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Epimetheus (moon)
Encyclopedia
Epimetheus icon is an inner satellite
of Saturn
. It is also known as Saturn XI. It is named after the mythological Epimetheus
, brother of Prometheus
.
Epimetheus occupies essentially the same orbit as the moon Janus
. Astronomers assumed that there was only one body in that orbit, and accordingly had difficulty determining their orbital characteristics. Observations were photographic and spaced widely apart in time, so that while the presence of two objects was not obvious, the observations were difficult to reconcile with a reasonable orbit.
Audouin Dollfus
observed a moon on December 15, 1966, which he proposed to be named "Janus". On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus. However, at the time, it was believed that there was only one moon, unofficially known as "Janus", in the given orbit.
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. This was confirmed in 1980 by Voyager 1
, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.
Epimetheus received its name in 1983. The name Janus was approved by the IAU
at the same time, although the name had been used informally since Dollfus proposed it shortly after the 1966 discovery.
Epimetheus and Janus
are co-orbital
: Janus's mean orbital radius from Saturn is as of 2006 (as shown by green color in the picture) only 50 km less than that of Epimetheus, a distance smaller than either moon's diameter. In accordance with Kepler's laws of planetary motion
, the closer orbit
is completed more quickly, but only by about 30 seconds. Each day the inner moon is an additional ¼° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction boosts the inner moon's momentum and saps the outer moon's momentum. With this added momentum, the inner moon's distance from Saturn and orbital period
are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons "trade" orbits, never approaching closer than about 10,000 km. The exchange takes place about once every four years; the last close approach occurred on January 21, 2006, the next was in 2010. At that time, Janus's orbital radius will increase by ~20 km, while Epimetheus's decreases by ~80 km; Janus's orbit is less affected because it is 4 times more massive than Epimetheus. As far as it is currently known, this arrangement is unique in the Solar System
.
The orbital relationship between Janus and Epimetheus can be understood in terms of the circular restricted three-body problem
, as a case in which the two moons (the third body being Saturn) are similar in size to each other.
, it seems likely that Epimetheus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.
The south pole shows what might be the remains of a large impact crater covering most of this face of the moon, and which could be responsible for the somewhat flattened shape of the southern part of Epimetheus.
There appear to be two terrain types: darker, smoother areas, and brighter, slightly more yellowish, fractured terrain. One interpretation is that the darker material evidently moves down slopes, and probably has a lower ice content than the brighter material, which appears more like "bedrock." Nonetheless, materials in both terrains are likely to be rich in water ice.
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 XI. It is named after the mythological Epimetheus
Epimetheus (mythology)
In Greek mythology, Epimetheus was the brother of Prometheus , a pair of Titans who "acted as representatives of mankind" . They were the inseparable sons of Iapetus, who in other contexts was the father of Atlas...
, brother of Prometheus
Prometheus
In Greek mythology, Prometheus is a Titan, the son of Iapetus and Themis, and brother to Atlas, Epimetheus and Menoetius. He was a champion of mankind, known for his wily intelligence, who stole fire from Zeus and gave it to mortals...
.
Discovery
Janus (moon)
Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus...
. Astronomers assumed that there was only one body in that orbit, and accordingly had difficulty determining their orbital characteristics. Observations were photographic and spaced widely apart in time, so that while the presence of two objects was not obvious, the observations were difficult to reconcile with a reasonable orbit.
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:...
observed a moon on December 15, 1966, which he proposed to be named "Janus". On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus. However, at the time, it was believed that there was only one moon, unofficially known as "Janus", in the given orbit.
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. This was confirmed in 1980 by 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...
, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.
Epimetheus received its name in 1983. The name Janus was approved by the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
at the same time, although the name had been used informally since Dollfus proposed it shortly after the 1966 discovery.
Orbital relationship between Epimetheus and Janus
Janus (moon)
Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus...
are co-orbital
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....
: Janus's mean orbital radius from Saturn is as of 2006 (as shown by green color in the picture) only 50 km less than that of Epimetheus, a distance smaller than either moon's diameter. In accordance with Kepler's laws of planetary motion
Kepler's laws of planetary motion
In astronomy, Kepler's laws give a description of the motion of planets around the Sun.Kepler's laws are:#The orbit of every planet is an ellipse with the Sun at one of the two foci....
, the closer orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
is completed more quickly, but only by about 30 seconds. Each day the inner moon is an additional ¼° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction boosts the inner moon's momentum and saps the outer moon's momentum. With this added momentum, the inner moon's distance from Saturn and orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons "trade" orbits, never approaching closer than about 10,000 km. The exchange takes place about once every four years; the last close approach occurred on January 21, 2006, the next was in 2010. At that time, Janus's orbital radius will increase by ~20 km, while Epimetheus's decreases by ~80 km; Janus's orbit is less affected because it is 4 times more massive than Epimetheus. As far as it is currently known, this arrangement is unique in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
.
The orbital relationship between Janus and Epimetheus can be understood in terms of the circular restricted three-body problem
Three-body problem
Three-body problem has two distinguishable meanings in physics and classical mechanics:# In its traditional sense the three-body problem is the problem of taking an initial set of data that specifies the positions, masses and velocities of three bodies for some particular point in time and then...
, as a case in which the two moons (the third body being Saturn) are similar in size to each other.
Physical characteristics
There are several Epimethean craters larger than 30 km in diameter, as well as both large and small ridges and grooves. The extensive cratering indicates that Epimetheus must be quite old. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites, but if this is the case the disruption must have happened early in the history of the satellite system. From its very low density and relatively high albedoAlbedo
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 Epimetheus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.
The south pole shows what might be the remains of a large impact crater covering most of this face of the moon, and which could be responsible for the somewhat flattened shape of the southern part of Epimetheus.
There appear to be two terrain types: darker, smoother areas, and brighter, slightly more yellowish, fractured terrain. One interpretation is that the darker material evidently moves down slopes, and probably has a lower ice content than the brighter material, which appears more like "bedrock." Nonetheless, materials in both terrains are likely to be rich in water ice.
Ring
A faint dust ring is present around the region occupied by the orbits of Epimetheus and Janus, 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 are particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.External links
- Epimetheus Profile by NASA's Solar System Exploration
- The Planetary Society: Epimetheus
- Cassini Images of Epimetheus
- 'Solar System Dynamics' by Murray and Dermott The standard text on the subject, describes the orbits in detail.
- Quicktime animation of co-orbital motion from Murray and Dermott
- Cassini image of Janus and Epimetheus near the time of their orbital swap.
- Epimetheus nomenclature from the USGS planetary nomenclature page