Orbits and Masses of the Small Satellites of Pluto
Abstract
We present a new orbit and mass solution for the four small satellites of Pluto: Styx, Nix, Kerberos, and Hydra. We have reanalyzed all available observations of the Pluto system obtained by the Hubble Space Telescope (HST) from 2005 to 2019 with the ACS, WFPC2, and WFC3 instruments, as well as the New Horizons LORRI images taken on approach to Pluto in 2015. We have used this high precision astrometry to produce updated orbits and mass estimates with uncertainties for all four of the small satellites. We find that the masses of Nix and Hydra are smaller than previously published estimates, with a dynamical mass of 1.80.4×10-3 km3/s2 (2.70.6×1016 kg) for Nix and 2.00.2×10-3 km3/s2 (3.00.3×1016 kg) for Hydra. These masses are 60% and 63% of the mean estimates by Brozovic et al. (2015), although still consistent with their 1-sigma uncertainties, and correspond to densities of 1.00.2 g/cm3 for Nix and 1.20.2 g/cm3 for Hydra given the moon volume estimates from Porter et al (2021). Although these densities are consistent with a range of ice-rock compositions, depending on the unknown bulk porosity in the moon interiors, the moons' high albedos and predominantly icy surfaces are most easily explained if their interiors are ice-rich. The tiny masses of Kerberos and Sytx remain very poorly constrained; we find 1-σ upper limits for the dynamical mass of Styx to be 3×10-5 km3/s2 (5×1014 kg) and for Kerberos 5×10-5 km3/s2 (8×1014 kg), consistent with densities of <2.1 g/cm3 for both bodies.
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