Phoebe (moon)

W. H. Pickering

Phoebe ( Saturn IX) is one of the larger and the outer moons of the planet Saturn.


Phoebe was discovered in 1899 by William Henry Pickering on photographic plates which had been exposed on 16 August 1898 by DeLisle Stewart in Arequipa (Peru ). It was the first moon, which was discovered by photographic means. Its precise orbit could be determined in 1905 by Frank Elmore Ross.

It was named after the Titan moon Phoebe from Greek mythology.

Web properties

For over 100 years was Phoebe as the outermost moon of Saturn, to several small satellites were discovered in 2000, are even further away.

Phoebe is four times farther from Saturn than its nearest large neighbor, the moon Iapetus, which is far greater than any other moon, which orbits Saturn at a similar distance.

Phoebe and Iapetus are the only large moon in the Saturn system, whose orbits do not lie in the equatorial plane of the planet. Phoebe's train is 175.3 ° to the ecliptic inclined by the Saturn represents the Laplace plane at this distance. So the web is retrograde, that is Phoebe runs counter to the direction of rotation of Saturn around the planet. In this case, their train a smaller inclination to the ecliptic than to the equatorial plane of Saturn.

Due to the irregular web properties Phoebe counts among Saturn 's moons to the Nordic group.

Structure and physical properties

Phoebe is approximately spherical and has an average diameter of 220 km. It rotates in 9 hours and 30 minutes around its own axis and thus in contrast to the other large moons of Saturn (except Hyperion ) no bound rotation. The rotational axis is inclined by 26.183 ° from the vertical. Prevail on its surface, depending on the sunlight, temperatures of -198 ° C to -161 ° C.

Most of Saturn's large moons have a very bright surface; however, is the extreme with a geometric albedo of 0.081 from dark Phoebe. In terms of their spherical albedo is only about 6% of the incident sunlight is reflected, so that the surface appears almost black. The dark coloration reminiscent of organic compounds, as they occur in primitive meteorites ( eg carbonaceous chondrites ). This led scientists to believe that it could be in Phoebe to be a captured asteroid.

In September 1981, the Voyager 2 spacecraft flew at a distance of 2.2 million kilometers past Phoebe and sent first pictures to Earth. Because of the great distance, the resolution of the images was low, so that no details were visible.

On June 11, 2004, the spacecraft Cassini -Huygens passed the moon at a distance of only 2068 km, and sent detailed images to Earth. The images show that Phoebe's surface is extremely heavily cratered, with impact craters up to 80 km in diameter are available. One of the crater has a rim wall of 16 km altitude.

An evaluation of the image data revealed that the surface of Phoebe has the highest so far observed crater density in the solar system. The crater density is a measure of the age of the surface of a celestial body. Phoebe is, according to NASA to be 4.5 billion years old so something like the solar system itself and therefore one of the objects that have occurred since its creation hardly changed.

Recent research, however, Phoebe said to have become short by internal heating after its formation so warm that he became a mostly round celestial bodies.

The images also show that the surface of Phoebe is covered by a thin dark layer having a thickness of 300 to 500 meters. At the crater edges, where the dark layer is torn as a result of the impact event, bright spots are visible. Here the underlying, almost appearing white material was ejected. In addition, traces of carbon dioxide were detected, a compound that previously could not be detected on any asteroid.

With 1.63 g/cm3 Phoebe has the second highest density among the major Saturnian moons Titan after. Your heart must except ice have a greater proportion of dense material, such as on siliceous rock.

The retrograde path and the composition suggest that Phoebe was originally a centaur, who was captured by the gravity of Saturn. Centaurs are a group of planetoids from the Kuiper belt, which move on eccentric orbits between the planets Jupiter and Neptune around the sun. Further evidence for this thesis are taken by the Cassini spacecraft surface spectra, by which it can be seen that frozen carbon dioxide and other organic compounds such as cyano compounds are present on Phoebe. These substances have so far been detected only on comets and Kuiper Belt bodies.

The hypothesis that the impact of micrometeorites released dark material of Phoebe's surface for the dark discoloration of the moons Hyperion and Iapetus could be responsible, got a first solid evidence than the infrared Spitzer Space Telescope, a huge ring with a very low density (10 to 20 particles per km ³ ) discovered, extending distance of 6 to 18 million kilometers from Saturn to the orbit of the moon Phoebe.

For larger impacts fragments may have been thrown into space, circling now as the moons Skathi, Mundilfari, Suttungr and Thrymr around Saturn. All of these moons that are less than 10 km, have a similar orbit as Phoebe.