby of Iapetus showed that a third process, thermal segregation, is probably the most responsible for Iapetus' dark hemisphere. Iapetus has a very slow rotation, longer than 79 days. Such a slow rotation means that the daily temperature cycle is very long, so long that the dark material can absorb heat from the sun and warm up. This heating will cause any volatile, or icy, species within the dark material to sublime out, and retreat to colder regions on Iapetus. This sublimation of volatiles causes the dark material to become even darker — and causes neighboring bright, cold regions to become even brighter. The moon may have experienced a (possibly small) influx of dark material from an external source, which could have warmed up and triggered this thermal segregation process.

   Iapetus is named after the Titan Iapetus from Greek mythology. The name was suggested by John Herschel in his 1847 publication in which he advocated naming the moons of Saturn after the Titans, brothers and sisters of the Titan Cronus (whom the Romans equated with their god Saturn).

This color-composite image shows the limb of Saturn's moon Iapetus, captured by Cassini on September 10, 2007. A ridge of 20 km-high mountains encircles the equator, possibly the remains of a former satellite that broke apart as its orbit degraded.
Credit: NASA/JPL-Caltech/SSI/Jason Major

Giovanni Domenico Cassini, discoverer of Iapetus. Lithograph by F.C., 1827. Credit: Wellcome Library

 

References:

NASA Solar System Exploration: Iapetus

© 2025, Andrew Mirecki