Early formation of moons around large trans-Neptunian objects via giant impacts

Abstract

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.

Heat Flux Constraints From Variance Spectra of Pluto and Charon Using Limb Profile Topography

“…Either of those orbital states can generate heat if the body can be tidally deformed. However, since we do not know the initial state of the orbit, we need to assume an initial state based on formation models (Arakawa et al., 2019; Canup, 2010). Unlike the distribution of increased temperature, the range of possible eccentricities is wide.…”

“…One of the more popular hypotheses for the formation of Charon involves a giant impact into a proto‐Pluto (Canup, 2010). A newly formed Charon would either coalesce out of orbiting debris or directly form from the largest intact fragments (Arakawa et al., 2019). In either case, depending on the parameters of the impact, Charon could form partially melted.…”

“…Possible eccentricities for a newly formed Charon can reach values near 1.0 (Arakawa et al., 2019). Eccentricities that high would reduce rapidly, especially if Charon were molten, and the heating during these periods would be brief.…”

“…In either case, depending on the parameters of the impact, Charon could form partially melted. Smooth particle hydrodynamics impact simulations from Canup (2010) and Arakawa et al (2019) record the state and temperature of particles through the simulations. Canup (2010) found that the temperature increase in the impact is concentrated toward the outer layers.…”

“…A lower k 2 /Q signals a more rigid body that cannot deform as easily and create heat. We assume, based on the results of Arakawa et al (2019), that Charon was initially partially molten (i.e. fluid).…”