The mass distribution of the Neptunian moons is the most lopsided of the satellite systems of the giant planets 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. The reason for the lopsidedness of the present Neptunian system is that Triton was captured from the
Kuiper belt well after the formation of Neptune's original satellite system, much of which was destroyed in the process of capture. Triton's orbit upon capture is presumed to have been highly
eccentric, which would have caused chaotic
perturbations in the orbits of the original inner Neptunian satellites, leading to the ejection of some moons and the collisional destruction of others. At least some of Neptune's present inner satellites are thought to have then
accreted from the resulting rubble after Triton's orbit was
circularized by
tidal deceleration. Among these re-accreted moons are
Proteus, the largest and outermost of Neptune's present inner moons. Proteus bears a large impact crater named
Pharos, which has a diameter around —more than half the diameter of Proteus itself. This unusually large size of Pharos relative to Proteus implies that the
impact event that formed the crater would have nearly
disrupted Proteus and ejected a significant amount of debris. The present orbit of Proteus is situated relatively close to that of Hippocamp, which orbits just interior of Proteus. Their orbital
semi-major axes differ by only ten percent, implying that both had likely originated from the same position in the past. This is further evidenced by accounting for the moons' respective outward
orbital migration rates, which also suggests that Hippocamp and Proteus were much closer together in the past. Ordinarily, two adjoining objects of disparate sizes would have either resulted in the smaller object being ejected or colliding with the larger object—this does not appear to be the case for Hippocamp and Proteus. Based on this evidence, Showalter and colleagues proposed that Hippocamp may have originated from debris ejected from Proteus by the
cometary impact that formed its largest crater, Pharos. In this scenario, Hippocamp would be considered as a third-generation satellite of Neptune, originating from impacts on Neptune's reformed
regular moons after the capture of Triton. The regular moons of Neptune are thought to have been disrupted by cometary impacts multiple times, with only Proteus surviving intact despite being nearly disrupted by the Pharos impact event. Some of the debris ejected by the impact settled into a stable orbit interior to Proteus, and coalesced into Hippocamp. However, Hippocamp only accounts for two percent of the missing volume of material generated by the Pharos impact event, and the reason for the absence of the rest of the debris remains unknown. As with the other small inner moons of Neptune, Hippocamp is thought to have been repeatedly disrupted by comet impacts after it had coalesced from debris ejected from Proteus. Based on the formation rate of large craters on Proteus, Hippocamp is estimated to have been disrupted about nine times in the past 4 billion years, re-accreting back after each disruption event. These disruption events substantially reduce the moon's orbital eccentricity and inclination, providing an explanation for Hippocamp's present circular orbit despite its proximity to Proteus. Hippocamp had also likely lost some of its mass during these disruption events, possibly accounting for some of the missing volume of material ejected from the Pharos impact event. Proteus has since receded over from Neptune owing to
tidal interactions with the planet, while Hippocamp remained close to its initial position where it formed as it migrates more slowly due to its smaller size. == Physical characteristics ==