The Surfaces of Small to Midsize Plutinos: Evidence of an Association between Inclination and Surface Type

Being one of the most populated mean motion resonances (MMRs) with Neptune and lying close to the inner boundary of the present-day cold classical disk, observations of the orbital and surface class distributions of the plutinos in the 3:2 MMR provide constraints on Neptune’s migration and insight into the compositional structure of the premigration planetesimal disk. Here, we present observations of the surface reflectance of 43 small to midsize ( H V ≳ 5) trans-Neptunian objects (TNOs) through the grz wavelength range, 14 of which are plutinos. We classify the surfaces of these TNOs using the two-surface class model (FaintIR and BrightIR surface classes) proposed by Fraser and collaborators, where the FaintIR surface class is dominated by cold classicals. Incorporating similar observations of plutinos from the literature for a total sample size of 43 plutinos, we find that the osculating inclination distributions ( i osc ) of the FaintIR and BrightIR plutinos are statistically distinguishable at 99.3% significance, where six out of seven of the plutinos with i osc < 4 . ° 5 have FaintIR surfaces. This is most easily explained if the FaintIR and BrightIR planetesimals were radially partitioned in the primordial planetesimal disk before being captured into the 3:2 resonance. While this could be evidence that the primordial cold classical disk with FaintIR surfaces was broader in the past by ∼3 au in the sunward direction, we cannot rule out the alternative explanation that these FaintIR plutinos were scattered from the ∼42.5–48 au region from the Sun and captured into the 3:2 resonance.