Irregular satellite

Saturn's irregular moons, including Titan in red

Irregular satellites , also known as irregular moons , are moons whose orbits have greater inclinations , eccentricities and distances from their planets than regular satellites. Today there are over 100 known irregular moons in the solar system .

Discoveries

Before 1997, when the irregular Uranus moons Caliban and Sycorax were discovered, only eleven irregular moons including Triton were known. Around the turn of the millennium, a rapid series of discoveries began due to the higher light sensitivity of the emerging large-area CCD sensors .

definition

The moons of the planets of the solar system show a natural division in terms of their orbital properties :

Regular moons orbit their planet on narrow, almost circular orbits (eccentricities of about 0.01) with low inclinations of a few degrees. They basically orbit their planet in a prograd manner , i.e. in the same direction in which the planet orbits the sun .

irregular moons, on the other hand, orbit much wider with large eccentricities (0.1 to 0.7) and apparently randomly distributed inclinations of up to 180 °.

In order to be able to compare the distances of the orbits from your planet across planets, the distances are given in multiples of the Hill radius r _H ; this is the distance up to which the gravitational force of the planet dominates that of the sun:

regular satellites are found on orbits with large semiaxes below 0.05 r _H ,
irregular satellites have distances up to about 0.5 r _H , which corresponds roughly to the maximum possible distance a moon can have.

Burns, for example, defined an alternative, more quantitative delimitation in 1986 using the precession of the orbits:

according to this, a moon is an irregular moon if the precession of its orbit is mainly caused by the sun and no longer by the planet.

Due to the strong dichotomy of the lunar orbitals, the above differ. Definitions in the classification of known moons are usually not. The only exception here is Neptune's moon Triton , which has a special role (see below).

properties

Orbits of the irregular moons. Above: inclinations depending on the major semi-axis. Below: Eccentricities depending on the major semi-axis

Irregular moons have been found around all four giant planets Jupiter , Saturn , Uranus and Neptune .

If you look at the orbital properties of the irregular moons as shown in the adjacent figure, you can see some of their properties:

The eccentricities, inclinations and distances normalized to r _H are widely spread over almost the entire range of possible values. However, there are groups of irregular moons that have extremely similar orbits. It is believed that these were caused by collisions between two irregular moons. This assumption is supported by the fact that the colors of moons whose orbits form a group are very similar to one another.
The distances normalized to the Hill sphere decrease from planet to planet with increasing distance from the sun. The reason for this is still unknown, a possible explanation could be collisions of irregular planets.
Prograde irregular moons exist only up to a distance of about 0.35 r _H , while retrograde moons are also found further out. This can be explained by the fact that the stability limit of the orbits - i.e. the distance up to which a moon can theoretically have a stable orbit - is 0.53 r _H for prograde moons and 0.69 r _H for retrograde moons .
While all regular planets have prograde orbits without exception, most of the irregular moons of all planets have retrograde orbits: the ratio of retrograde to prograde satellites is about 4.5: 1. Since most of the formation models of irregular planets (see below) predict a symmetrical distribution on pro- and retrograde orbits, it is assumed that some particularly large moons on retrograde orbits destroyed several moons on prograde orbits, or the asymmetry due to the higher stability of moons irregular trajectories.

The colors of the irregular moons range from neutral to reddish. The fine color distribution of the moons of the different planets does not differ, which suggests that the moons did not arise around the planet, but have a common origin.

The sizes of most of the irregular moons could only be determined imprecisely. The radii of the previously known irregular moons are between a few kilometers and about 106 kilometers ( Phoebe ), whereby it is possible that even smaller moons may not have been found yet. The fact that more and smaller irregular moons are known from the closer planets is also due to the distortion caused by the observation possibilities.

The number of moons whose diameter is smaller than or equal to a certain diameter (English size-frequency distribution , SFD for short) can be described, as with most objects in the solar system, by two power functions . The exponent for small moons (i.e. with radii less than 10 km) is around 3.5 and for objects with radii greater than 10 kilometers it is around 2.

Triton

The Neptune moon Triton plays a special role. It is often counted among the regular moons because it has a narrow orbit (0.003 r _H ) with low eccentricity (0.00002), which is usual for regular moons, and an unusually large radius for irregular moons (2706 ± 2 km). However, it orbits Neptune on a retrograde orbit (inclination of 156.8 °), and must therefore also have been captured.

Creation models

While regular satellites were formed by accretion from the planetary gas disk according to current knowledge , irregular moons can not be formed in this way for several reasons :

they are spatially too separated from the regular moons to have originated from the same gas disk,
such large eccentricities cannot arise through accretion,
above all, no retrograde orbiting moons can arise from a gas disk.

The origin of the irregular moons is therefore still unclear. They must have been captured from heliocentric orbit by the planet in some way. The system of sun-planet-capturing bodies is not sufficient, however, since it is reversible in time and therefore every path of the body from the orbit around the sun to a path around the planet is also a possible way back.

Gas drag

As a possible explanation to break the temporal reversibility, it was suggested that the body energy through friction (Engl. Drag ) to the planet loses the surrounding gas.

While this model can explain some of Jupiter's irregular moons, it cannot explain all of them, as some are further away than the gas disk reached. This model probably cannot be applied to the irregular moons of Neptune and Uranus either, since their gas disks are probably not suitable for this model.

Migration models such as the Nice model (see below) lead to another major problem for gas drag models: if the irregular moons thus created are too close to a large planetesimal or a planet, they are swept out of the planet's system extremely efficiently. While this used to be used as an argument against models in which planets come close to each other, since the success of the Nice model at the latest, it has mostly been used as an argument against the gas-drag models.

Another problem with the gas-drag models is the colors of the irregular moons: if the moons came from the local area around the planets, then all the moons of a planet would have to be of a similar color, and these colors would have to depend on the solar distance of the planet.

Nice model

The Nice model assumes that shortly after the dissolution of the protoplanetary gas disk from which the giant planets emerged, triggered by the interaction of the planets with a massive disk of planetesimals surrounding the solar system at that time, a resonance occurs between the orbits of Jupiter and Saturn. This destabilizes the system before it stabilizes again after about a hundred million years. In the chaotic phase, planets keep coming close, and at the same time numerous planetesimals fly through the solar system.

In the context of the Nice model, Nesvorný and colleagues explained the capture of irregular moons via three-body reactions : two planets come so close that they penetrate each other's hill radius, some of which also become planetesimals that also fly through the hill radius of the planets as a result trapped on distant orbits around one of the planets and now orbit the planet as irregular moons.

In the simulations by Nesvorný and colleagues, the Nice model generates far too many irregular planets, and the size distribution (SFD) does not match the observations very well either. The explanation for this is that the chaotic orbits of the irregular mode led to numerous collisions in which many moons were destroyed. Simulations that take this into account can successfully explain the irregular moons of all four giant planets.

Web links

Commons : Irregular satellite collection of images, videos and audio files

Individual evidence

↑ Philip D. Nicholson, Matija Cuk, Scott S. Sheppard, David Nesvorny, Torrence V. Johnson: Irregular satellites of the giant planets. In: M. Antonietta Barucci (Ed.): The Solar System Beyond Neptune. 1, 2008, pp. 411-424.
↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k David Jewitt, Nader Haghighipour : Irregular Satellites of the Planets: Products of Capture in the Early Solar System. In: Annual Review of Astronomy and Astrophysics. 45, No. 1, 2007, pp. 261-295, doi: 10.1146 / annurev.astro.44.051905.092459 .
^ ^A ^b Douglas P. Hamilton, Alexander V. Krivov: Dynamics of Distant Moons of Asteroids. In: Icarus. 128, No. 1, 1997, pp. 241-249, doi: 10.1006 / icar.1997.5738 .
^ Joseph A. Burns: The evolution of satellite orbits. In: Satellites. Vol. 1, 1986.
↑ ^a ^b ^c ^d ^e David Nesvorný, David Vokrouhlický, Alessandro Morbidelli: Capture of Irregular Satellites during Planetary Encounters. In: The Astronomical Journal. 133, No. 5, 2007, pp. 1962-1976, doi: 10.1086 / 512850 .
^ J. Pollack: Formation of the Giant Planets by Concurrent Accretion of Solids and Gas. In: Icarus. 124, 1996, pp. 62-85. ISSN 0019-1035
^ William F. Bottke, David Nesvorný, David Vokrouhlický, Alessandro Morbidelli: The Irregular Satellites: The Most Collisionally Evolved Populations in the Solar System. In: The Astronomical Journal . 139, No. 3, 2010, pp. 994-1014, doi: 10.1088 / 0004-6256 / 139/3/994 .

[1] Philip D. Nicholson, Matija Cuk, Scott S. Sheppard, David Nesvorny, Torrence V. Johnson: Irregular satellites of the giant planets. In: M. Antonietta Barucci (Ed.): The Solar System Beyond Neptune. 1, 2008, pp. 411-424.

[review-2] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k David Jewitt, Nader Haghighipour : Irregular Satellites of the Planets: Products of Capture in the Early Solar System. In: Annual Review of Astronomy and Astrophysics. 45, No. 1, 2007, pp. 261-295, doi: 10.1146 / annurev.astro.44.051905.092459 .

[Hamilton-3] A ^b Douglas P. Hamilton, Alexander V. Krivov: Dynamics of Distant Moons of Asteroids. In: Icarus. 128, No. 1, 1997, pp. 241-249, doi: 10.1006 / icar.1997.5738 .

[4] Joseph A. Burns: The evolution of satellite orbits. In: Satellites. Vol. 1, 1986.

[Nesvorný2007-5] David Nesvorný, David Vokrouhlický, Alessandro Morbidelli: Capture of Irregular Satellites during Planetary Encounters. In: The Astronomical Journal. 133, No. 5, 2007, pp. 1962-1976, doi: 10.1086 / 512850 .

[6] J. Pollack: Formation of the Giant Planets by Concurrent Accretion of Solids and Gas. In: Icarus. 124, 1996, pp. 62-85. ISSN 0019-1035

[7] William F. Bottke, David Nesvorný, David Vokrouhlický, Alessandro Morbidelli: The Irregular Satellites: The Most Collisionally Evolved Populations in the Solar System. In: The Astronomical Journal . 139, No. 3, 2010, pp. 994-1014, doi: 10.1088 / 0004-6256 / 139/3/994 .