Irregular moon
In astronomy, an irregular moon, irregular satellite, or irregular natural satellite is a natural satellite following an orbit that is irregular in some of the following ways: distant; inclined; highly elliptical; retrograde. They have often been captured by their parent planet, unlike regular satellites that formed in orbit around them. Irregular moons have a stable orbit, unlike temporary satellites which often have similarly irregular orbits but will eventually depart. The term does not refer to shape; Triton, for example, is a round moon but is considered irregular due to its orbit and origins.
, 358 irregular moons are known, orbiting all four of the outer planets. The largest of each planet are Himalia of Jupiter, Phoebe of Saturn, Sycorax of Uranus, and Triton of Neptune. Triton is rather unusual for an irregular moon; if it is excluded, then Nereid is the largest irregular moon around Neptune. It is currently thought that the irregular satellites were once independent objects orbiting the Sun before being captured by a nearby planet, early in the history of the Solar System. An alternative suggests that they originated further out in the Kuiper belt and were captured after the close flyby of another star.
Definition
There is no widely accepted precise definition of an irregular satellite. Informally, satellites are considered irregular if they are far enough from the planet that the precession of their orbital plane is primarily controlled by the Sun, other planets, or other moons.In practice, the satellite's semi-major axis is compared with the radius of the planet's Hill sphere,. Irregular satellites have semi-major axes greater than 0.05 with apoapses extending as far as to 0.65. The radius of the Hill sphere is given in the adjacent table: Uranus and Neptune have larger Hill sphere radii than Jupiter and Saturn, despite being less massive, because they are farther from the Sun. However, no known irregular satellite has a semi-major axis exceeding 0.47.
Earth's Moon seems to be an exception: it is not usually listed as an irregular satellite even though its precession is primarily controlled by the Sun and its semi-major axis is greater than 0.05 of the radius of Earth's Hill sphere. On the other hand, Neptune's Triton, which is probably a captured object, is usually listed as irregular despite being within 0.05 of the radius of Neptune's Hill sphere, so that Triton's precession is primarily controlled by Neptune's oblateness instead of by the Sun. Neptune's Nereid and Saturn's Iapetus have semi-major axes close to 0.05 of the radius of their parent planets' Hill spheres: Nereid is usually listed as irregular, but not Iapetus.
Orbits
Current distribution
The orbits of the known irregular satellites are extremely diverse, but there are certain patterns. Retrograde orbits are far more common than prograde orbits. No satellites are known with orbital inclinations higher than 60° ; moreover, apart from Nereid, no irregular moon has inclination less than 26°, and inclinations greater than 170° are only found in Saturn's system. In addition, some [|groupings] can be identified, in which one large satellite shares a similar orbit with a few smaller ones.Given their distance from the planet, the orbits of the outer satellites are highly perturbed by the Sun and their orbital elements change widely over short intervals. The semi-major axis of Pasiphae, for example, changes as much as 1.5 million km in two years, the inclination around 10°, and the eccentricity as much as 0.4 in 24 years. Consequently, mean orbital elements are used to identify the groupings rather than osculating elements at the given date.
Origin
Irregular satellites may have been captured from heliocentric orbits.. Alternatively, trans-Neptunian objects may have been injected due to the close passing star and a fraction of these injected TNOs captured by the giant planets. For this to occur, at least one of three things needs to have happened:- energy dissipation
- a substantial extension of the planet's Hill sphere in a brief period of time
- a transfer of energy in a three-body interaction. This could involve:
- * a collision of an incoming body and a satellite, resulting in the incoming body losing energy and being captured.
- * a close encounter between an incoming binary object and the planet, resulting in one component of the binary being captured. Such a route has been suggested as most likely for Triton.
Long-term stability
The current orbits of the irregular moons are stable, in spite of substantial perturbations near the apocenter.The cause of this stability in a number of irregulars is the fact that they orbit with a secular or Kozai resonance.
In addition, simulations indicate the following conclusions:
- Orbits with inclinations between 50° and 130° are very unstable: their eccentricity increases quickly resulting in the satellite being lost
- Retrograde orbits are more stable than prograde
Retrograde satellites can be found further from the planet than prograde ones. Detailed numerical integrations have shown this asymmetry. The limits are a complicated function of the inclination and eccentricity, but in general, prograde orbits with semi-major axes up to 0.47 rH can be stable, whereas for retrograde orbits stability can extend out to 0.67 rH.
The boundary for the semimajor axis is surprisingly sharp for the prograde satellites. A satellite on a prograde, circular orbit placed at 0.5 rH would leave Jupiter in as little as forty years. The effect can be explained by so-called evection resonance. The apocenter of the satellite, where the planet's grip on the moon is at its weakest, gets locked in resonance with the position of the Sun. The effects of the perturbation accumulate at each passage pushing the satellite even further outwards.
The asymmetry between the prograde and retrograde satellites can be explained very intuitively by the Coriolis acceleration in the frame rotating with the planet. For the prograde satellites the acceleration points outward and for the retrograde it points inward, stabilising the satellite.
Temporary captures
The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon. The only observed examples are and, which were temporary satellites of Earth discovered in 2006 and 2020, respectively.Physical characteristics
Size
Because objects of a given size are more difficult to see the greater their distance from Earth, the known irregular satellites of Uranus and Neptune are larger than those of Jupiter and Saturn; smaller ones probably exist but have not yet been observed. Bearing this observational bias in mind, the size distribution of irregular satellites appears to be similar for all four giant planets.The size distribution of asteroids and many similar populations can be expressed as a power law: there are many more small objects than large ones, and the smaller the size, the more numerous the object. The mathematical relation expressing the number of objects,, with a diameter smaller than a particular size,, is approximated as:
The value of q is determined through observation.
For irregular moons, a shallow power law is observed for sizes of 10 to 100 km,† but a steeper law is observed for objects smaller than 10 km. An analysis of images taken by the Canada–France–Hawaii Telescope in 2010 shows that the power law for Jupiter's population of small retrograde satellites, down to a detection limit of ≈ 400 m, is relatively shallow, at q ≃ 2.5. Thus it can be extrapolated that Jupiter should have moons 400 m in diameter or greater.
For comparison, the distribution of large Kuiper belt objects is much steeper. That is, for every object of 1000 km there are a thousand objects with a diameter of 100 km, though it's unknown how far this distribution extends. The size distribution of a population may provide insights into its origin, whether through capture, collision and break-up, or accretion.
†For every object of 100 km, ten objects of 10 km can be found.
Around each giant planet, there is one irregular satellite that dominates, by having over three-quarters the mass of the entire irregular satellite system: Jupiter's Himalia, Saturn's Phoebe, Uranus's Sycorax, and Neptune's Nereid. Nereid also dominates among irregular satellites taken altogether, having about two-thirds the mass of all irregular moons combined. Phoebe makes up about 17%, Sycorax about 7%, and Himalia about 5%: the remaining moons add up to about 4%.
Colours
The colours of irregular satellites can be studied via colour indices: simple measures of differences of the apparent magnitude of an object through blue, visible i.e. green-yellow, and red filters. The observed colours of the irregular satellites vary from neutral to reddish.| albedo | neutral | reddish | red |
| low | C 3–8% | P 2–6% | D 2–5% |
| medium | M 10–18% | A 13–35% | |
| high | E 25–60% |
Each planet's system displays slightly different characteristics. Jupiter's irregulars are grey to slightly red, consistent with C, P and D-type asteroids. Some groups of satellites are observed to display similar colours. Saturn's irregulars are slightly redder than those of Jupiter.
The large Uranian irregular satellites are light red, whereas the smaller Prospero and Setebos are grey, as are the Neptunian satellites Nereid and Halimede.
Spectra
With the current resolution, the visible and near-infrared spectra of most satellites appear featureless. So far, water ice has been inferred on Phoebe and Nereid and features attributed to aqueous alteration were found on Himalia.Rotation
Regular satellites are usually tidally locked. In contrast, tidal forces on the irregular satellites are negligible given their distance from the planet, and rotation periods in the range of only ten hours have been measured for the biggest moons Himalia, Phoebe, Sycorax, and Nereid. Such rotation rates are in the same range that is typical for asteroids. Triton, being much larger and closer to its parent planet, is tidally locked.Families with a common origin
Some irregular satellites appear to orbit in 'groups', in which several satellites share similar orbits. The leading hypothesis is that these objects constitute collisional families, parts of a larger body that broke up.Dynamic groupings
Simple collision models can be used to estimate the possible dispersion of the orbital parameters given a velocity impulse Δv. Applying these models to the known orbital parameters makes it possible to estimate the Δv necessary to create the observed dispersion. A Δv of tens of meters per seconds could result from a break-up. Dynamical groupings of irregular satellites can be identified using these criteria and the likelihood of the common origin from a break-up evaluated.When the dispersion of the orbits is too wide :
- either more than one collision must be assumed, i.e. the cluster should be further subdivided into groups
- or significant post-collision changes, for example resulting from resonances, must be postulated.
Colour groupings
When the colours and spectra of the satellites are known, the homogeneity of these data for all the members of a given grouping is a substantial argument for a common origin. However, lack of precision in the available data often makes it difficult to draw statistically significant conclusions. In addition, the observed colours are not necessarily representative of the bulk composition of the satellite.Observed groupings
Irregular satellites of Jupiter
Typically, the following groupings are listed- Prograde satellites
- * The Himalia group shares an average inclination of 28°. They are confined dynamically. They are homogenous at visible wavelengths and at near infrared wavelengths
- * The prograde satellites Themisto and Valetudo are not part of any known group.
- Retrograde satellites
- * The Carme group shares an average inclination of 165°. It is dynamically tight. It is very homogenous in colour, each member displaying light red colouring consistent with a D-type asteroid progenitor.
- * The Ananke group shares an average inclination of 148°. It shows little dispersion of orbital parameters. Ananke itself appears light red but the other group members are grey.
- * The Pasiphae group is very dispersed. Pasiphae itself appears to be grey, whereas other members are light red.
Pasiphae and Sinope are also trapped in secular resonances with Jupiter.
Irregular satellites of Saturn
The following groupings are commonly listed for Saturn's satellites:- Prograde satellites
- * The Gallic group shares an average inclination of 34°. Their orbits are dynamically tight, and they are light red in colour; the colouring is homogenous at both visible and near infra-red wavelengths.
- * The Inuit group shares an average inclination of 46°. Their orbits are widely dispersed but they are physically homogenous, sharing a light red colouring.
- Retrograde satellites
- * The Norse group is defined mostly for naming purposes; the orbital parameters are very widely dispersed. Sub-divisions have been investigated, including
- ** The Phoebe group shares an average inclination of 174°; this sub-group too is widely dispersed, and may be further divided into at least two sub-sub-groups
- ** The Skathi group is a possible sub-group of the Norse group
Irregular satellites of Uranus and Neptune
| Planet | rmin |
| Jupiter | 1.5 km |
| Saturn | 3 km |
| Uranus | 7 km |
| Neptune | 16 km |
According to current knowledge, the number of irregular satellites orbiting Uranus and Neptune is smaller than that of Jupiter and Saturn. However, it is thought that this is simply a result of observational difficulties due to the greater distance of Uranus and Neptune. The table at right shows the minimum radius of satellites that can be detected with current technology, assuming an albedo of 0.04; thus, there are almost certainly small Uranian and Neptunian moons that cannot yet be seen.
Due to the smaller numbers, statistically significant conclusions about the groupings are difficult. A single origin for the retrograde irregulars of Uranus seems unlikely given a dispersion of the orbital parameters that would require high impulse, implying a large diameter of the impactor, which is incompatible in turn with the size distribution of the fragments. Instead, the existence of two groupings has been speculated:
These two groups are distinct in their distance from Uranus and in their eccentricity.
However, these groupings are not directly supported by the observed colours: Caliban and Sycorax appear light red, whereas the smaller moons are grey.
For Neptune, a possible common origin of Psamathe and Neso has been noted. Given the similar colours, it was also suggested that Halimede could be a fragment of Nereid. The two satellites have had a very high probability of collision over the age of the solar system.
Exploration
To date, the only irregular satellites to have been visited close-up by a spacecraft are Triton and Phoebe, the largest of Neptune's and Saturn's irregulars respectively. Triton was imaged by Voyager 2 in 1989 and Phoebe by the Cassini probe in 2004. Voyager 2 also captured a distant image of Neptune's Nereid in 1989, and Cassini captured a distant, low-resolution image of Jupiter's Himalia in 2000. New Horizons captured low-resolution images of Jupiter's Himalia, Elara, and Callirrhoe in 2007. Throughout the Cassini mission, many Saturnian irregulars were observed from a distance: Albiorix, Bebhionn, Bergelmir, Bestla, Erriapus, Fornjot, Greip, Hati, Hyrrokkin, Ijiraq, Kari, Kiviuq, Loge, Mundilfari, Narvi, Paaliaq, Siarnaq, Skathi, Skoll, Suttungr, Tarqeq, Tarvos, Thrymr, and Ymir.The Tianwen-4 mission is planned to focus on the regular moon Callisto around Jupiter, but it may fly-by several irregular Jovian satellites before settling into Callistonian orbit.