The Hubble Wide Field Camera 3 Test of Surfaces in the Outer Solar System: The Compositional Classes of the Kuiper Belt

We present the first results of the Hubble Wide Field Camera 3 Test of Surfaces in the Outer Solar System. The purpose of this survey was to measure the surface properties of a large number of Kuiper Belt objects and attempt to infer compositional and dynamical correlations. We find that the Centaurs and the low-perihelion scattered disk and resonant objects exhibit virtually identical bifurcated optical color distributions and make up two well-defined groups of objects. Both groups have highly correlated optical and NIR colors that are well described by a pair of two-component mixture models that have different red components but share a common neutral component. The small, H_606 ≳ 5.6 high-perihelion excited objects are entirely consistent with being drawn from the two branches of the mixing model, suggesting that the color bifurcation of the Centaurs is apparent in all small excited objects. On the other hand, objects larger than H_606 ~ 5.6 are not consistent with the mixing model, suggesting some evolutionary process avoided by the smaller objects. The existence of a bifurcation amongst all excited populations argues that the two separate classes of object existed in the primordial disk before the excited Kuiper Belt was populated. The cold classical objects exhibit a different type of surface that has colors that are consistent with being drawn from the red branch of the mixing model, but with much higher albedos