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Janus (moon)

Janus (moon)

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Janus is an inner satellite
Inner satellite
In astronomy, an inner moon is a natural satellite following a prograde, low inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet...

 of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

. It is also known as Saturn X (Roman numeral X = 10). It is named after the mythological Janus
Janus (mythology)
In ancient Roman religion and mythology, Janus is the god of beginnings and transitions, thence also of gates, doors, doorways, endings and time. He is usually a two-faced god since he looks to the future and the past...

.

Discovery and orbit

The following is a summary. For more detailed information about Janus and Epimetheus' unusual shared orbit, see Epimetheus
Epimetheus (moon)
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...

.


Janus occupies practically the same orbit as the moon Epimetheus
Epimetheus (moon)
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...

. This caused some confusion for astronomers, who assumed that there was only one body in that orbit, and for a long time struggled to figure out what was going on. It was eventually realized that they were trying to reconcile observations of two distinct objects as a single object.

The discovery of Janus is attributed to its first observer: Audouin Dollfus
Audouin Dollfus
Audouin Charles Dollfus was a French astronomer and aeronaut, specialist in studies of the solar system and discoverer of Janus, a moon of Saturn.-Astronomical Career and Research:...

, on December 15, 1966. The new object was given the temporary designation . Previously, Jean Texereau had photographed Janus on October 29, 1966 without realising it; Dollfus named it at the same occasion. On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus
Epimetheus (moon)
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...

.

Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

 confirmed this in 1980. (See Co-orbital moon
Co-orbital moon
In astronomy, a co-orbital configuration refers to two or more celestial objects that orbit at the same, or very similar, distance from their parent object as each other, i.e. they are in a 1:1 mean motion resonance....

 for a more detailed description of their unique arrangement.)

Janus was observed on subsequent occasions and given different provisional designations
Provisional designation in astronomy
Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calculated...

. It was observed by the Pioneer 11
Pioneer 11
Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...

probe when it passed near Saturn on September 1, 1979: three energetic particle detectors observed its "shadow" (.) Janus was observed by Dan Pascu on February 19, 1980 (,) and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on the 23rd (.)

All of these people thus share, to various degrees, the title of discoverer of Janus.

Name


Janus is named after Janus
Janus (mythology)
In ancient Roman religion and mythology, Janus is the god of beginnings and transitions, thence also of gates, doors, doorways, endings and time. He is usually a two-faced god since he looks to the future and the past...

, the two-faced Roman
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...

 god. Although the name was informally proposed soon after the initial 1966 discovery, it was not officially given this name until 1983. Epimetheus
Epimetheus (moon)
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...

 received its name at the same time.

According to the OED
Oxford English Dictionary
The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

, the adjectival form
Adjective
In grammar, an adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified....

 of the moon's name is Janian.

Physical characteristics


Janus is extensively cratered with several craters larger than 30 km but few linear features. The Janian surface appears to be older than Prometheus
Prometheus (moon)
-Animations:-External links:* at ** anaglyph view of Prometheus...

' but younger than Pandora
Pandora (moon)
Pandora is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated '. In late 1985 it was officially named after Pandora from Greek mythology. It is also designated as '.Pandora is the outer shepherd satellite of the F Ring...

's. From its very low density and relatively high albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, it seems likely that Janus is a very porous
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

 and icy rubble pile
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...

. The moon is also highly non-spherical.

Ring


A faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006. The ring has a radial extent of about 5000 km. Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.

See also


External links