Constraints on Uranus formation from its D/H ratio

The formation of the ice giants Uranus and Neptune remains poorly understood, with several competing hypotheses attempting to explain their observed compositions. In particular, the carbon enrichment and nitrogen depletion observed in these planets challenge traditional models of planet formation. However, the measurement of the deuterium-to-hydrogen (D/H) ratio in Uranus by the Herschel Space Telescope provides a critical constraint on its bulk composition, including the CO/H2O ratio, providing valuable insights into the planet's formation and evolution.

D/H measurements in comets and planets are crucial for understanding their formation history. In the protosolar nebula, water ice is enriched in deuterium in the colder, outer regions and depleted in the warmer, inner regions relative to protosolar hydrogen. For example, D/H measurements from gas giants, which are predominantly composed of hydrogen, typically reflect or closely resemble the protosolar hydrogen D/H ratio. In contrast, D/H measurements from ice giants like Uranus and Neptune show supersolar D/H ratios in their atmospheres. The leading hypothesis to explain this is that their envelopes formed through the mixing of protosolar hydrogen with deuterium--rich primordial ices that they accreted during their formation. 

Under this assumption, the atmospheric D/H ratio of Uranus can be directly linked to the D/H ratio of its building block ices, depending on models of its internal structure. Assuming a cometary D/H ratio for the primordial ices accreted by Uranus enables the estimation of the planet's bulk composition, particularly its CO/H2O ratio. The objective of this study is to compare the inferred CO/H2O ratio of Uranus, derived from D/H remote sensing measurements, with values predicted for the protosolar nebula using a protoplanetary disk model. These findings provide critical constraints on the timing and location of Uranus's formation within the early Solar System and offer valuable insights into the processes that shaped its evolution.