Epimetheus (moon)
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As imaged by Cassini on December 3, 2007
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Discovery
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| Discovered by | Richard Walker |
| Discovery date | December 18, 1966 |
| Epoch 31 December 2003 (JD 2453005.5) | |
| Semi-major axis | 151,410 ± 10 km |
| Eccentricity | 0.0098 |
| Orbital period | 0.694333517 d |
| Inclination | 0.351 ± 0.004° to Saturn's equator |
| Satellite of | Saturn |
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Physical characteristics
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| Dimensions | 135 × 108 × 105 km[2] |
| Mean radius | 57 ± 3 km[2] |
| Surface area | ~40,000 km² |
| Volume | ~760,000 km³ |
| Mass | 5.304 ± 0.013 ×1017 kg[1] |
| Mean density | 0.69 ± 0.11 g/cm³ |
| Equatorial surface gravity | ~0.0078 m/s2 |
| Escape velocity | ~0.032 km/s |
| Rotation period | synchronous |
| Axial tilt | zero |
| Albedo | 0.73 ± 0.03 (geometric)[3] |
| Temperature | ~78 K |
Epimetheus (pronounced /ˌɛpɨˈmiːθiəs/ EP-i-MEE-thee-əs, also /-θiuːs/ -thee-oos, or as Greek Επιμηθεύς) is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus.
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[edit] Discovery
Epimetheus occupies essentially the same orbit as the moon Janus. Astronomers assumed that there was only one body in that orbit, and accordingly had a hard time figuring out 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,[4] which he proposed to be named "Janus".[5] On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus.[6] 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.[7] This was confirmed in 1980 by Voyager 1[citation needed], and so Larson and Fountain officially share the discovery of Epimetheus with Walker.
Epimetheus received its name in 1983.[8] The name Janus was officially approved by the IAU at the same time, although the name had been used informally since Dollfus proposed it a few days after the 1966 discovery.
[edit] Orbital relationship between Epimetheus and Janus
Epimetheus and Janus are co-orbital: Janus' orbital radius from Saturn is currently only 50 km less than that of Epimetheus. Since closer orbits have higher velocities, the two moons must inevitably approach each other, and since Epimetheus' mean diameter is 115 km and Janus' is 178 km it would seem at first glance that a collision were inevitable. But as the inner moon catches up with the outer moon their gravitational attraction boosts the inner moon's momentum and raises its orbit, while the outer moon equally loses momentum and drops into a lower orbit. The moons thus "trade" orbits and begin to move apart again, since the forward moon is now in the lower, faster orbit. The nearest they ever approach is some 10,000 km. The exchange takes place about once every four years; the last close approach was on 21 January 2006,[9] the next will be in 2010. At that time, Janus' orbital radius will increase by ~20 km, while Epimetheus' decreases by ~80 km; Janus' 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. Other examples of the three-body problem include the Lagrangian points, Trojan asteroids and Trojan moons, the "horseshoe" orbit of Cruithne with respect to Earth, and potentially dozens of other objects in similar orbits.[10]
[edit] 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 albedo, 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.[11]
[edit] 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 5,000 km.[12] Its source are particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.[13]
[edit] See also
[edit] References
- ^ a b Spitale, J. N.; et al. (2006). "The orbits of Saturn's small satellites derived from combined historic and Cassini imaging observations". The Astronomical Journal 132: 692. doi:.
- ^ a b Porco, C. C.; et al. (2006). "Physical Characteristics and Possible Accretionary Origins for Saturn's Small Satellites". Bulletin of the American Astronomical Society 37: 768.
- ^ Verbiscer, A.; French, R.; Showalter, M.; and Helfenstein, P.; Enceladus: Cosmic Graffiti Artist Caught in the Act, Science, Vol. 315, No. 5813 (February 9, 2007), p. 815 (supporting online material, table S1)
- ^ IAUC 1987: Probable New Satellite of Saturn 1967 January 3 (discovery)
- ^ IAUC 1995: Saturn X (Janus) 1967 February 1 (naming Janus)
- ^ IAUC 1991: Possible New Satellite of Saturn 1967 January 6
- ^ Fountain, J. W.; and Larson, S. M.; Saturn's ring and nearby faint satellites, Icarus, Vol. 36 (October 1978), pp. 92–106
- ^ Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (confirms Janus, names Epimetheus, Telesto, Calypso) (mentioned in IAUC 3872: Satellites of Jupiter and Saturn 1983 September 30)
- ^ NASA JPL, Cassini-Huygens Multimedia: The Dancing Moons, May 3, 2006
- ^ Lloyd, R.; More Moons Around Earth? Its Not So Loony, Space.com, 29 October 1999
- ^ Catalog Page for PIA09813
- ^ NASA Planetary Photojournal PIA08328: Moon-Made Rings
- ^ Cassini-Huygens press release NASA Finds Saturn's Moons May Be Creating New Rings, 11 October 2006
[edit] External links
- Epimetheus Profile by NASA's Solar System Exploration
- The Planetary Society: Epimetheus
- 'Solar System Dynamics' by Murray and Dermott The standard text on the subject, describes the orbits in detail.
- Quicktime illustration of co-orbital motion from Murray and Dermott
- Cassini image of Janus and Epimetheus near the time of their orbital swap.
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