Pluto has 5 satellites: massive charon, plus 4 small exterior moons, styx, NIX, kerberos, and hydra. The small moon masses and densities were extremely uncertain prior to new horizons, and remain…

This federal contract award, valued at $477,625.80, was granted to Southwest Research Institute (SwRI) by the National Aeronautics and Space Administration (NASA) for research related to the properties of Pluto's small moons. The work involves analyzing New Horizons spacecraft data and archival Hubble Space Telescope observations to better determine the masses and densities of Pluto's outer moons, which are critical inputs for modeling the formation of the Pluto-Charon system. This research will support improving our understanding of the origin and evolution of the Pluto system. SwRI, an independent, nonprofit research organization, will perform the work under this grant, which has an ultimate completion date of July 11, 2023. The contract does not have a set-aside designation.

Pluto has 5 satellites: massive charon, plus 4 small exterior moons, styx, NIX, kerberos, and hydra. The small moon masses and densities were extremely uncertain prior to new horizons, and remain poorly constrained even after the analysis of data by the new horizons team detailed in weaver ET AL. (2016). That work reported approximate sizes for the moons, but not their masses or densities, due to mass uncertainties as high as 100%. Better determination of the outer moon properties is crucial to interpreting the history of these objects and the entire pluto system. Charon likely formed by a giant impact with pluto. The small moon orbits are nearly CO-planar and circular, suggesting they formed from a disk produced by the same impact that formed charon. However, successful descriptions of this process have proved elusive, and prior models have been undermined by uncertainty in the small moon masses and the disk properties. Accurate mass estimates are needed to determine how the radial surface density profile of a disk produced by an impact relates to the initial positions of the outer moons, and therefore to the degree of radial migration required for the moons to reach their current locations. We will determine the physical properties of pluto s small moons through RE-analysis of new horizons data combined with HST observations (task 1), and based on these results, we will develop much more accurate models of a pluto-system forming impact (task 2). The two tasks are interconnected: task 1 will provide the small moon masses, which are critical inputs to task 2, and successful task 2 results will constrain the ICE-rock content of the extended disk, which will be incorporated into the interpretation of the small moon densities in task 1 that may be insufficiently constrained by shape models alone for some of the moons. Our combined study will provide needed initial conditions for models of the origin and evolution of pluto s small moons. More broadly, IT will provide much higher fidelity and better constrained models of the pluto-system forming impact, central to understanding pluto and charon s thermal and dynamical evolutions. In task 1, we will analyze new horizons (NH) approach data from the long range reconnaissance imager (lorri) framing camera (contained in the first PDS data release), in combination with archival HST observations of the small moons tied to the gaia catalog, to provide greatly improved astrometry relative to that reported previously. This, in combination with markov chain monte carlo dynamical simulations, will be used to determine mass estimates for the small moons. The mass estimates will be combined with satellite shape models developed from publically available NH images to yield initial density estimates. In task 2, we will perform smoothed particle hydrodynamics (SPH) simulations of giant impacts with pluto with up to a factor of 100 times finer mass resolution than nearly all prior models. These will allow US to determine 1) the outer edge of the disk at the mass level of outermost hydra, and 2) the disk ICE VS. Rock composition. We will determine (1) and (2) as a function of impact parameters and the bulk properties of proto-pluto and the impactor prior to the impact. We will identify the subset of impacts whose outcomes are consistent with charon s mass and density and the small moon masses determined in task 1, favoring those cases that produce the most extended disks that minimize the needed small moon orbital migration. The ICE-rock content of the extended disks produced in the most successful cases will be incorporated into task s 1 interpretation of the small moon densities.