Color patterns in the Kuiper Belt: A possible primordial origin

As a result of our continuing photometric survey, we report here optical colors for 36 Kuiper Belt objects, increasing our sample size to 91 objects. We find that certain dynamical classes of objects exhibit distinctive colors - 21 out of 21 objects on small-inclination and small-eccentricity orbits with perihelion distances larger than 40 AU exhibit red surface colors (B-R>1.5), while 17 out of 20 objects on large-inclination and large-eccentricity orbits with aphelion distances larger than 70 AU exhibit gray surface colors (B-R<1.5). Our observations are consistent with a primordial origin for Kuiper Belt surface colors, if we assume that gray objects formed closer to the Sun than red objects, and as Neptune migrated outward it scattered gray objects onto dynamically hot orbits. By this model, the contrasting dynamically cold and red objects beyond 40 AU remained far enough away from Neptune that they were never perturbed by the planet.