Irregular moon

In Astronomy, an irregular satellite is a natural satellite following a distant, inclined, often retrograde orbit and believed to be captured as opposed to a regular satellite, formed in situ.

Phoebe, the largest irregular satellite of Saturn.

Eighty-six irregular satellites have been discovered since 1997 orbiting all four giant planets. Only fourteen others were discovered earlier. This includes Phoebe, the largest irregular satellite of Saturn, and Himalia, the largest irregular satellite of Jupiter. It is thought that the irregular satellites have been captured from heliocentric orbits, near their current locations, early after the planet formation. Their origin in Kuiper Belt space is not supported by the current observations.

Definition

There's no widely accepted precise definition of the irregular satellite. Informally, the 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.

Typically, the semi-major axis is compared with the Hill sphere (the gravitational influence sphere) $r_{H}$. Irregular satellites have semi-major axes greater than 0.05 $r_{H}$ and apoapsis up to 0.65 $r_{H}$.

Orbits

Irregular satellites of Jupiter (red), Saturn (yellow), Uranus (green) and Neptune (blue)

Current distribution

The diagram illustrates the orbits of the irregular satellites of the giants planets discovered so far. The semi-major axes are expressed as a fraction of the planet’s Hill sphere’s radius and the inclination is represented on Y axis. The satellites above the X axis are prograde, the satellites beneath are retrograde.

Origin

The capture from a heliocentric orbit requires one of the following

• energy dissipation (e.g. in the primordial gas cloud)
• a substantial (40%) extension og the planet's Hill sphere in a brief period of time (thousands of years)
• transfer of energy in a three-body interaction.

The last mechanisms, involving a collision (or close encounter) of two satellites could result in one of them losing energy and be captured.

Long-term stability

Remarkably, the current orbits prove stable in numerical simulations, in spite of substantial perturbations near apocentre . A number of irregulars are proven to be in secular or Kozai resonance believed to be the cause of this stability. In addition, the simulations indicate that

• orbits with inclinations higher than 50° (130° for retrograde orbits) are not stable, their eccentricity growing quickly resulting in the satellite being lost
• retrograde orbits are more stable than prograde (stable retrograde orbits can be found further from the planet)

Physical characteristics

Colours

Observed colours vary from neutral to reddish but not as red as Kuiper Belt objects. Each planet's system displays slightly different characteristics; Jupiter's irregulars are consistent with C and D-type asteroids , Saturn's irregulars are redder, similar to Iapetus (but different from Phoebe). Only the irregulars of Uranus show the very red colours typical for classical KBOs.

Families with a common origin

The questions of a possible common origin arise naturally given very similar orbital parameters of some satellites.

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 possible to estimate the δV necessary to create the observed dispersion. It is believed δV of tens of meters per seconds (5-50m/s) could result from a break-up. Dynamical groupings of irregular satellites can be identified using these criteria and their common origin from a break-up can be postulated.

When the dispersion of the orbits is too wide (i.e. it would require δV in the order of hundreds of meters per second)

• 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 the common origin. However, the precision of the available data makes often difficult to draw statistically significant conclusions. In addition, the observed colours are not necessarily representative for the bulk composition of the satellite.

Observed groupings

Irregular satellites of Jupiter

Irregular satellites of Jupiter.

Typically, the following groupings are listed (dynamically tight groups displaying homogenous colours are listed in bold)

• Prograde satellites
  • Himalia group (inclination 28° cluster): confined dynamically (δV~150m/s); very homogenous: neutral colours similar to C-type asteroids.
  • Themisto (isolated so far)
  • Carpo (isolated so far)
• Retrograde satellites
  • Carme group (165° cluster): dunaically tight (5<δV<50m/s)very homogenous, light-red colour consistent with a D-type progenitor
  • Ananke group (148° cluster): little dispersion of orbital parameters (15<δV<80m/s); Ananke itself appears light-red while the satellites following similar orbits are grey
  • Pasiphae group: dispersed; Pasiphae appears to be grey while other members are light-red ¹.

¹Sinope, sometimes included into Pasiphae group, is thought to be independent, trapped in a secular resonance with Pasiphae

Irregular satellites of Saturn

Irregular satellites of Saturn.

Typically, the following groupings are listed

• Prograde satellites
  • Gallic group (inclination 34° cluster): tight dynamically (δV~50m/s), homogenous ((light-red colours)
  • Inuit group: (34° cluster) dispersed (δV~350 m/s) but homogenous (light-red colours)
• Retrograde satellites
  • Norse group is defined mostly for naming purposes; the orbital parameter’s dispersion is large and different sub-divisions have been investigated, including
    • Phoebe group (174° cluster); large dispersion suggesting at least two sub-groupings
    • Skathi sub-group