Presentation #203.01 in the session “Centaurs and Kuiper Belt Objects: Physical Characterization”.
Spectral surveys of the outer Solar System are starting to reveal the signatures of the early dispersal of the protoplanetesimal disk in the compositional-dynamical structure of the current planetesimal populations. The distribution of the known surface types of Kuiper Belt Objects (KBOs) are now being used to infer the compositional structure of the protoplanetesimal disk itself. We present a new idea regarding the compositional properties of KBOs that is predicated on the correlated optical-NIR colours of the two dynamically excited compositional classes. Specifically, we hypothesize that nearly all KBOs belonging to the less-red class share origins with some of the carbonaceous asteroids. We make use of the ugrizJ colours sample acquired by the Colours of the Outer Solar System Origins Survey to probe what compositional components are compatible with that idea. The broad spectral behaviour of the less-red class can be very well accounted for with a C-type spectrum mixed with many laboratory organic materials, such as tholins and other disordered organic ices, and requires the presence of water-ice. Variation of the non-carbonaceous additions can account for the correlated optical-NIR colours of KBOs. Increasing concentrations of the non-carbonaceous materials not only increases the optical spectral slopes, but increasingly masks the presence of the absorption features common to carbonaceous asteroids, which are seen on only the most neutral coloured KBOs, and produce nearly linear spectra like those observed. Within this model, Phoebe occupies a special category, possessing a high concentration of water-ice, and is completely devoid of the reddening agent. If this idea is true, it follows that the less-red class of KBOs are cosmogonically related to certain classes of carbonaceous asteroids, and likely shared a similar origin within the protoplanetesimal disk. As the Kuiper Belt is dominated by mass from the less-red class, our model implies that the majority of KBOs are simply reddened carbonaceous asteroids. Confirmation of this idea will come from the identification of the absorption features common to the C-types, which must also be common to most small KBOs, albeit with a more modest presence.