Astronomy:Shepherd moon

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Short description: Satellite associated with a planetary ring
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, also called a herder moon or watcher moon, is a small natural satellite that clears a gap in 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

Uranus also has shepherd moons on its ε ring, Cordelia and Ophelia. They are interior and exterior shepherds, respectively.[8] Both moons are well within Uranus's synchronous orbit radius, and their orbits are therefore slowly decaying due to 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 centaurs have been identified. Chariklo's rings are remarkably well-defined and are suspected to either be very young or kept in place by a shepherd moon similar in mass to the rings.[12] 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. ↑ Goldreich, Peter; Tremaine, Scott (1979). "Towards a theory for the Uranian rings.". Nature 277 (5692): 97–99. doi:10.1038/277097a0. Bibcode: 1979Natur.277...97G. https://www.nature.com/articles/299209a0.pdf?origin=ppub. 
  2. ↑ "Voyager 1". https://solarsystem.nasa.gov/missions/voyager-1/in-depth/. 
  3. ↑ Faure, Gunter; Mensing, Teresa (2007). Introduction to Planetary Science: The Geological Perspective. Springer. ISBN 978-1-4020-5233-0. 
  4. ↑ "On the masses and motions of mini-moons: Pandora's not a". http://www.planetary.org/blogs/emily-lakdawalla/2014/07010001-ringmoons-shepherds.html. 
  5. ↑ "NASA - Cassini Finds New Saturn Moon That Makes Waves" (in en). http://www.nasa.gov/mission_pages/cassini/media/cassini-051005.html. 
  6. ↑ Showalter, Mark R. (1991-06-27). "Visual detection of 1981S13, Saturn's eighteenth satellite, and its role in the Encke gap" (in en). Nature 351 (6329): 709–713. doi:10.1038/351709a0. Bibcode: 1991Natur.351..709S. 
  7. ↑ Moutamid, Maryame El; Nicholson, Philip D.; French, Richard G.; Tiscareno, Matthew S.; Murray, Carl D.; Evans, Michael W.; French, Colleen McGhee; Hedman, Matthew M. et al. (2015-10-01). "How Janus' Orbital Swap Affects the Edge of Saturn's A Ring?". Icarus 279: 125–140. doi:10.1016/j.icarus.2015.10.025. Bibcode: 2016Icar..279..125E. 
  8. ↑ Esposito, Larry W. (2002-01-01). "Planetary rings" (in en). Reports on Progress in Physics 65 (12): 1741–1783. doi:10.1088/0034-4885/65/12/201. ISSN 0034-4885. Bibcode: 2002RPPh...65.1741E. http://stacks.iop.org/0034-4885/65/i=12/a=201. 
  9. ↑ Karkoschka, Erich (2001-05-01). "Voyager's Eleventh Discovery of a Satellite of Uranus and Photometry and the First Size Measurements of Nine Satellites". Icarus 151 (1): 69–77. doi:10.1006/icar.2001.6597. Bibcode: 2001Icar..151...69K. 
  10. ↑ Miner, Ellis D.; Wessen, Randii R.; Cuzzi, Jeffrey N. (2007). "Present knowledge of the Neptune ring system". Planetary Ring System. Springer Praxis Books. ISBN 978-0-387-34177-4. https://archive.org/details/planetaryringsys0000mine. 
  11. ↑ Salo, Heikki; Hanninen, Jyrki (1998). "Neptune's Partial Rings: Action of Galatea on Self-Gravitating Arc Particles". Science 282 (5391): 1102–1104. doi:10.1126/science.282.5391.1102. PMID 9804544. Bibcode: 1998Sci...282.1102S. 
  12. ↑ Braga-Ribas, F.; Sicardy, B.; Ortiz, J. L.; Snodgrass, C.; Roques, F.; Vieira-Martins, R.; Camargo, J. I. B.; Assafin, M. et al. (April 2014). "A ring system detected around the Centaur (10199) Chariklo". Nature 508 (7494): 72–75. doi:10.1038/nature13155. PMID 24670644. Bibcode: 2014Natur.508...72B. 
  13. ↑ Ortiz, J. L.; Duffard, R.; Pinilla-Alonso, N.; Alvarez-Candal, A.; Santos-Sanz, P.; Morales, N.; Fernández-Valenzuela, E.; Licandro, J. et al. (2015). "Possible ring material around centaur (2060) Chiron". Astronomy & Astrophysics 576: A18. doi:10.1051/0004-6361/201424461. ISSN 0004-6361. Bibcode: 2015A&A...576A..18O. 
  14. ↑ Kenworthy, Matthew A.; Mamajek, Eric E. (January 22, 2015). "Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons?". The Astrophysical Journal 800 (2): 126. doi:10.1088/0004-637X/800/2/126. Bibcode: 2015ApJ...800..126K. 
  15. ↑ Sutton, P. J. (2019). "Mean motion resonances with nearby moons: an unlikely origin for the gaps observed in the ring around the exoplanet J1407b". Monthly Notices of the Royal Astronomical Society 486 (2): 1681–1689. doi:10.1093/mnras/stz563. Bibcode: 2019MNRAS.486.1681S. 

Further reading

  • Arnold Hanslmeier: Einführung in Astronomie und Astrophysik. Spektrum, Berlin/Heidelberg 2007, ISBN:978-3-8274-1846-3.



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