The Small Satellites of Pluto as Observed by New Horizons

TL;DR

New Horizons observed Pluto's small moons, revealing their sizes, shapes, high albedos, surface ages, and rapid rotations, supporting a collision origin hypothesis.

Contribution

This study provides detailed measurements of Pluto's small moons, including sizes, shapes, surface properties, and rotation states, offering new insights into their origins.

Findings

Moons are small, elongated, with high albedos indicating water-ice surfaces.

Crater densities suggest surface ages over 4 billion years.

Moons rotate faster than synchronous, with poles nearly orthogonal to Pluto-Charon system.

Abstract

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 $\approx$40 km for Nix and Hydra and ~10 km for Styx and Kerberos. They are also highly elongated, with maximum to minimum axis ratios of $\approx$2. All four moons have high albedos ( $\approx$50-90 %) suggestive of a water-ice surface composition. Crater densities on Nix and Hydra imply surface ages $\gtrsim$ 4 Ga. 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.