Iapetus Moon Facts

There are two different pronunciations of Iapetus: Eee-a-Pa-tus or I-a-PA-tus. Either of these pronunciations are accepted.

Iapetus Moon Profile

Discovered By:G.D. Cassini October 25, 1671
Diameter:1,471.2 km
Mass:1.81 × 10^21 kg (2.5% Moon)
Orbit Distance:3,560,851 km
Orbit Length:79.3 days
Surface Temperature:-143 to -183 °C

Who Discovered Iapetus?

Iapetus is one of the four moons of Saturn that were discovered in 1672 by Giovanni Cassini.

Iapetus was a bit difficult to find due to the dark and light that contrasted and hid the moon for many years.

Iapetus is tidally locked to Saturn, which means it keeps the same face turned toward Saturn and the same opposite face away from Saturn.


The unusual positioning of Iapetus kept it in the shadow, only allowing the bright side to be seen from Earth when it traveled on Saturn’s west side.

The most contrasting feature about Iapetus is that the side facing Saturn is brilliantly light, whereas the side facing away is coal black.

Cassini originally named the four moons as part of the “Sidera Lodoicea” or “Stars of Louis” in honor of King Louis XIV. However, not too many people liked the names and they were referred to in their numerical form based on their distance to Saturn.

Almost 200 years later, Williams Herschel’s son, John Herschel, suggested that the 4 moons be named after the mythological Greek Titans and their families (Saturn to the Romans).

Greek mythology

Iapetus in Greek mythology is the son of Gaia and Uranus and the father of Prometheus and Atlas.

Since Prometheus was supposed to have given fire to mankind, Iapetus is considered to be the father of the human race. The density of Iapetus is so low that it is just a bit more than liquid water.

What do Scientists think?

Scientists believe that the moon is made up of mostly water and about ¼ rock. Iapetus also has a weird walnut-shape with a center that bulges and poles that are squashed.

The shape is an indicator that the moon has an odd rotation, once every ten hours as compared to the orbit which takes 79 days to complete a revolution.

It’s believed that early in the life of this moon it was rapidly spinning and a thick crust could have frozen over its surface.

Over many years, the moon began to slow down in its spin and became tidally locked.


Iapetus is Saturn’s third-largest moon, with a diameter of 914 mi/1,471.2 km. Around its equator it has a chain of high mountains that stretch over halfway around it for over 800 mi/1,300 km.

It has mountain peaks that reach over 12 mi/20 km which make them some of the highest mountains in our solar system.

Facts about Iapetus

  • In the 1980’s the Voyager spacecraft discovered a line of white dots that make up the ridge.
  • Originally called the Voyager Mountains, the later Cassini probe found that the white dots were ice regions.
  • Iapetus doesn’t suffer from the effects of tidal heating from Saturn.
  • Due to the lack of internal tidal heat, the surface of Iapetus doesn’t show any of the smooth features that would indicate resurfacing or melting.
  • In 2012, scientists found that they had detected landslides on Iapetus that were nearing fifty miles. This means that the surface does experience some change.
  • Iapetus does have a very light gravity.
  • Both of Iapetus’ hemispheres show a variety of craters in many sizes due to impacts.
  • The largest crater on Iapetus is 360 mi/580 km
  • Scientists have a lot of theories for why the two sides of Iapetus are exact opposites: light and dark.
  • Scientists may have evidence for one theory for the moon’s contrasting colors as in 2009 they discovered a ring of material that was believed to come from Phoebe and surrounds Saturn; and with rotations, heat, and gravity some of the materials were deposited on the dark side of Iapetus.
  • The bright side of Iapetus continues to get brighter and the dark side continues to get darker.
  • Scientists are looking at Iapetus as a model to explain various theories for both celestial bodies and those on Earth.


  • What is the most noticeable feature of Iapetus?
    One side is brightly colored white and the other side is coal-black?
  • What do scientists think Iapetus is made up of?
    Ice and rock
  • Do scientists think that Iapetus is affected by Saturn’s gravity causing tidal heat?
  • What is around Iapetus’ equator?
    A chain of mountains
  • Why does only one side of Iapetus face Saturn?
    They are tidally locked
  • What is the shape of Iapetus?