Satellite associated with a planetary ring
A
shepherd 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
[
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]
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
[
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]
Jupiter
[
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]
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
[
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]
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
[
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]
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
[
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]
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
[
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]
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
[
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]
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
[
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]
References
[
edit
]
- ^
Goldreich, Peter; Tremaine, Scott (1979).
"Towards a theory for the Uranian rings"
(PDF)
.
Nature
.
277
(5692): 97?99.
Bibcode
:
1979Natur.277...97G
.
doi
:
10.1038/277097a0
.
S2CID
4232962
.
- ^
"Voyager 1"
.
- ^
Faure, Gunter; Mensing, Teresa (2007).
Introduction to Planetary Science: The Geological Perspective
. Springer.
ISBN
978-1-4020-5233-0
.
- ^
"On the masses and motions of mini-moons: Pandora's not a"
.
www.planetary.org
. Retrieved
2016-06-14
.
- ^
"NASA - Cassini Finds New Saturn Moon That Makes Waves"
.
www.nasa.gov
. Retrieved
2016-06-14
.
- ^
Showalter, Mark R. (1991-06-27). "Visual detection of 1981S13, Saturn's eighteenth satellite, and its role in the Encke gap".
Nature
.
351
(6329): 709?713.
Bibcode
:
1991Natur.351..709S
.
doi
:
10.1038/351709a0
.
S2CID
4317496
.
- ^
Moutamid, Maryame El; Nicholson, Philip D.; French, Richard G.; Tiscareno, Matthew S.;
Murray, Carl D.
; Evans, Michael W.; French, Colleen McGhee; Hedman, Matthew M.; Burns, Joseph A. (2015-10-01). "How Janus' Orbital Swap Affects the Edge of Saturn's A Ring?".
Icarus
.
279
: 125?140.
arXiv
:
1510.00434
.
Bibcode
:
2016Icar..279..125E
.
doi
:
10.1016/j.icarus.2015.10.025
.
S2CID
51785280
.
- ^
Esposito, Larry W. (2002-01-01).
"Planetary rings"
.
Reports on Progress in Physics
.
65
(12): 1741?1783.
Bibcode
:
2002RPPh...65.1741E
.
doi
:
10.1088/0034-4885/65/12/201
.
ISSN
0034-4885
.
S2CID
250909885
.
- ^
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.
Bibcode
:
2001Icar..151...69K
.
doi
:
10.1006/icar.2001.6597
.
- ^
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
.
- ^
Salo, Heikki; Hanninen, Jyrki (1998). "Neptune's Partial Rings: Action of Galatea on Self-Gravitating Arc Particles".
Science
.
282
(5391): 1102?1104.
Bibcode
:
1998Sci...282.1102S
.
doi
:
10.1126/science.282.5391.1102
.
PMID
9804544
.
- ^
Braga-Ribas, F.; Sicardy, B.; Ortiz, J. L.; Snodgrass, C.; Roques, F.; Vieira-Martins, R.; Camargo, J. I. B.; Assafin, M.; Duffard, R. (April 2014). "A ring system detected around the Centaur (10199) Chariklo".
Nature
.
508
(7494): 72?75.
arXiv
:
1409.7259
.
Bibcode
:
2014Natur.508...72B
.
doi
:
10.1038/nature13155
.
PMID
24670644
.
S2CID
4467484
.
- ^
Ortiz, J. L.; Duffard, R.; Pinilla-Alonso, N.; Alvarez-Candal, A.; Santos-Sanz, P.; Morales, N.; Fernandez-Valenzuela, E.; Licandro, J.; Bagatin, A. Campo (2015). "Possible ring material around centaur (2060) Chiron".
Astronomy & Astrophysics
.
576
: A18.
arXiv
:
1501.05911
.
Bibcode
:
2015A&A...576A..18O
.
doi
:
10.1051/0004-6361/201424461
.
ISSN
0004-6361
.
S2CID
38950384
.
- ^
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.
arXiv
:
1501.05652
.
Bibcode
:
2015ApJ...800..126K
.
doi
:
10.1088/0004-637X/800/2/126
.
S2CID
56118870
.
- ^
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.
arXiv
:
1902.09285
.
Bibcode
:
2019MNRAS.486.1681S
.
doi
:
10.1093/mnras/stz563
.
S2CID
119546405
.
Further reading
[
edit
]
- Arnold Hanslmeier:
Einfuhrung in Astronomie und Astrophysik
. Spektrum, Berlin/Heidelberg 2007,
ISBN
978-3-8274-1846-3
.