A shepherd moon (also herder moon or watcher moon) is a small
natural satellite
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
that clears a gap in
planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the "herd" of the ring particles as a shepherd.
Due to their
gravitational
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
effect, they pick up particles and deflect them from their original orbits through
orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
. This causes gaps in the ring system, such as the particularly striking
Cassini Division
The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirel ...
, as well as other characteristic bands, or strange "twisted" deformation of rings.
Discovery
The existence of shepherd moons was theorized in early 1979. 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.
Examples
Jupiter
Several of
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
's small innermost moons, namely
Metis
Metis or Métis may refer to:
Ethnic groups
* Métis, recognized Indigenous communities in Canada and America whose distinct culture and language emerged after early intermarriage between First Nations peoples and early European settlers, prima ...
and
Adrastea, are within Jupiter's
ring system
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as ...
and are also within Jupiter's
Roche limit
In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forc ...
. It is possible that these rings are composed of material that is being pulled off these two bodies by Jupiter's
tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
s, possibly facilitated by impacts of ring material on their surfaces.
Saturn
The complex
ring system of Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
has several such satellites. These include
Prometheus
In Greek mythology, Prometheus (; , , possibly meaning "forethought")Smith"Prometheus". is a Titan god of fire. Prometheus is best known for defying the gods by stealing fire from them and giving it to humanity in the form of technology, know ...
(F ring),
Daphnis
In Greek mythology, Daphnis (; grc, Δάφνις, from , ''daphne'', "Bay Laurel") was a Sicilian shepherd who was said to be the inventor of pastoral poetry.
Family
According to tradition, he was the son of Hermes and a nymph, despite which ...
(Keeler Gap),
Pan (Encke Gap),
Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
, and
Epimetheus
In Greek mythology, Epimetheus (; grc-gre, Ἐπιμηθεύς, , afterthought) was the brother of Prometheus (traditionally interpreted as "foresight", literally "fore-thinker"), a pair of Titans who "acted as representatives of mankind". The ...
(both A ring).
Uranus
Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
also has shepherd moons on its
ε ring,
Cordelia
Cordelia is a feminine given name. It was borne by the tragic heroine of Shakespeare's ''King Lear'' (1606), a character based on the legendary queen Cordelia. The name is of uncertain origin. It is popularly associated with Latin '' cor'' ( geni ...
and
Ophelia
Ophelia () is a character in William Shakespeare's drama ''Hamlet'' (1599–1601). She is a young noblewoman of Denmark, the daughter of Polonius, sister of Laertes and potential wife of Prince Hamlet, who, due to Hamlet's actions, ends up in ...
. They are interior and exterior shepherds, respectively. Both moons are well within Uranus's synchronous orbit radius, and their orbits are therefore slowly decaying due to
tidal deceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from ...
.
Neptune
Neptune's rings
Ring may refer to:
* Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry
* To make a sound with a bell, and the sound made by a bell
:(hence) to initiate a telephone connection
Arts, entertainment and media Film and ...
are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but
Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
's images showed them to be complete rings with bright clumps. It is thought
that the gravitational influence of the shepherd moon
Galatea
Galatea is an ancient Greek name meaning "she who is milk-white".
Galatea, Galathea or Gallathea may refer to:
In mythology
* Galatea (Greek myth), three different mythological figures
In the arts
* ''Aci, Galatea e Polifemo'', cantata by H ...
and possibly other as-yet undiscovered shepherd moons are responsible for this clumpiness.
Minor planets
Rings around some
centaurs
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as being ...
have been identified.
Chariklo's rings
Ring may refer to:
* Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry
* To make a sound with a bell, and the sound made by a bell
:(hence) to initiate a telephone connection
Arts, entertainment and media Film and ...
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.
Chiron
In Greek mythology, Chiron ( ; also Cheiron or Kheiron; ) was held to be the superlative centaur amongst his brethren since he was called the "wisest and justest of all the centaurs".
Biography
Chiron was notable throughout Greek mythology ...
is also thought to have rings similar in form to those of Chariklo.
See also
*
Kirkwood gap
A Kirkwood gap is a gap or dip in the distribution of the semi-major axes (or equivalently of the orbital periods) of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter.
For example, there ar ...
*
Subsatellite (a moon of a moon)
References
Further reading
* Arnold Hanslmeier: ''Einführung in Astronomie und Astrophysik''. Spektrum, Berlin/Heidelberg 2007, {{ISBN, 978-3-8274-1846-3.
Celestial mechanics