Charon (moon)

Charon
Charon, captured by the New Horizons spacecraft on July 14, 2015
Provisional or systematic name	S / 1978 P 1 Pluto I.
Central body	Pluto
Properties of the orbit
Major semi-axis	(19,571.4 ± 4.0 planetocentric; 17,181.0 ± 4.0 barycentric) km
Periapsis	19,570.0 km
Apoapsis	19,572.8 km
eccentricity	0.000000 ± 0.000070
Orbit inclination	0.001 ° (equatorial plane) 119.591 ± 0.014 ° (orbital plane) 112.783 ± 0.014 ° ( ecliptic ) °
Orbital time	6.3872304 ± 0.0000011 d
Mean orbital velocity	0.223 km / s
Physical Properties
Albedo	0.372 ± 0.02
Apparent brightness	17.26 mag
Medium diameter	1,208.0 ± 3.0 km
Dimensions	1.586 ± 0.015 x 10 21 kg
surface	4,400,000 km 2
Medium density	1.65 ± 0.07 g / cm 3
Sidereal rotation	6.3872304 ± 0.0000011 days
Axis inclination	0.000 °
Acceleration of gravity on the surface	0.28 ± 0.01 m / s 2
Escape speed	604 m / s
Surface temperature	−210 ° C / 63 K
discovery
Explorer	JW Christy
Date of discovery	June 22, 1978
Remarks	Largest pluto satellite; Pluto-synchronized rotation
Size comparison between the earth - moon and Pluto - Charon pairs (bottom right) on the same scale (photo montage).

Charon ( [ˈçaːrɔn] , also [ˈkaːrɔn] ) is the innermost and largest of the five known moons of the dwarf planet Pluto . It was discovered in 1978. Its mean diameter is 1208 kilometers, which is slightly more than half the diameter of Pluto. Compared to other moons in the solar system , Charon is unusually large in relation to its main body and the common center of gravity is far outside of Pluto.

In August 2006 the International Astronomical Union (IAU) discussed whether Charon and Pluto should jointly receive the status of a dwarf planet ( Plutoid ) as a double planet . However, a decision was only made on Pluto; Charon has remained in the satellite status for the time being .

discovery

Discovery photo (negative) of Charon

Charon was discovered on June 22, 1978 by the astronomer James Walter Christy of the United States Naval Observatory in Washington, DC while evaluating photographic plates made months earlier by the 1.55-meter Kaj Strand Astrometric Reflector . Christy noted that Pluto would periodically show a slight bulge in the photographs (see picture). This phenomenon, which indicated a moon of Pluto, could later be demonstrated on photographic plates that had already been exposed on April 29, 1965. The period of the bulge corresponded to the period of rotation of Pluto, which was known from its light curve , and thus indicated a synchronous orbit . All doubts were removed when Pluto and Charon repeatedly covered each other between 1985 and 1990 , which only happened in two stages within the 248-year period of Pluto. So it was a happy coincidence that this happened so soon after Charon's discovery. The discovery of a Pluto's moon allowed astronomers to more accurately determine Pluto's mass and size. Before that, Pluto was considered to be significantly larger.

designation

The discovery was announced on July 7, 1978; the new celestial body was given the provisional designation S / 1978 P 1 .

On January 3, 1986, the moon was officially named by the IAU after the ferryman Charon , who in Greek mythology brings the dead across the Styx River to the realm of the god of the dead Hades ( Pluto in Latin ). The IAU preferred Christy's suggested name, which also alluded to the first four letters of his wife's name, Charlene , whom he called Char . The name is therefore sometimes pronounced Sharon .

When naming, at the suggestion of Christy's colleagues from the Naval Observatory, Persephone was also up for discussion, Pluto's wife in mythology.

Unlike the Earth's moon or Pluto, Charon does not have an official astronomical symbol or one that is commonly used. Charon symbols (e.g. ) that are circulating on the Internet are drafts by private individuals. Official recognition is not in sight, as astronomical symbols only play a subordinate role in modern astronomy.

There is a slight risk of confusion with the asteroid and comet (2060) Chiron , which was discovered about a year earlier, but which was named after a centaur from mythology.

Track properties

Orbits of Pluto, Charon, Nix and Hydra around the bary center

Orbit

Pluto and Charon orbit one another in a retrograde , almost perfectly circular orbit with respect to the solar system at an average distance of 17,536 km ± 4 km (19,571.4 km distance between the centers of the two bodies, about 17 Pluto and 32.3 Charon radii) around the common center of gravity , which is about 1200 km above the surface of Pluto (2360 km from the center) because of the relatively small mass difference. So Charon and Pluto are physically a double system. This results in an average distance between the two surfaces of 17,812 km. The orbital eccentricity is not more than 0.00007, the web is 0.001 ° relative to the equator of Pluto inclined . Like the Pluto equator, Charon's orbital plane is therefore inclined retrograde at 119.591 ° to the orbital plane of the dwarf planet.

Charon and Pluto orbit each other in 6 days, 9 hours, 17 minutes and 36.7 ± 0.1 seconds, which corresponds to around 14,186.63 orbits in a Pluto year (around 248.09 earth years).

rotation

Charon also rotates in 6 days, 9 hours, 17 minutes and 36.7 ± 0.1 seconds around its own axis and in the same period and with the same direction of rotation around the barycentre . Like the earth's moon, it shows a bound rotation and always shows its main body the same side. In contrast to the earth and moon, the rotation times of Pluto and Charon were slowed down and synchronized by tidal forces on both sides; therefore Pluto always turns the same side to Charon. This is the only confirmed case of doubly bound rotation in the solar system, known as dumbbell rotation .

Physical Properties

Charon has a mean diameter of 1208 km ± 3.0 km (according to other data 1207.2 km ± 2.8 km), which corresponds to about 52.6 percent, i.e. slightly more than half of the central body. Charon's mass is about 12.2 percent of the mass of Pluto. It is the twelfth largest moon in the solar system .

internal structure

Two models of the internal structure of Charon

Charon's mean density was determined to be 1.65 g / cm³. It should therefore consist of around 55–60% rock and 40–45% water ice ; an obvious difference to Pluto, whose rock content is around 70%.

There are two theories about the internal structure of Charon: either Charon is a differentiated body with a rock core and ice cover, or it consists of a uniform mixture of ice and rock. With the discovery of evidence of cryovolcanism , the first theory is favored. This makes Charon an ice moon .

The relatively high proportion of rocky material and the lack of a noticeable atmosphere support the assumption that this relatively large satellite, analogous to the formation of the Earth's moon, is the product of the great collision of a predecessor of Pluto with another Pluto-sized body of the Kuiper belt .

brightness

Seen from the earth, Charon is very faint with a brightness of 16 ^m . Viewed from Pluto, it is very bright due to its size and reaches about −10.6 ^m in the full Charon . The ash-gray light of the New Charon is −4 ^m brighter than that of the Earth's moon with −2.8 ^m . Viewed from Charon, Pluto reaches the brightness of our full moon at −12.5 ^m at full Pluto . The reason is the small distance between Pluto and Charon in terms of size. The resulting tidal forces are almost 20 times as strong as in the earth-moon system.

surface

Charon's surface has a size of approximately 4,400,000 km ² . It has a reflectivity of 37.2%. This is quite bright compared to other Kuiper Belt objects and is roughly the same as Earth's albedo . Unlike Pluto's surface, which is covered in frozen nitrogen and methane , Charon's surface appears to be made of the less volatile water ice. In addition, in contrast to the inhomogeneously red-brownish colored Pluto, Charon appears in a uniformly neutral gray.

The surface temperature was determined to be −220 ° C on July 11, 2005 when the stars were eclipsed by Charon  . This corresponds to the radiation equilibrium to be expected at this distance . The pressure of a possibly extremely thin atmosphere can be at most 0.011  Pa .

The Gemini Observatory announced on July 17, 2007 that it had discovered cryovolcanoes on Charon , which bring a mixture of crystalline water ice and ammonium hydroxide to the surface, which then deposits globally. That the ice is still present in crystalline form, points to recent deposits, since the ultraviolet rays of the sun and cosmic rays , the ice within about 30,000 years ago in an amorphous state have weathered would have.

What is striking is the almost complete absence of any major impact craters in the images of the New Horizons space probe. This speaks for a young surface, which until recently was reshaped by geological or other processes, whereby the older impact craters usually present were covered or eroded, especially in the southern hemisphere.

Charon's reddish polar region is particularly striking. It was given the informal name Mordor Macula, after the land of Mordor in JRR Tolkien's novel The Lord of the Rings . Methane that has escaped from Pluto's atmosphere is deposited there during the more than 100-year-old polar night, which, before it can evaporate again, is gradually converted into more stable, red-brown tholines by high-energy radiation until the summer . According to this explanation, analyzes of the Pluto light reflected from Charon's polar night side indicate that there is also a dark discoloration in the South Pole region.

For the nomenclature of the formations on Charon, the IAU has defined the areas of origin of possible namesake on goals and milestones of fictional explorations as well as fictional and mythological vehicles, travelers and researchers. On April 12, 2018, the first twelve names were officially confirmed.

exploration

The first images that showed Pluto and Charon as two separate bodies came in the 1990s with the Hubble space telescope . Later this became possible through the use of adaptive optics also with earth-based telescopes.

Stages of the passage of the Pluto system through New Horizons

The New Horizons space probe, launched on January 19, 2006, flew past Pluto at a distance of 12,500 km on July 14, 2015 and a few minutes later passed its orbit point closest to the more distant Charon at a minimum distance of 28,800 km. Its further trajectory led the probe through the shadows of the two celestial bodies, so that both an eclipse by Pluto and one by Charon could be observed with it.

Charon in fiction

In the video game series Mass Effect , Charon turns out to be a portal for interstellar space travel .

See also

• List of moons of planets and dwarf planets
• List of moons from asteroids
• Transneptunian object

literature

• Alan Stern, Jacqueline Mitton: Pluto and Charon. Ice Worlds on the Ragged Edge of the Solar Syste. 2nd edition, revised and updated. Wiley-VCH, Weinheim 2005, ISBN 3-527-40556-9 .

Web links

Comparison of some large trans-Neptunian objects with the earth (mostly fantasy drawings. Caption as of June 2015) . To get to the corresponding article, click on the object ( large display ).

Commons : Charon  - collection of images, videos and audio files

• Wm. Robert Johnston: (134340) Pluto, Charon, Nix, and Hydra
• Charon at Monde.de
• Cool companion in the realm of shadows ( Memento from April 19, 2005 in the Internet Archive ), Wiener Zeitung , June 13, 2003 (25th anniversary of discovery)
• Marc W. Buie, Lowell Observatory saw the phases of Charon of Pluto from (English)
• Marc W. Buie, Lowell Observatory: The Surfaces of Pluto and Charon (English)

Individual evidence

1. ↑ [ˈçaːrɔn] in Helmut Boor (ed.): Theodor Siebs - Deutsche Hochsprache (18th edition), de Gruyter, Berlin 1966, p. 263 .
2. ↑ [ˈçaːrɔn] in Max Mangold (edit.): Duden. The pronunciation dictionary (6th edition), Dudenverlag, Mannheim 2005, ISBN 3-411-04066-1 , p. 238.
3. ↑ [çˈaːʁɔn] in Eva-Maria Krech , Eberhard Stock , Ursula Hirschfeld , Lutz-Christian Anders: German pronunciation dictionary , de Gruyter, Berlin 2009, ISBN 978-3-11-018202-6 , p. 408 .
4. ↑ "Kahron" in Carl Venator: The common foreign words in our language (3rd edition), L. Pabst, Darmstadt 1838, p.79 .
5. ^ "Karon" in Pierer's Universal-Lexikon Volume 3, Altenburg 1857, p. 874 .
6. ↑ IAU.org: Pluto and the Developing Landscape of Our Solar System . Quote: For now, Charon is considered just to be Pluto's satellite. The idea that Charon might qualify to be called a dwarf planet in its own right may be considered later.
7. ↑ James W. Christy, Robert S. Harrington: The satellite of Pluto . In: Astronomical Journal . Vol. 83, 1978, pp. 1005-1008, doi: 10.1086 / 112284 , online
8. ↑ IAUC 3241: 1978 P 1 July 7, 1978 (discovery).
9. ↑ IAUC 4157: Satellites of Saturn and Pluto January 3, 1986 (designation).
10. ↑ Example of a private website with a collection of symbol designs : Denis Moskowitz: Astronomical / Astrological symbols for other planets' moons. April 13, 2014, accessed May 19, 2015 . 
11. ↑ Marc W. Buie, William M. Grundy: Orbits and photometry of pluto's satellites: charon, S / 2005 P1, and S / 2005 P2.
12. ^ The Pluto system: Initial results from its exploration by New Horizons. Table 1, arxiv : 1510.07704v1
13. ↑ MJ Person, JL Elliot, AAS Gulbis, JM Pasachoff, BA Babcock, SP Souza, J. Gangestad: Charon's Radius and Density from the Combined Data Sets of the 2005 July 11 Occultation. In: Astronomical Journal . 03/2006; 132 (4), pp. 1575-1580, ISSN  1538-3881 , doi: 10.1086 / 507330 , arxiv.org .
14. ↑ Hans-Arthur Marsiske: The shadow of Charon. In: Telepolis January 8, 2006 (size of Charon accurate to a few kilometers).
15. ↑ Bruno Sicardy et al. a .: Charon's size and an upper limit on its atmosphere from a stellar occultation . In: Nature . Vol. 439, 2006, pp. 52-54, doi: 10.1038 / nature04351 .
16. ↑ Measuring the Size of a Small, Frost World In: ESO press release January 4, 2006 (English).
17. ↑ Charon: An Ice Machine in the Ultimate Deep Freeze In: Gemini Observatory . July 12, 2007 (cryovolcanism).
18. ↑ Rainer Kayser: Why Charon wears a red cap. ( Memento of the original from February 7, 2018 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. 20th September 2016. 
19. ↑ WM Grundy et al., The formation of Charon's red poles from seasonally cold-trapped volatiles , Nature 2016 doi : 10.1038 / nature19340
20. ↑ Names on Pluto Moon. From Spock Crater to Mordor and back. In: Spiegel Online , July 29, 2015
21. ^ IAU: Naming of Astronomical Objects . Last accessed on October 15, 2016
22. ↑ USGS , Astrogeology Science Center: First Names Approved for Charon.
23. ↑ Charon in the Gazetteer of Planetary Nomenclature of the IAU (WGPSN) / USGS