Moons of Neptune - Formation

Formation

The mass distribution of the Neptunian moons is the most lopsided of any group of satellites in the Solar System. One moon, Triton, makes up nearly all of the mass of the system, with all other moons together comprising only one third of one percent. This may be because Triton was captured well after the formation of Neptune's original satellite system, much of which would have been destroyed in the process of capture. (Saturn's satellite system is the next most lopsided, with most of its mass being in its largest moon Titan. Jupiter and Uranus have more balanced systems.)

Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. This means it is likely that Neptune's present inner satellites are not the original bodies that formed with Neptune. Only after Triton's orbit became circularised could some of the rubble re-accrete into the present-day regular moons. It is possible that because of this great perturbation, the satellite system of Neptune does not follow the 10,000:1 ratio of mass between the parent planet versus all its moons seen in all other gas giants.

The mechanism of Triton’s capture has been the subject of several theories over the years. One of them postulates that Triton was captured in a three-body encounter. In this scenario, Triton is the surviving member of a binary disrupted by its encounter with Neptune.

Numerical simulations show that there is a 0.41 probability that the moon Halimede collided with Nereid at some time in the past. Although it is not known whether any collision has taken place, both moons appear to have similar ("grey") colors, implying that Halimede could be a fragment of Nereid.