Iapetus (moon)

Iapetus as seen by the Cassini probe - 20071008.jpg

Iapetus or Japetus (also Saturn VIII) is the third largest moon of the planet Saturn. It was discovered in 1671, orbits relatively far out and has two different hemispheres, whose brightnesses are in the ratio 1:12 to each other.

Table of Contents

Orbit and rotation

Iapetus orbits Saturn at an average distance of 3.561.300 km in 79 days, 7 hours and 55 minutes. The orbit has an eccentricity of 0.0283 and is inclined 7.52° with respect to the Laplace plane, which at this distance from Saturn is inclined about 14.8° with respect to the equatorial plane. Besides the moon Phoebe, Iapetus is the only large moon of Saturn whose orbit has a significant inclination. Of all large moons over 500 km in diameter in the solar system, it is the one with the largest orbital inclination (with the exception of the irregular Neptune moon Triton, which orbits the planet in retrograde), the largest semi-major axis, and the longest orbital period.


Iapetus has a rotation tied to its orbital period of 79 days, 7 hours, and 55 minutes as well. Its axis of rotation is exactly perpendicular to its orbital plane.


Physical and chemical structure

Iapetus has a mean diameter of 1436 km. Its low density of 1.27 g/cm 3 indicates that it is composed almost entirely of water ice with small amounts of silicate rock.


Iapetus’ surface has two distinctly different regions. The anterior hemisphere in the direction of motion is as dark as coal, with an albedo of 0.03 to 0.05, with a slightly reddish tint, while the following hemisphere has an albedo of 0.5 to 0.6, The trailing hemisphere is thus almost as bright as Jupiter’s moon Europa; its northern part has been christened Roncevaux Terra and its southern part Saragossa Terra. The dark region has been named Cassini Regio after Iapetus’ discoverer, Giovanni Domenico Cassini. The difference in brightness is so great that Cassini reported being able to observe the moon with his telescope only on one side of Saturn. If the moon turned the dark region to the earth, it remained invisible for the observer at that time. Iapetus has the highest brightness contrast of all known bodies in the solar system.

The bright side is icy and heavily cratered. The polar regions are also free of dark materials.

The dark materials could be deposits of organic compounds such as those found in primitive meteorites (for example, carbonaceous chondrites) or on the surface of comets. In addition, it could contain cyanides such as frozen-out hydrogen cyanide polymers. This is indicated by Earth-based observations. The origin of the dark material is not clear, so far there are several theories about it. The thickness of the material is also unclear. If the dark layer were thin, it would have to be constantly renewed, as lighter material would be ejected from the subsurface in the event of an impact.

The dark material could have come from the interior of the moon and reached the surface through a combination of impact events and volcanism. This theory is supported by the concentrated occurrence at the bottom of craters. Iapetus formed at a far distance from Saturn and was exposed to less high temperatures during solar system formation, allowing it to incorporate volatile components such as methane or ammonia in its interior. These may have been brought to the surface later by geological processes such as cryovolcanism (cold volcanism) and converted into dark compounds by UV radiation from the Sun, ionized particles, or cosmic rays. At the boundary between the light and dark hemispheres, a dark ring 100 km in diameter is visible, reminiscent of structures on Earth’s Moon or Mars where volcanic lava has flowed into impact craters with a central mountain.

One theory (based on the Cassini flyby of 10. September 2007) says that a very thin layer from the outside could have reached the actually white surface of Iapetus and by the higher energy absorption of the dark material a melting or sublimation effect could have occurred, which brought darker rock masses to the surface. In addition, smaller (30-60 m diameter) bright impact craters were observed, which provide clear evidence of layer thickness and origin of the dark material. Since for a crater 60 m in diameter the crater depth is approx. 10 m, it is clear that the dark material must be thinner.

According to another theory the dark material comes from the moon Phoebe. It may have been released by the impact of micrometeorites and collected on Iapetus’ leading hemisphere. This theory is strengthened by the discovery of another very extended Saturn ring, the Phoebe ring, by the Spitzer Space Telescope on 6. October 2009. It is assumed that the material of this ring originates from Phoebe, because its orbit runs quite exactly inside the ring. This retrograde orbiting ring begins at a Saturn distance of about 6 million kilometers. Thus, the right-running Iapetus is moving in the opposite direction through the rim regions of the ring, which would plausibly explain the transfer of material.

However, Phoebe’s coloring is somewhat different from the coloring of the deposits on Iapetus. The theory that the deposits came from Phoebe is rejected by some researchers (T. Owen et al.). Based on spectroscopic measurements, they favor the origin of the dark material from Saturn’s moon Titan.

Another big mystery is a ridge discovered on the Cassini images, which extends exactly on the geographic equator through Cassini Regio except for a few degrees of latitude. On the photos the phenomenon is clearly visible as a broad band, through which the moon on its dark colored side almost appears to be composed of two parts ("walnut shape" of Iapetus). The mountain range could be observed on a length of 1300 kilometers so far. It reaches a width of up to 20 kilometers and a maximum height of 13 kilometers.

How the mountain range was formed is still in the darkness. Scientists consider two main theories possible: First, the uplift could have been formed by tectonic processes, i.e. by unfolding similar to the European Alps on Earth. On the other hand, through a crack in the moon’s crust, liquid material from the subsurface could have reached the surface and accumulated to its present appearance. According to a completely different hypothesis (Wing-Huen Ip) it is the debris of a crashed ring, which was either a remnant of the gas and dust disk from which Iapetus was formed, or the result of the impact of a large asteroid and the material ejected by it.

According to newer researches the ridge is supposed to have been formed by the fact that Iapetus rotated fast in its youth and was not yet frozen, because it was heated up by radioactive substances ( 26 aluminum and 60 iron) with relatively short half-lives. The fast rotation gave it a bulging shape. The isotopic activity decreased and Iapetus froze before the rotation period increased to the present value. The bulging shape had to go back now actually. But this was no longer possible due to freezing. The material therefore accumulated at the former highest point, the equator.

Of the 58 named craters of Iapetus, 20 have a diameter of at least 100 km. By far the largest crater, named Abisme, measures 767.74 kilometers – more than half of Iapetus’ diameter. It is located in the middle of the northern half of the leading hemisphere in the dark terrain. Inside the large, but optically hardly noticeable depression, Iapetus’ fifth largest crater Falsaron stands out more clearly with a diameter of 424 km.


Iapetus was discovered on 25. October 1671 discovered by Giovanni Domenico Cassini.

The moon was named after the Titan Iapetos from Greek mythology. The name "Iapetus" and the names of seven other moons of Saturn were given by Wilhelm Herschel’s son, the astronomer John Herschel, in a publication published in 1847 "Results of Astronomical Observations made at the Cape of Good Hope." proposed.

Exploration by space probes

As the first space probe Pioneer 11 came to him on 29. August 1979 with a smallest distance of 1033 Mm relatively close. From the deflection of the probe by the gravitational field of the moon the mass could be roughly estimated.

After Voyager 1 in November 1980 Voyager 2 passed the Saturn system in August 1981. This made the first detailed recordings of Iapetus on the 22nd of September. August 1981, the smallest distance between Iapetus and the spacecraft was 909.000 km. During their one-time flyby, individual structures could already be recorded and the Trabant then mapped with low resolution.

On 1. January 2005 the space probe Cassini passed Iapetus for the first time in a distance of 123.000 km and delivered the first high-resolution images of the moon. Another targeted flyby took place on 10. September 2007. Here the probe flew past the moon in only 1640 kilometers distance and made still clearly more detailed photographs.

In Arthur C. Clarke’s novel 2001: A Space Odyssey Iapetus is a stargate constructed by extraterrestrials, resp. wormhole, chosen because of the unusual and unexplained properties of the moon.

in Jack McDevitt’s novel God’s machines An ice statue of extraterrestrial origin found on Iapetus plays a role in the story.

Kim Stanley Robinson describes in the novel 2312 a (human) city that stretches along a road on the ridge of Iapetus around the entire moon.

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