巨大衝突を通じた大きな太陽系外縁天体をとりまく衛星の初期形成

Recent studies1,2 have revealed that all large (over 1,000 km in diameter) trans-Neptunian objects (TNOs) form satellite systems. Although the largest Plutonian satellite, Charon, is thought to be an intact fragment of an impactor directly formed via a giant impact3, whether giant impacts can explain the variations in secondary-to-primary mass ratios (the ratio between the body and its main satellite) and spin/orbital periods among all large TNOs remains to be determined. Here we find that hydrodynamic simulations of giant impacts can reproduce the secondary-to-primary mass ratio of the satellite systems of large TNOs when the impact velocity is approximately the same as the escape velocity. We also reveal that the satellite systems’ current distribution of spin/orbital periods and small eccentricity can be most easily explained when their spins and orbits tidally evolve, initially as fluid-like bodies and finally as rigid bodies. The preferred duration of fluid-like behaviour is approximately 10⁴−10⁶ yr, and it depends on the secondary-to-primary mass ratio and the initial orbital elements. These results suggest that all satellites of large TNOs were formed via giant impacts before the outward migration of Neptune4 and that they were fully or partially molten during the giant impact era.