The small satellites of Pluto as observed by New Horizons

摘要

The New Horizons mission has provided resolved measurements of Pluto's moons Styx, Nix, Kerberos, and Hydra. All four are small, with equivalent spherical diameters of similar to 40 kilometers for Nix and Hydra and similar to 10 kilometers for Styx and Kerberos. They are also highly elongated, with maximum to minimum axis ratios of similar to 2. All four moons have high albedos (similar to 50 to 90%) suggestive of a water-ice surface composition. Crater densities on Nix and Hydra imply surface ages of at least 4 billion years. The small moons rotate much faster than synchronous, with rotational poles clustered nearly orthogonal to the common pole directions of Pluto and Charon. These results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary.