Triton and Pluto: the Long Lost Twins of Active Worlds

Assessing the origin of Pluto and Triton has profound implications for the bigger picture of solar system formation and evolution. In such a context, this chapter reviews our current knowledge of the formation conditions of Pluto and Triton’s constitutive building blocks in the protosolar nebula (PSN), which can be derived from their known or estimated volatile contents. Assuming that the ultravolatiles carbon monoxide and dinitrogen detected in Pluto and Triton are primordial, the presence of these molecules suggests that the two bodies accreted material originating from the vicinity of the carbon monoxide and dinitrogen icelines. Dinitrogenrich and water-poor comets such as comet C/2016 R2 (PanSTARRS) obviously present a compositional link with Pluto and Triton, indicating that their building blocks formed in nearby regions of the PSN, despite the variation of the water abundance among those bodies. Also, the assumption of Triton’s growth in Neptune’s circumplanetary disk requires that its building blocks formed at earlier epochs in the PSN to remain consistent with its estimated composition.