Shepherd moon

Source: Wikipedia, the free encyclopedia.
For the album by Enya, see Shepherd Moons.
Prometheus (right) and Pandora (left) both orbit near Saturn's F ring, but only Prometheus is thought to act as a shepherd.
Operation of a shepherd moon– particles are located in front or behind the Moon in its orbit, so these are either accelerated in the direction of the moon and thrown to the outside, or they are slowed on their path and pulled inwards.

A shepherd moon is a small

planetary ring
material or keeps particles within a ring contained. The name is a result of their limiting the "herd" of the ring particles as a shepherd.

Due to their gravitational influence, shepherd moons deflect ring particles from their original orbits due to proximity or through orbital resonances. This can carve gaps in the ring system, such as the Encke Gap maintained by Saturn's moon Pan, or lead to the confining of narrow ringlets, such as Saturn's F ring.

Discovery

The existence of shepherd moons was theorized in early 1979.[1] Observations of the rings of Uranus show that they are very thin and well defined, with sharp gaps between rings. To explain this, Goldreich and Tremaine suggested that two small satellites that were undetected at the time might be confining each ring. The first images of shepherd satellites were taken later that year by Voyager 1.[2]

Examples

Jupiter

Several of Jupiter's small innermost moons, namely Metis and Adrastea, are within Jupiter's ring system and are also within Jupiter's Roche limit.[3] It is possible that these rings are composed of material that is being pulled off these two bodies by Jupiter's tidal forces, possibly facilitated by impacts of ring material on their surfaces.

Saturn

The complex ring system of Saturn has several such satellites. These include Prometheus (F ring),[4] Daphnis (Keeler Gap),[5] Pan (Encke Gap),[6] Janus, and Epimetheus (both A ring).[7]

Uranus

tidal deceleration.[9]

Neptune

Neptune's rings are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but Voyager 2's images showed them to be complete rings with bright clumps.[10] It is thought that the gravitational influence of the shepherd moon Galatea and possibly other as-yet undiscovered shepherd moons are responsible for this clumpiness.[11]

Minor planets

Rings around some

Chiron is also thought to have rings similar in form to those of Chariklo.[13]

Exoplanets

A major gap in the large ring system of the V1400 Centauri b object at about 61 million km (0.4 AU) from its center is considered to be indirect evidence of the existence of an exomoon with mass up to 0.8 Earth masses.[14][15]

See also

References

  1. S2CID 4232962
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  2. ^ "Voyager 1".
  3. ISBN 978-1-4020-5233-0
    .
  4. ^ "On the masses and motions of mini-moons: Pandora's not a". www.planetary.org. Retrieved 2016-06-14.
  5. ^ "NASA - Cassini Finds New Saturn Moon That Makes Waves". www.nasa.gov. Retrieved 2016-06-14.
  6. S2CID 4317496
    .
  7. S2CID 51785280
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  8. S2CID 250909885
    .
  9. doi:10.1006/icar.2001.6597
    .
  10. ISBN 978-0-387-34177-4
    .
  11. PMID 9804544
    .
  12. S2CID 4467484
    .
  13. S2CID 38950384
    .
  14. S2CID 56118870
    .
  15. S2CID 119546405
    .

Further reading
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