In a paper published in Science, a new mechanism of moon formation is described. The moons are progressively spawned at the roche limit of the planetary rings:
Crida and Charnoz "Formation of Regular Satellites from Ancient Massive Rings in the Solar System" 2012 Science 338, p1196-1199:
"When a planetary tidal disk—like Saturn’s rings—spreads beyond the Roche radius (inside which planetary tides prevent aggregation), satellites form and migrate away. Here, we show that most regular satellites in the solar system probably formed in this way. According to our analytical model, when the spreading is slow, a retinue of satellites appear with masses increasing with distance to the Roche radius, in excellent agreement with Saturn’s, Uranus’, and Neptune’s satellite systems. This suggests that Uranus and Neptune used to have massive rings that disappeared to give birth to most of their regular satellites. When the spreading is fast, only one large satellite forms, as was the case for Pluto and Earth. This conceptually bridges the gap between terrestrial and giant planet systems."
This mechanism was suggested by the repartition of moons around giant planets (© SAp, Animea):
- Moon-repartition.jpg (853.08 KiB) Viewed 7109 times
There is a clear relationship between the moons size and their orbit size. According to this mechanism, a moon forms progressively at the roche limit of an initial spreading massive ring and migrates away. When the newly formed moon becomes too far from the ring, a new moonlet start to form at the roche limit, and so on until the ring is depleted. The initial moon are larger because the ring initially contained more matter.
Since there are good evidence that some moons have been considerably expanding (Europa, Ganymede, Enceladus...), a possible alternative hypothesis that would explain the observed pattern and account for the expansion, is that moonlets are initially spawned from the spreading rings at the roche limit, migrate away but grow in size due to inner expansion.
Also, the rings could be replenished by micrometeoroids striking the moons surface, as commonly admitted, and by dust and icy particles ejected by the most active moons (e.g. Enceladus feeds the E-ring of Saturn; Kempf et al 2010 Icarus). 206, p446-457).