Internal satellite

Currently, there are about thirty known internal satellites orbiting all four giant planets (Jupiter, Saturn, Uranus and Neptune). Due to the small size and brilliance of the planet, internal satellites are difficult to observe from Earth. Some of the internal satellites, such as Pan and Daphnis at Saturn, were observed only by spacecraft.

The first internal satellite observed was Amalthea , discovered by Edward Barnard in 1892. The next were Saturn's satellites Epimetheus and Janus , observed in 1966. These two satellites are in the same orbit, which led to confusion that was resolved only after the flight of Voyager 1 in 1980. Most of the remaining known internal satellites were discovered by the Voyager-1 and Voyager-2 devices during their flights past Jupiter (1979), Saturn (1980), Uranus (1986) and Neptune (1989).

All internal satellites are in almost circular prograde orbits. The median eccentricity is 0.0012; the satellite with the highest eccentricity of the orbit is Phoebe , e = 0.0177. The inclination of the orbit to the plane of the equator of the planet is also very small. For all known internal satellites, except for one, the slope is less than a degree, the median value is 0.1 °. Naiad, the closest satellite to Neptune, is an exception: its orbit is inclined at an angle of 4.75 ° to the plane of the equator of Neptune.

The most internal satellites circulate in the region of planetary rings, inside the Roche limit for liquid bodies, and only internal forces and friction prevent the destruction of satellites under the action of tidal forces. This means that if you put a stone on the surface of the satellite at a point farthest from the planet, then the tidal force will be greater than the attractive force of the stone by the satellite, as a result of which the stone will fly away from the surface. For this reason, the surface images of such satellites show that they are free of dust and stones on the surface.

The most extreme cases are the Pan satellites, orbiting inside the rings within 70% of the Roche limit, as well as Naiad. The density of Naiad is unknown, therefore, the exact Roche limit for it is also unknown, but if we assume a density of 1100 kg / m³, then Naiad is even smaller relative to the Roche limit from the planet.

Those of the satellites that have periods of revolution less than the period of rotation of the planet experience tidal deceleration, leading to a gradual approach to the planet in a spiral. In the distant future, such satellites will fall on the planet or be destroyed by tidal forces. Examples of such satellites are Metida and Adrastea at Jupiter, as well as most of the known internal satellites of Uranus and Neptune - to (including) Perdita and Larissa, respectively. However, not one of Saturn’s moons possesses this property, since Saturn itself rotates rapidly.

Dimensions

Internal satellites are small compared to large planet moons. All known internal satellites are too small to acquire and maintain a spheroidal shape. Many of them are very elongated, such as Amalthea, whose length is twice the width. The largest internal satellite is Proteus , reaching a size of 440 km and close in shape to a ball, but still not large enough to consider a form acquired only under the influence of gravitational compression. Most of the known internal satellites have sizes from 50 to 100 km, the smallest is Daphnis with sizes from 6 to 8 km. Unconfirmed objects that approach close to Saturn’s ring F , such as S / 2004 S 6 , can be even smaller satellites, unless they are temporary dust clumps. The Cassini-Huygens spacecraft found evidence (small dust rings) that even smaller satellites could circulate in the Cassini gap . ^[2] The sizes of the smallest known internal satellites increase with the distance of the planets from the Sun, but this dependence is likely due to the difficulty in detecting small satellites from distant planets.

Rotation

Internal satellites are in tidal capture , so their orbital period is synchronized with the rotation period, so they are always turned with one side to the planet. The large axes of the satellite figures are usually directed toward the planet.

Surfaces

All internal moons of Jupiter, Uranus and Neptune have a very dark surface with an albedo from 0.06 ( Metis ) to 0.10 ( Adrastea ). Saturn's satellites, in contrast, have a bright surface with an albedo of 0.4 to 0.6. It is believed that the surfaces of these satellites are covered with particles of ice that has arisen in the ring system where the satellites circulate. The internal satellites of other planets could darken due to the effects of space weathering . None of the known internal satellites has an atmosphere.

Craters

The internal satellites whose images were obtained have surfaces with a large number of craters. The rate of formation of craters at objects orbiting near a giant planet is higher than for main and external satellites, since there is a phenomenon of gravitational focusing: objects orbiting the Sun when passing in the vicinity of gas giants deviate toward the planet due to its attraction, therefore, the probability of collisions in the case of The satellite’s position is close to the planet much higher than for external satellites and interplanetary space. As a result, it is estimated that very small satellites near planets should collapse upon collisions on time scales substantially less than the age of the solar system. Such estimates provide the minimum value of satellite sizes that can be preserved.

The accumulation of substance rings

At least two of Saturn’s internal moons (Atlas and Prometheus) have equatorial ranges. The ridge on the Atlas is especially pronounced. Also, Pandora is covered with a substance that probably fell on her. It is assumed that the reason is the accretion of the substance of the rings to the satellites. Other evidence for this phenomenon may be low satellite density and high albedo. It was observed that Prometheus attracts the substance of the ring F during periodic close passages.

Jupiter's internal moons

Jupiter has the smallest set of internal satellites, including only four of the following:

• Methida
• Adrastea
• Amalthea
• Phoebe

Saturn's internal satellites

Saturn’s seven internal moons are closely related to its ring system, many of which circulate within the rings, creating gaps or small rings between the moons.

• Pan - Shepherd’s satellite in Enck’s slit of Saturn’s ring A
• Daphnis - Shepherd’s companion in the Keeler Slit of Saturn’s Ring A
• Atlas — Shepherd’s satellite at the outer edge of Saturn’s ring A
• Prometheus and Pandora - the internal and external companions-shepherds of the ring F of Saturn
• Epimetheus and Janus - co-orbiting satellites periodically exchanging energy

Some objects whose position is known, such as S / 2004 S 3 , S / 2004 S 4 and S / 2004 S 6 , sometimes surrounded by a dust halo, were observed in the vicinity of ring F, but it is currently unclear whether these satellites are transient clots or they contain solid objects (satellites).

Uranus Internal Satellites

Uranus has a large system of internal satellites, currently containing 13 satellites:

• Cordelia and Ophelia - internal and external companions-shepherds of the brightest ring of Uranus
• Bianca
• Cressida
• Desdemona
• Juliet
• A portion
• Rosalind
• Cupid
• Belinda
• Fart
• Pack
• Mab - connected to the ring μ.

Neptune's internal satellites

Neptune has seven internal satellites:

• Naiad
• Thalassa
• Despina
• Galatea is an internal satellite of the very narrow Adams ring, broken into arcs
• Larissa
• Hippocampus
• Proteus

It is believed that these satellites are << heaps of rubble >>, reassembled from fragments of the original satellites of Neptune. These satellites were heavily influenced by Triton shortly after the satellite was captured into the initial orbit with a large eccentricity. Disturbances led to collisions of satellites, and that part of the fragments that was not lost, reassembled into the currently observed satellites after the orbit of Triton became circular. ^[one]

For most of the internal satellites, Voyager 1 and Voyager 2 images were received. Most internal satellites appear in the images as separate pixels or have a diameter of only a few pixels. However, some satellites show surface details: