Recent studies 1,2 have revealed that all large (over 1000 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 impact 3 , whether giant impacts can explain the variations in secondary-to-primary mass ratios and spin/orbital periods among all large TNOs remains to be determined. Here we systematically perform hydrodynamic simulations to investigate satellite formation via giant impacts. We find that the simulated secondary-to-primary mass ratio varies over a wide range, which overlaps with observed mass ratios. We also reveal that the satellite systems' current distribution of spin/orbital periods and small eccentricity can be explained only when their spins and orbits tidally evolve: initially as fluid-like bodies, but finally as rigid bodies. These results suggest that all satellites of large TNOs were formed via giant impacts in the early stage of solar system formation, before the outward migration of Neptune 4 , and that they were fully or partially molten during the giant impact era.

Understanding the collisional behavior of dust aggregates consisting of submicron-sized grains is essential to unveiling how planetesimals formed in protoplanetary disks. It is known that the collisional behavior of individual dust particles strongly depends on the strength of viscous dissipation force; however, impacts of viscous dissipation on the collisional behavior of dust aggregates have not been studied in detail, especially for the cases of oblique collisions. Here we investigated the impacts of viscous dissipation on the collisional behavior of dust aggregates. We performed numerical simulations of collisions between two equal-mass dust aggregates with various collision velocities and impact parameters. We also changed the strength of viscous dissipation force systematically. We found that the threshold collision velocity for the fragmentation of dust aggregates barely depends on the strength of viscous dissipation force when we consider oblique collisions. In contrast, the size distribution of fragments changes significantly when the viscous dissipation force is considered. We obtained the empirical fitting formulae for the size distribution of fragments for the case of strong dissipation, which would be useful to study the evolution of size and spatial distributions of dust aggregates in protoplanetary disks.

Several pieces of evidence suggest that silicate grains in primitive meteorites are not interstellar grains but condensates formed in the early solar system. Moreover, the size distribution of matrix grains in chondrites implies that these condensates might be formed as nanometer-sized grains. Therefore, we propose a novel scenario for rocky planetesimal formation in which nanometer-sized silicate grains are produced by evaporation and recondensation events in early solar nebula, and rocky planetesimals are formed via aggregation of these nanograins. We reveal that silicate nanograins can grow into rocky planetesimals via direct aggregation without catastrophic fragmentation and serious radial drift, and our results provide a suitable condition for protoplanet formation in our solar system.

Context. The thermal conductivity of highly porous dust aggregates is a key parameter for many subjects in planetary science; it is not yet fully understood, however. Aims. We investigate the thermal conductivity of fluffy dust aggregates with filling factors lower than 10 −1 . Methods. We determined the temperature structure and heat flux of the porous dust aggregates calculated through N-body simulations of static compression in the periodic boundary condition. Results. We derive an empirical formula for the thermal conductivity through the solid network k sol as a function of the filling factor of dust aggregates φ. The results reveal that k sol is approximately proportional to φ 2 , and the thermal conductivity through the solid network is significantly lower than previously assumed. In light of these findings, we must reconsider the thermal histories of small planetary bodies.