What is an Outer Large Moon?
The Saturnian system is a big family composed of 82 moons. The number is only expected to grow as there are still other bodies that are not yet confirmed. Some of the moons of Saturn are very small while others are also exceptionally large. We can put these natural satellites in different orbital groups to remember them better. One of them is the outer large moons group, and it is in this group that we find the Iapetus moon.
Moons of Saturn – Orbital Groups
Ring moonlets – Very small moons
Ring shepherds – Small moons that orbit within or just beyond Saturn’s rings
Co-orbitals – Moons that share and swap orbits
Inner large moons – Large moons within the E ring
Alkyonides – Little satellites between the bigger moons Mimas and Enceladus
Trojan moons – Moons that are located in the stable Lagrange points of larger moons
Outer large moons – Large moons outside the E ring
Irregular moons – Moons that are very distant from Saturn. There are three subgroups:
- Inuit group
- Gallic group
- Norse group
The main rings of Saturn, from closest to farthest, are D, C, B, A, F, G, and E Ring. They will help us in locating the different orbital groups.
The Iapetus Moon
Iapetus Moon: Facts and Figures
All About the Name
- Pronunciation – /aɪˈæpɪtəs/
- Behind the name – Īapetus (A Titan in Greek mythology, son of the primordial gods Uranus and Gaia)
- Adjective/s – Iapetian
- Other designation/s – Saturn VIII
- Discoverer/s – G. D. Cassini
- Discovery date – October 25, 1671
- Parent planet – Saturn
- Orbital period – 79.3215 days
- Average orbit distance – 3,560,851 km
- Mean orbit velocity – 11,748.8 km/h
- Eccentricity – 0.0293
- Orbital inclination – 15.47° (to Saturn’s equator)
- Dimensions – 1,492.0 × 1,492.0 × 1,424 km
- Equatorial radius – 734.5 ± 2.8 km (456.4 ± 1.7 mi)
- Equatorial circumference – 4,621.9 km
- Volume – 1,667,300,080 km3
- Mass – 1,805,952,411,282,580,000,000 kg
- Surface area – 6,799,755.63 km2
- Mean density – 1.088±0.013 g/cm³
- Surface gravity – 0.223 m/s2
- Escape velocity – 2,061 km/h
- Rotation – (synchronous)
- Axial tilt – Zero
- Temperature – 90–130 K
- Albedo – 0.03–0.05 (leading hemisphere) 0.5–0.6 (trailing hemisphere)
- Apparent magnitude – 11.9 (leading hemisphere) 10.2 (trailing hemisphere)
Iapetus is a large astronomical body. It is Saturn’s third-largest moon after Titan and Rhea. This moon is also designated Saturn VIII. It was discovered in the 17th century and is the farthest of Saturn’s major moons.
One of the most striking characteristics of this moon is the yin-yang-like color of its surface. These contrasting bright and dark hemispheres of Iapetus made it a challenge for its discover, G.D. Cassini, to view it on both sides of Saturn.
More and more features of this moon were discovered when the Voyager twins and the Cassini mission visited the Saturnian system. The Cassini orbiter discovered a long mountain range straddling along its equator. This ridge somewhat makes the moon look like a walnut!
Also, Iapetus has a highly inclined orbit. Because of this, the rings of Saturn would not look edge-on from this moon.
How Did the Moon Iapetus Form?
The origin story of Iapetus is most likely similar to the moon Titan. Studies indicate that they formed as a result of giant impacts that involved primordial moons. So, it is most probable that they are ancient moons.
Most of the moons of Saturn, however, are likely the product of co-accretion. In this process, the moons started as disks of gas surrounding the then young planet Saturn. Time goes on until they coalesced into solid bodies, becoming the moons.
Iapetus was discovered on October 25, 1671. It was the Italian-French astronomer Giovanni Domenico Cassini who discovered this moon.
Upon discovery, he was able to observe Iapetus on Saturn’s western side. He tried to view it many times on the eastern side but was unable to see it.
It was only in 1705 that Cassini was able to view Iapetus on the eastern side of Saturn. When he did so, he found out that the moon is darker on that side compared to the other. This made him suspect that the moon has two distinct areas, with one being darker than the other.
Additionally, he assumed that the moon is most likely tidally locked to the planet. His assumptions were later confirmed when improved telescope technology allowed for a better view of the moon. The twin probes of Voyager I and II also verified his theories.
Visits to Iapetus
The Cassini orbiter observed Iapetus up close on September 10, 2007. It was the only targeted flyby made by the spacecraft. With a distance of about 1,227 km (762 miles), Cassini was able to capture features such as the mountains making up the moon’s bulging equator.
Iapetus’s shape is greatly affected by its equatorial ridge. This bulge has made it diverge from having a perfectly spherical body to having more of a walnut-shaped appearance. The moon’s poles also look like they are somewhat flattened and squashed.
Iapetus has an equatorial radius of about 735 km (457 mi) with the dimensions 1,492.0 × 1,492.0 × 1,424 km. This large body is not in hydrostatic equilibrium.
Compared to the Earth’s moon, Iapetus is 2.4 times smaller. It is also smaller than the largest moon of Saturn, Titan, by about 3.5 times.
The density of Iapetus is 1.088 ± 0.013 g/cm³. This low density can tell us that the moon is largely composed of water ice.
Just like the moon Rhea, about 75% of Iapetus is believed to be made up of ice. The remaining quarter is rocky material.
The reflectivity or albedo of Iapetus is not uniform throughout the surface of the moon. The leading hemisphere has an albedo of 0.03–0.05 which is roughly as dark as coal. On the other hand, the albedo of the trailing side ranges from 0.5 to 0.6.
In terms of apparent magnitude, or its brightness as observed here on Earth, Iapetus’s hemispheres also vary. The leading hemisphere is less bright at approximately 11.9 while the trailing hemisphere is about 10.2.
Both figures indicate that we cannot see the moon with the naked eye, since the dimmest object we can see is around magnitude 6.0.
The temperature on Iapetus ranges from 90 to 130 K. In the daytime, the bright region of the moon is at about 113 K while the dark region is hotter at 129 K.
Iapetus orbits Saturn at a distance of 3,560,851 km. It makes a complete orbit in about 79 days. And since it is in synchronous rotation with the ringed planet, its rotation period is also 79 days. It has the longest revolution among the moons of Saturn.
The orbit of Iapetus is very different from the other regular moons in the system. It is inclined by 15.47° to Saturn’s equator. Also, it is in resonance with the moon Titan.
What Does the Moon Iapetus Look Like?
Just like the other outer large moons, the surface of Iapetus has lots of craters. Its great distance from the tidal forces of Saturn did not allow for resurfacing to happen. This way, ice could have melted and deposited in the impact basins where they will freeze again, thus filling the depression.
Iapetus has been likened to the yin yang symbol because of its contrasting bright and dark areas. Its leading side has a different tone compared to the rest of the moon’s surface. It has a reddish-brown color, making it darker than the trailing hemisphere.
Did You Know?
With an albedo of 0.5 – 0.6, the trailing hemisphere of Iapetus is nearly as reflective as the moon Europa of Jupiter.
The distinguishable dark region of Iapetus is called Cassini Regio. The bright areas are divided into the northern and southern parts of the equator, named Roncevaux Terra and Saragossa Terra respectively.
The Two-Colored Iapetus
One side of Iapetus is bright while the other is dark. Behind this mystery is a layer of dark material on its surface. This dark material could be from the dark moon Phoebe, which is much farther from Saturn. Another source that astronomers also consider is ice volcanism.
In 2007, the Cassini spacecraft captured images of dark materials in the craters of Iapetus. These are in the transition regions between the dark and bright regions.
New findings of the Cassini probe suggest that the most probable reason for the varying color of Iapetus must be thermal segregation. It is likely a result of two things.
First is that a different moon collided with a meteoroid, producing debris that was swept up by Iapetus’s leading side. Initially darkened, the area then becomes hotter which causes ice to sublime, leading to thermal segregation.
The dark material that initially started the two colors of Iapetus is believed to be from another moon. This moon suffered some impacts and blasted material in space.
As Iapetus travels its orbit, its leading hemisphere caught the material, making it darker than the other hemisphere.
The moon Phoebe is the most probable source of the dark material on Iapetus. It has a broad disk that is said to be even bigger than that of Saturn’s.
The dark material is believed to be lag deposits left when the ice on the moon underwent sublimation. Remember that when ice sublime, it means that it changes from solid to gas state directly.
The dark region, having darker color because of material blasted from moon collision, absorbs more heat and therefore causes more sublimation on its ice. The vapor then goes back to its solid state by undergoing deposition. The deposits would transfer to colder areas, outside Cassini Regio.
As the process goes on, the dark area will become darker as its ice sublime, making the brighter areas even brighter through the process of deposition.
The moon’s slow rotation also contributes to this. Since a day on this moon is about 79 days, it means that the surface will also be exposed to the Sun for a long time!
Iapetus have four types of geological features. Most of their names were inspired by The Song of Roland, an epic poem from French literature.
- Montes (singular: mons)
- Regiones (singular: regio)
- Terrae (singular: terra)
- Craters (singular: crater)
The montes are mountain features while a regio is a part of the Iapetian surface which differs in reflectivity. The word terrae is used in large areas of Iapetus that are highly reflective. And the impact craters are, of course, circular depressions caused by impact.
Below are some of the most prominent geological features of Saturn’s moon Iapetus.
Cassini Regio is the distinct dark area of the moon Iapetus. It covers a big part of the moon’s leading hemisphere. And because this region is darker compared to the rest of the moon, its albedo is also lower.
Roncevaux Terra and Saragossa Terra
The reflective region of Iapetus is divided into two: the north and the south part. The northern part is called Roncevaux Terra while the southern region is called Saragossa Terra. The large crater called Engelier is in Saragossa Terra.
Turgis is a very large crater located in the dark part of Iapetus, Cassini Regio. It is nearly 600 km across, making it the largest crater discovered on the yin-yang moon.
Did You Know?
The dark hemisphere of Iapetus, Cassini Regio, is named after the moon’s discoverer Giovanni Domenico Cassini.
The Curious Ridge
The Cassini spacecraft uncovered a previously hidden feature of Iapetus—an equatorial ridge. Earlier observations were not able to reveal it because it lies in the dark area of Cassini Regio.
The ridge of Iapetus, which is roughly 1,300 km long, was discovered on December 31, 2004. Approximately 10 km high, the mountains that make up this ridge are some of the tallest in the solar system.
Did You Know?
It was Voyager I and Voyager II that hinted at the possibility of mountains on Iapetus. Because of that, the peaks are also called the Voyager Mountains.
Astronomers proposed some theories that might help explain the curious bulge of Iapetus. A hypothesis holds that there may have been a ring system around the moon in the distant past, and the moon accreted material from it. Another state that it could be traces of its oblate shape from when it was young.
Moreover, it is interesting to note that the peaks of Iapetus are somewhat concentrated along the equator, specifically in the dark-colored areas.
Why does the ridge lie only in the equatorial area? And why are they specific in Cassini Regio? The mystery of the Iapetian ridge remains a mystery for the most part.
Behind the Name
The name Iapetus has a lesser-known version, Japetus.
English astronomer John Herschel, son of renowned William Herschel, proposed the name Iapetus on this moon. This is in line with his suggestion that the moons of Saturn be named after the Titans of Greek mythology.
The naming system was made because the Titans are the siblings of Cronus, whose Roman equivalent is Saturn.
Before Iapetus became Saturn VIII, it was called Saturn V first then Saturn VII. The changes in the assigned Roman numeral designation had something to do with the discovery of the Saturnian moons.
The earliest known moon of Saturn is Titan, discovered by Christiaan Huygens. This feat was followed by Giovanni Cassini when he discovered the four moons, Iapetus, Rhea, Tethys, and Dione. He called them Sidera Lodoicea to honor King Louis XIV.
Astronomers referred to the first known five moons as Saturn I to Saturn V. Being the furthest, Iapetus was given the label Saturn V. Two new moons, Mimas and Enceladus, were discovered and Iapetus became Saturn VII.
In 1848, another moon before it was discovered and became Saturn VII. Thus Iapetus was assigned a bigger Roman numeral and became Saturn VIII as we know it today.
Iapetus in Greek Mythology
Iapetus is one of the twelve Titans of Greek mythology. Their parents were the ancient gods Gaia (Earth) and Uranus (Sky).
According to different works, Iapetus’s wife is either the Oceanids Asia or Clymene. His sons are some of the most well-known characters in Greek mythology as they are considered the ancestors of mankind. They are Epimetheus, Prometheus, Atlas, and Menoetius.
The offsprings of Iapetus have their own qualities that uniquely defined them. Epimetheus and Prometheus are opposites, as one is foolish while the other is clever. We know Atlas as a mighty and strong character. Menoetius, however, is known for his violence.
Iapetus fought with his brother Cronus in the war against Zeus and the Olympian gods. Their efforts proved unsuccessful and they, together with the other Titans, ended up in Tartarus.
- Iapetus_(moon) – By NASA/JPL/Space Science Institute – PIA08384: The Other Side of Iapetus, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2885080
- The Two-Colored Iapetus – https://solarsystem.nasa.gov/system/content_pages/main_images/932_PIA11690.jpg
- Cassini Regio – By NASA (Cassini probe), Matt McIrvin: Public Domain, https://commons.wikimedia.org/w/index.php?curid=49955
- Roncevaux Terra and Saragossa Terra – NASA image, Public Domain, https://commons.wikimedia.org/w/index.php?curid=16292400
- The Curious Ridge – Public Domain, https://commons.wikimedia.org/w/index.php?curid=2740019
- Iapetus in Greek Mythology – https://study.com/cimages/multimages/16/a39afde7-d380-41e3-b2cc-4dd913342289_time.jpg