Elemental Composition of Uranus and Neptune: Insights from Building Blocks

This study aims to predict the bulk volatile compositions of Uranus and Neptune, aligning with the preparatory framework for the Uranus Orbiter Probe mission. We employ a protoplanetary disk model to trace the evolution of volatile species through vapor, dust, and pebble transport. It is assumed that both planets formed near the CO iceline in the protosolar nebula, in a way consistent with interior models reconciling heavy element abundances with measured atmospheric D/H ratios.The time-dependent abundances of primary volatile species are calculated at the CO iceline location, gradually evolving inward. This yields abundance ratios of key elements, including heavy noble gases, within Uranus and Neptune's feeding zones. Predictions for their deep compositions are formulated, assuming negligible compositional gradients in the envelope.Despite potential interior heterogeneity, estimates of the Xe/Ar or Kr/Ar abundance ratios prove valuable for evaluating our model. This is due to the absence of known differentiation mechanisms among these noble gases while they remain in vapor form within the atmospheres.