Saturn has 62 moons that are confirmed but only 53 have received official names. This makes the orbit of Saturn a rather crowded place.

Iapetus is a very strange moon. It is oddly shaped and colored, and is the third largest moon orbiting around the planet Saturn. Iapetus has one hemisphere that is light colored and another hemisphere that is incredible darkly colored with a bit of a red tint. Because of its odd coloration, it’s been called the yin and yang of the Saturn moons.


Iapetus may be quite a distance from its parent planet but it is also tidally locked. This means that it only shows one side of itself to Saturn all the time. However, due to its distance and the inclination of its orbit, it may be the only large moon that has the best view of Saturn’s rings.

Iapetus has an orbit of 2,213,000 mi/3,561,000 km from Saturn and this far distance doesn’t allow it to get any of the tidal forces from Saturn or even most of the other moons. The lack of this affect also means that Iapetus hasn’t had any of the resurfacing or melting situations that other moons have had that are closer to Saturn.

While Iapetus is Saturn’s third largest moon, its radius is only two-fifths of the radius of our own moon. Unlike our moon that is made of rocky materials, Iapetus is mostly ice and the composition means that it’s around 2 percent as massive as our moon.

Iapetus Statistics:

  • Orbits: Saturn
  • Discovered By: G.D. Cassini
  • Discovery Date: October 25, 1671
  • Diameter: 1,471.2 km
  • Mass: 1.81 × 10^21 kg (2.5% Moon)
  • Orbital Period: 79.3 days
  • Orbit Distance: 3,560,851 km
  • Surface Temperature: – 143 to -183 degrees C


On October 25, 1671, the Italian astronomer, Giovanni Cassini discovered Iapetus. Even though this small moon was only a tiny dot to most astronomers, it had a brightness that moved from faint to brighter as it orbited around Saturn.

Cassini’s original name for the group of four moons that he discovered was Sidea Lodoicea, which means “Stars of Louis” for King Louis XIV. His discovery included the moons Iapetus, Tethys, Dione, and Rhea. Other astronomers wanted the moons to have numbers so the moons were given Roman numerals. As more moons were discovered, it became apparent that this naming arrangement wasn’t going to work.

John Hershel, sun of the famous astronomer William Herschel, suggested that the moons of Saturn be named after the mythological brothers and sisters of the Greek god Kronus. The Roman equivalent of the god was known as Saturn.

The name Iapetus was the mythological son of Gaia and Uranus, brother of Kronus, and father of Prometheus and Atlas. Greeks thought of Iapetus as the father of the human race since he was the father of Prometheus.

Formation, Structure and Surface:

Scientists believe that Saturn’s moons were formed in a process called “co-accretion,” similar to the process that is thought to have formed all of our solar system planets. When a gas giant is young and first forming it’s surrounded by discs of material that gradually bond together to form moons. All the moons of Saturn except Titan are believed to have formed this way. Titan is thought to be the result of giant impacts that occurred between existing moons that eventually formed the giant. Both Rhea and Iapetus were once thought to be formed with the leftover materials from the collisions. In more recent research, there is a suggestion that all of the moons inward of Titan are no more than 100 million years old. Iapetus is not likely to have been formed in the same collision series as Rhea, but instead may have been earlier satellites.


As so many of the moons of Saturn, Iapetus is in “resonance” with Titan, Saturn’s largest moon. This means that both Iapetus and Titan speed up and reduce speed as they pass each other in their complex set of orbit. However, Iapetus isn’t as large as Titan, and with a diameter of less than a third of the giant moon, Titan’s orbit and rotation are affected on a lesser scale than that of Iapetus.

Scientists were aware that one side of Iapetus is so much darker than the other hemisphere as well as when compared to surfaces of other objects in Saturn’s system. It was thought that Iapetus might be picking up particles from the even farther moon of Phoebe. If this was the case, the darkened shade should remain constantly dark as new particles land, however, there are brighter craters that are being detected, which doesn’t support that theory. Another idea is that Iapetus may have ice volcanism that is erupting and spreading darkened material to the surface of that region. Eruptions of hydrocarbons could create a dark surface, especially if there are solar radiation chemical reactions. While both of the hemispheres do contain craters that vary in size, Turgis is the largest crater and has a 360 mi/580 km diameter.

The flyby of the Cassini spacecraft in 2007 of Iapetus revealed a third process called thermal segregation. Scientists are more apt to believe that this is the reason for the darkened hemisphere. Iapetus has a slow rotation that is longer than 79 days. This means that the temperature cycle is also very long and the dark material has a longer amount of time to absorb the sun’s heat. The heating process will cause any icy material to melt or go to colder areas. This allows the dark areas to be even darker and the lighter areas to retain their brightness. Scientists haven’t ruled out that Iapetus could be receiving material from some other external source that could be warming up and triggering the process.

In 2009, scientists did detect a ring of material around Saturn that was believed to come from Phoebe. Infrared observations in 2015 by NASA’s WISE spacecraft demonstrated that the ring around Phoebe was bigger than previously thought. It has a distance that ranges from 100-270 times Saturn’s radius.

Scientists are reviewing models of Iapetus and are trying to compare it to the Earth-tracked microwave emissions. The problem with this is that ice on Earth turns into liquid water. Comparison with other locations that are more appropriate include Jupiter’s moons and some of the smaller icy Kuiper Belt bodies that are beyond Neptune.

One of the major features that is noticeable on Iapetus is the chain of 6 mi/10 km high mountains that form an “equatorial ridge.” These mountains are on the equator of the moon and seem to spread around the side of the moon that doesn’t face Saturn. On that side the ridge seems to break up and the mountains are particularly bright. Both the Voyager 1 and Voyager 2 spacecraft sent back data on the mountains as they observed white dots on the eastern sides. The dots turned out to be ice regions and this area nicknamed the “Voyager Mountains.” Some astronomers feel that the ridge was formed during the early creation of Iapetus as it rotated more quickly that it does today. Other astronomers feel that the ridge might be the remains of material from a ring that collapsed.

The peaks of the mountains reach as high as 12 mi/20 km and this makes them part of the highest mountains in the entire solar system. Since Iapetus hasn’t had the tidal heating that other moons experience, the surface shows no evidence of resurfacing or melting. Without any environmental or weather conditions, the mountain range doesn’t break down or get smaller.

The density of Iapetus is just a bit more than liquid water and this means that Iapetus is more than likely made up of mainly water and rock combinations, with the rock percentage being around one quarter of the total. The moon has an odd walnut shape and isn’t spherical like many moons. Iapetus has an equator that bulges and both of its poles appear squashed. The odd structure of Iapetus has led to many conspiracy theory ideas including that it isn’t a natural satellite or that it was created or modified by an alien civilization. However, scientists can explain that in the early life of Iapetus, when it was spinning more rapidly, the crust could have developed on the moon and frozen it into its strange shape. Time would have allowed it to even out a bit, but it would still be too frozen as well as tidally locked to change completely.

In 2012, scientists did report seeing some surface changes when they detected landslides. These were more than likely caused from material that fell from great heights and then moved lower due to the very light gravity. Scientists are studying the landslides so that they can better explain similar situations that happen on Mars.

North and South hemispheres of Iapetus

With one side light and the other side dark, Iapetus represents a striking moon that is completely different from other moons. The Cassini spacecraft made note that when Iapetus is facing towards Earth, its dark leading region, called the Cassini Regio, helps to keep it hidden. It was given that name to honor the famed Italian astronomer.

Atmosphere and Magnetosphere:

Iapetus doesn’t have an atmosphere. It’s believed that the moon may have a core but it’s made up of mostly ice with a little rock. It therefore doesn’t have any magnetic field.

Could Life Exist?

There are specific qualifications needed to start and support life and Iapetus doesn’t seem to have a majority of them. If it did have liquid water in its past it is believed that is became frozen. Iapetus is not a candidate as a location to support life.


The 1980 and 1981 Voyager 1 and Voyager 2 spacecraft confirmed the observations that Cassini originally made to explain that each of the hemispheres of Iapetus had different levels of reflectivity.

In 2007, the Cassini spacecraft’s flyby collected detailed data and information about Iapetus revealing a process called thermal segregation to explain the darkening of one hemisphere.

Facts about Iapetus Moon for Kids:

  • Some of the geological features on Iapetus have been named after the epic French poem “The Song of Roland.”
  • The tradition of naming the satellites after the mythological creatures, characters and Greek gods followed suit with naming Iapetus, who was one of the Greek Titans.
  • The biggest impact crater on Iapetus has a basin up to 580 km across.
  • Some of the dark patches on Iapetus are thought to be made up of organic materials that are found in early comets or meteorites.
  • The ridge of Iapetus extends across the equator 1,300 km through the moon’s central region and then seems to break up a bit as it enters the darker hemisphere.
  • Scientists are not really sure how the equatorial ridge was formed.
  • The orbit of Iapetus is inclined to Saturn’s plane and yet is really far away so that it is the only large moon that can easily observe Saturn’s rings.

Pop Culture:

In Arthur C. Clarke’s 1968 book and film: 2001: A Space Odyssey, the monolith at the end of the movie is located on Iapetus.