Iapetus (moon)

Giovanni Cassini

Iapetus or Iapetus ( Saturn VIII) is the third- largest moon of the planet Saturn.


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

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

Web properties

Iapetus orbits Saturn at an average distance of 3.5613 million kilometers with bound rotation in 79 days, 7 hours and 55 minutes. The track has an eccentricity of 0.0283 and 7.52 ° relative to the inclined Laplaceebene which is inclined relative to the equatorial plane in the range from about 14.8 ° Saturn. Iapetus is next to the moon Phoebe the only large moon of Saturn, whose orbit has a significant inclination. Of all the large moons over 500 km in diameter in the solar system he is the one with the greatest orbital inclination ( with the exception of the irregular moon Triton Neptune, the planet orbits the decline ), the largest semi-axis and the longest orbital period.

Structure and physical properties

Iapetus has an average diameter of 1436 km. Its low density of 1.27 g/cm3 indicates that it is composed almost entirely of water ice with small amounts of silicate rock. The axis of rotation is inclined at 14.84 ° from the vertical.

Iapetus ' surface can be divided according to their discoloration in two distinct regions. The leading hemisphere is very dark and reddish with an albedo from 0.03 to 0.05. The dark region has been named Cassini Regio after the discoverer. The following hemisphere is as bright as Jupiter 's moon Europa with an albedo of 0.5, she was baptized Roncevaux Terra. The difference in brightness is so striking that Cassini reported to be able to watch the moon with his telescope on only one side of Saturn. Turned Earth's Moon, the dark region, it remained invisible. Iapetus has by all bodies in the solar system 's largest brightness contrast.

The Voyager 2 spacecraft passed Iapetus on 22 August 1981 in a distance of 966,000 km. This individual structures could be added to the dark region. The bright side is icy and heavily cratered. The polar regions are free of dark materials.

The dark materials could be deposits of organic compounds, as they occur in primitive meteorites (eg carbonaceous chondrites ) or on the surface of the comet. In addition, they may contain cyanides such as frozen- Cyanwasserstoffpolymere. Then indicate ground-based observations. The origin of the dark material is not clarified, so far there are several theories to it. The layer thickness of the material is unclear. If the dark layer to be thin, so it would have to be renewed constantly, as in a lighter material impact would be ejected from the ground.

The dark material could have come from the interior of the moon and be passed through a combination of Impaktereignissen and volcanism at the surface. This theory is supported by the concentrated deposits at the bottom of craters. Iapetus formed in a wide distance of Saturn and was less exposed to high temperatures in the formation of the solar system, so that he could incorporate volatile components such as methane or ammonia in its interior. This may have been later by geological processes such as the Kryovulkanismus ( Kältevulkanismus ) reaches the surface and is converted by UV radiation from the sun, ionized particles or cosmic rays in dark connections. At the boundary between the bright and the dark hemisphere a dark ring of 100 km in diameter can be seen, reminiscent of structures on the Earth's Moon or Mars where volcanic lava has flowed into impact crater with a central peak.

One theory ( based on the Cassini flyby of 10 September 2007) states that a very thin layer could be reached from the outside to the really white surface of Iapetus, and could have occurred by the higher Energieabsorbanz of the dark material is a melting or Sublimation, the darker rock masses brought to the fore. In addition, smaller (30-60 m diameter) were observed bright impact crater, which provide strong evidence for layer thickness and origin of the dark material. Since the crater depth is at a 60 -meter diameter crater approximately 10 m, it is clear that the dark material has to be thinner.

Another theory the dark material originated from the moon Phoebe. It could have liberated by the impact of micrometeorites and collected on Iapetus ' leading hemisphere. This theory is strengthened by the discovery of another, very extensive Saturn ring by the Spitzer Space Telescope on October 6, 2009. It is assumed that the material of this ring of Phoebe comes because its orbit passes pretty much within the ring. This decline circumferential ring starts at a Saturn distance of about 6 million kilometers. The rechtläufige Iapetus thus moves in the opposite direction through the edges of the ring, which would explain the material transfer plausible.

However, Phoebe's coloring is slightly different from the color of the deposits on Iapetus. The theory that the deposits originate from Phoebe is rejected by some researchers ( T. Owen et. Al. ). They favor the origin of the dark material from Saturn 's moon Titan due to spectroscopic measurements.

Another great puzzle represents a discovered on Cassini images ridge that extends to within a few degrees of latitude exactly on the geographical equator of Iapetus. In the photos, the phenomenon is clearly seen as a broad band, almost composed as of two parts through which the moon appears ( " walnut " shape of Iapetus ). The ridge has been observed over a length of 1300 kilometers. He reaches a width of up to 20 kilometers and a maximum altitude of 13 kilometers.

As the mountain range is created, is still in the dark. Scientists consider mainly two theories possible: First, the survey could have been formed by tectonic processes, ie by folding similar to the European Alps on earth. On the other could be kicked to the surface from the underground through a crack in the crust of the moon liquid material and have accumulated up to the present appearance. After a completely different hypothesis ( Wing- Huen Ip) is the wreckage of a crashed ring which is either been a remnant of the gas and dust disk from the Iapetus has formed, or, and the consequence of the impact of a large asteroid of thereby ejected material.

According to the latest researches of the mountain range should be caused by the fact that Iapetus in his youth quickly rotated and was not frozen because it was heated by radioactive substances ( 26Aluminium and 60Eisen ) with a relatively short half-life. Due to the rapid rotation he got a baggy shape. The activity of the isotope decreased and Iapetus froze before the rotation period extended to today's value. The baggy shape had to go back now actually. But that was no longer possible due to freezing. The material thus gathered at the former highest point, the equator, at.

Exploration by space probes

As the first spacecraft Voyager 2 came Iapetus on August 22, 1981 relatively close ( 966,000 km ) and thus mapped Iapetus with low resolution.

On 1 January 2005, the space probe Cassini Iapetus happened for the first time at a distance of 123,000 km and delivered the first high-resolution images of the moon. Another targeted flyby on 10 September 2007. Here the probe flew in just 1640 kilometers distance to the moon over and still made ​​it much more detailed shots.


In Arthur C. Clarke's novel 2001: A Space Odyssey Iapetus is a contrived by aliens stargate or wormhole, chosen because of the unusual and unexplained characteristics of the moon.