Phoebe (moon)
| Phoebe | |
|---|---|
|
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| Saturn moon Phoebe, taken by Cassini-Huygens on June 11, 2004 | |
| Central body | Saturn |
| Properties of the orbit | |
| Major semi-axis | 12,952,000 km |
| Periapsis | 10,841,000 km |
| Apoapsis | 15,063,000 km |
| eccentricity | 0.163 |
| Orbit inclination | 175.8 ° |
| Orbital time | 550.479 d |
| Mean orbital velocity | 1.70 km / s |
| Physical Properties | |
| Albedo | 0.081 ± 0.002 |
| Apparent brightness | 16.5 mag |
| Medium diameter | 213.0 km |
| Dimensions | 8.3 × 10 18 kg |
| surface | 150,000 km 2 |
| Medium density | 1.638 g / cm 3 |
| Sidereal rotation | 9.2735 ± 0.0006 h |
| Axis inclination | 26.183 ° |
| Acceleration of gravity on the surface | 0.038 - 0.050 m / s 2 |
| Escape speed | 93.4 m / s |
| Surface temperature | 78 to 112 K |
| discovery | |
| Explorer | |
| Date of discovery | August 16, 1898 |
Phoebe (also Saturn IX ) is one of the larger and outer moons of the planet Saturn . It was discovered in 1899, making it the last Saturn moon to be found before the space probe era.
As the only main satellite of the ring planet, it orbits it in opposite directions and with an unusually long orbital period of 1½ years. Since it has a very porous structure and extremely dark surface, it is presumably a trapped cometary nucleus from the Edgeworth-Kuiper Belt .
discovery
Phoebe was discovered by William Henry Pickering in 1899 on photographic plates that had been exposed on August 16, 1898 by DeLisle Stewart in Arequipa ( Peru ). It was the first moon to be discovered photographically. Its exact orbit could not be determined until 1905 by Frank Elmore Ross .
The moon was named after the titan Phoibe from Greek mythology .
Track properties
For over 100 years, Phoebe was considered the outermost moon of Saturn, until space probes discovered numerous small satellites between 2000 and 2006, most of which are further away.
Phoebe is four times farther from Saturn than its closest larger neighbor, the moon Iapetus , being far larger than any other moon orbiting at a comparable distance.
Phoebe and Iapetus are the only large moons in the Saturn system whose orbits are not close to the planet's equatorial plane. Phoebe's orbital plane, the so-called Laplace plane , is inclined by 175.3 ° in relation to the orbital plane of Saturn and is therefore retrograde , i.e. H. Phoebe runs around the planet against the direction of rotation of Saturn.
Because of the irregular orbit characteristics, Phoebe belongs to the Nordic group of Saturn's moons .
Structure and physical properties
Phoebe is approximately spherical and has a mean diameter of 213 km (the lengths of the main axes are 218.6 km × 216.8 km × 203.6 km). It rotates around its own axis in 9 hours and 17 minutes and, like the other outer moons of Saturn, has no bound rotation . The axis of rotation is aligned almost exactly parallel to the axis of rotation of Saturn. This means that although the orbit is retrograde, Phoebe is rotating in the same direction as Saturn. Depending on the amount of sunlight, temperatures of −198 ° C to −161 ° C prevail on their surface.
Most of Saturn's large moons have very bright surfaces; in contrast, Phoebe's is extremely dark with a geometric albedo of 0.081. With regard to its spherical albedo, only about 6% of the incident sunlight is reflected, so that the surface appears almost black. The dark color is reminiscent of organic compounds such as those found in primitive meteorites (e.g. carbonaceous chondrites ). This led scientists to suspect that Phoebe might be a captured asteroid .
In September 1981 the Voyager 2 spacecraft flew past Phoebe at a distance of 2.2 million kilometers and sent the first photos to Earth. Due to the great distance, the resolution of the images was low, so that almost no details could be seen.
On June 11, 2004, the Cassini-Huygens spacecraft passed the moon at a distance of only 2068 km and sent detailed images to Earth; which showed that Phoebe's surface is extremely cratered. The largest impact crater called Jason measures 101 km in diameter. This makes Jason almost half the size of Phoebe's diameter. It is located at the equator of the moon. Its walls are up to about 18 km high and are connected to the maximum angle of incline of 47 °.
An evaluation of Cassini's image data showed that the surface of Phoebe has the highest crater density in the solar system that has been determined to date . The crater density is an indicator of the age of the surface of a celestial body. According to NASA, Phoebe is roughly the same age as the solar system itself, at 4.5 billion years, making it one of the objects that have barely changed since its formation.
According to further research, Phoebe is said to have become so warm due to internal heating shortly after her formation that she became a largely round celestial body.
The images also show that Phoebe's surface is covered by a thin, dark layer that is 300 to 500 meters thick. At the crater edges, where the dark layer tore open as a result of the impact event , light spots can be seen. Here the underlying, almost white-appearing material was ejected. In addition, traces of carbon dioxide were found, a compound that has not yet been found on any asteroid.
With 1.63 g / cm³ Phoebe has the second highest density of the large Saturn moons after Titan . Apart from ice, their interior must have a larger proportion of dense material, such as silicate rock.
The retrograde orbit and composition suggest that Phoebe was originally a centaur who was captured by the gravity of Saturn. Centaur are a group of asteroid from the Kuiper belt , which on eccentric orbits between the planets Jupiter and Neptune around the sun move. Further evidence of this thesis are the surface spectra recorded by the Cassini space probe, which show that there are carbon dioxide frozen on Phoebe and other organic compounds such as cyano compounds. These substances have so far only been detected on comets and objects in the Kuiper belt.
The hypothesis that upon impact of micro-meteorites released dark material from the surface of Phoebe with respect to the dark discoloration of the moons Hyperion and Iapetus could play an essential role, got another boost when the Infrared Space Observatory Spitzer a huge torus, the so-called Phoebe ring , with very low density (10 to 20 particles per km³) that extends 6 to 12 million km from Saturn to Phoebe's orbit. In the meantime (2015) it was found with the space telescope WISE that the ring even reaches a distance of 6–16 million km from Saturn. It consists mainly of very fine, dark dust.
Web links
- Raumfahrer.net: Is Phoebe a comet?
- Raumfahrer.net: Cassini passes the Saturn moon Phoebe
- Pronunciation of the word Phoebe
Individual evidence
- ↑ Ulrich Köhler, K. Stephan, R. Wagner: In den eisigen Welten des Saturn , In Bührke / Wengenmayr (eds.) Mysterious Kosmos , Wiley-Verlag 2014
- ↑ DLR Institute for Planetary Research: Cassini-Huygens ( Memento of the original from May 2, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Datasheet of Saturn's moons
- ^ Bauer, JM, Buratti, BJ, Simonelli, DP, Owen, WM: Recovering the Rotational Lightcurve of Phoebe . In: The Astronomical Journal . No. 610 , 2004, p. L57-L60 .
- ↑ http://ssd.jpl.nasa.gov/?sat_phys_par
- ↑ Phoebe in the Gazetteer of Planetary Nomenclature of the IAU (WGPSN) / USGS ; last accessed on September 27, 2016.
- ↑ File: Phoebe 2005 Mercator PIA07795.jpg
- ^ B. Giese, G. Neukum, T. Roatsch, T. Denk, CC Porco: Topographic modeling of Phoebe using Cassini Images. In: Planetary and Space Science 54 (2006), May 4, 2006, p. 1160 (PDF, last accessed on September 27, 2016)
- ↑ Stefan Deiters: PHOEBE, News from the mysterious Saturn moon , Date: April 27, 2012, Accessed : May 1, 2012
- ↑ Nasa telescope discovers huge ring of Saturn. In: Spiegel Online. October 7, 2009, accessed December 5, 2014 .
- ↑ Saturn's giant ring is even bigger , in scinexx.de, accessed: June 15, 2015 , the article states as the source: Douglas P. Hamilton, Michael F. Skrutskie, Anne J. Verbiscer, Frank J. Masci: Small particles dominate Saturn's Phoebe ring to surprisingly large distances , in Nature 522, 185-187 (11 June 2015)
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Semi- major axis (km) Phoebe 12,944,000 |
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