Iapetus Facts

Iapetus is the 3rd largest moon of Saturn. The name Iapetus comes from Greek mythology. Iapetus was a Titan. He was the son of Gaia and Uranus i.e. the Earth and the sky.

Iapetus Profile

Interesting Iapetus Facts

  • Iapetus was discovered by Giovanni Domenica Cassini in 19671.
  • At first, Cassini named the four moons of Saturn as part of the “Sidera Lodoicea” or “Stars of Louis” to honor King Louis XIV.
  • It was renamed Iapetus by Williams Herschel’s son, John Herschel after 200 years.
  • Iapetus is named after a Titan from Greek mythology. He was the son of Uranus and Gaia.
  • Iapetus has a radius of 736 km.
  • It is the largest body in the solar system to not be in hydrostatic equilibrium.
  • It is the 3rd largest moon of Saturn.
  • It is the 11th biggest moon in the entire solar system.
  • It is 3.5 million km away from Saturn.
  • It takes 79.3 days to complete one revolution around Saturn and one rotation about its own axis.
  • Iapetus is tidally locked to Saturn.
  • Iapetus has a unique appearance with easily distinguishable light and dark hemispheres.
  • Iapetus is composed of icy and rocky material.
  • The moon is not spherical or elliptical in shape.
  • It does not have a magnetic field.
  • Iapetus was featured in the climax of the novel 2001: A Space Odyssey by Arthur C. Clark.
  • It has been observed by NASA’s Cassini spacecraft.
  • All the geological features on the moon have been named after characters and locations from the poem, The Song of Roland.
  • In 1981, Voyager 2 took a series of photos of Iapetus in white dots forming a line of mountains on its Equator were noticed. These were named “Voyager Mountains”.
  • Later on, during Cassini’s flyby, it was discovered that these Voyager Mountains were ice regions.

History of Iapetus

Iapetus was discovered by the Italian astronomer Giovanni Domenica Cassini in 1671. He discovered it on the western side of Saturn but failed to view it from the eastern side a few months later. He finally succeeded in doing so in the year 1705.

He also discovered that the moon appeared to be dimmer on the eastern side of the planet. Hence, he concluded that the moon has a dark and bright hemisphere and always has the same face towards Saturn that is it is tidally locked to the planet.

The name Iapetus comes from Greek mythology. Iapetus was a Titan. He was the son of Gaia and Uranus i.e. the Earth and the sky.

Iapetus was also father to Atlas, Epimetheus, Menoetius, and Prometheus. The moon was named so in 1847 after being suggested by John Herschel, the son of William Herschel. 

It is interesting to know that geological features on the Iapetus have been named after characters and locations from the poem, The Song of Roland.

Appearance of Iapetus

The moon Iapetus is light grey in color. Its appearance is quite unique as the dark and bright hemispheres of the moon are clearly visible in its picture.

Its large crater Engelier is also dominantly visible in the image. The moon is neither ellipsoid nor spherical in shape. It has squashed poles and a bulging waistline 

Size of Iapetus

Iapetus has a mean radius of about 736 kilometers (or 457 miles). The moon is not in hydrostatic equilibrium that is the state at which the flow velocity of the fluids is constant. Iapetus is the largest object in the solar system to not be in this state.

It is the third-largest moon of Saturn and the 11th biggest moon in the entire solar system.

Surface and Composition of Iapetus

As mentioned before Iapetus has a greyish appearance. Its surface is heavily created with its largest crater, Engelier, dominantly visible in its image.

The moon is believed to have been formed through a process called co-accretion, which is also believed to be the process in which the planets were formed. The moons of the gas giants were formed by the disc of material that surrounded the young planets.

The equatorial range of the moon is one of its notable features. This is a range of mountains as high as 10 kilometres over the equator of the moon.

Iapetus is mainly composed of ice and a small amount of rocky material. It is heavily cratered and has large impact basins, some of which are even wider than 350 kilometers. The two hemispheres of the moon have a striking difference in color.

There are different theories that try to explain the reason behind this. One states that it is possible that Iapetus may be taking up particles from another dark moon Phoebe. Another theory says that the dark material on the surface could be distributed by ice volcanism.

Hydrocarbons coming from the volcanoes main form the dark surfaces after reacting due to solar radiation. Another theory to explain this color difference is the phenomenon of thermal segregation.

Very slow rotation means that the temperature cycle is quite long. So much so that this may be enabling the dark material to absorb heat from the Sun and warm up.

Magnetic field of Iapetus

Iapetus does not have any magnetic field. Even if the magnetic field is present it has not yet been detected.

Atmosphere on Iapetus

Iapetus is not known to have any atmosphere as yet. This is quite unusual as most of the other moons of Saturn are known to have at least a thin layer of atmosphere around them.

Orbit and Rotation of Iapetus

Iapetus is nearly 3.5 million kilometers or 2.2 million miles away from Saturn. Iapetus takes nearly 79.3 days to complete one revolution around Saturn and to rotate about its own axis. It is tidally locked to the planet. This means that only one face of the moon is always visible from Saturn.

Iapetus has a somewhat unusual orbit. It has the most inclined orbital plane of all the regular satellites.

Another interesting fact that we can derive from this is that the moon Iapetus is the only large moon from which a good view of Saturn’s rings can be seen. We do not know the cause of this inclined orbit.

Exploration of Iapetus

Before NASA’s Cassini spacecraft, Iapetus was only observed through telescopes. It is difficult to observe this moon as it is quite far away from the planet Saturn.

However, Cassini managed to conduct one close flyby of Iapetus in 2007 in which it went as close as 1227 km from the moon.

No spacecraft has ever been able to land on the surface of the moon as yet. It has obviously also never been visited by human beings due to obvious reasons of extreme temperature and pressure conditions.

However, scientists are making continuous efforts to explore the moons of different planets in more detail.

Potential for Life on Iapetus

No evidence of life has ever been found on Iapetus. This is due to the extreme temperature and pressure conditions. If any life does exist on the moon, it would be very different from that on the Earth. The possibility of finding life forms elsewhere in the universe can never be ruled out.

Iapetus in Pop Culture

People continue to be fascinated by the planets in the solar system as well as their moons. Iapetus moon has been mentioned in several elements of pop culture such as films, television shows, books, video games, etc, either individually, or collectively as a part of the Saturnian moon system.

It was most notably featured in the climax of the novel 2001: A Space Odyssey by Arthur C. Clark.


  • 1671: Iapetus is discovered by Giovanni Cassini
  • 1705: Iapetus is viewed from the western side
  • 1847: Iapetus is given its name
  • 2007: NASA’s Cassini conducts a flyby of Iapetus


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