Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways?

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The formation of Uranus and Neptune remains poorly constrained, largely due to poorly known deep elemental abundances. Carbon monoxide (CO), a disequilibrium species in the upper troposphere, provides an indirect constraint on the deep oxygen abundance. We wish to improve our knowledge of the deep O/H and C/O ratios of the ice giants and their impact on formation scenarios by accounting for meridional variations in the atmospheric structure and assessing the impact of chemical uncertainties on these ratios. We extended a 1D thermochemical and diffusion model into a pseudo-2D model by including the latitudinal variation in several key input model parameters. The deep oxygen abundance was inferred by matching the modeled CO mole fractions to observations in the upper troposphere and was compared to the deep carbon abundance to compute the C/O ratio. For Uranus, varying the tropospheric methane mole fraction alone yielded O/H ∼ 47--57 protosolar, whereas allowing K_zz to vary expanded the inferred range to O/H ∼ 62--177 protosolar. For Neptune, the corresponding sets of simulations led to O/H ∼ 182--215 protosolar and O/H ∼ 222--342 protosolar, respectively, supporting the hypothesis that oxygen is depleted in Uranus relative to Neptune. This agrees with previous works. The uncertainties associated with the chemical network introduce a more modest effect that amounts to ∼ 10% of the retrieved oxygen ranges at most. The uncertainties are on the same order as the uncertainty associated with the measurement of the CO abundance on Neptune, and they are higher on Uranus. We computed C/O latitudinal ranges and found mathrm C/O ∼ 0.06--0.52 on Uranus and mathrm C/O ∼ 0.02--0.12 on Neptune. When compared with a protoplanetary disk model, these results suggest a different formation or evolution pathway for Uranus and Neptune. The results highlight the dominant role of vertical mixing in constraining the deep oxygen abundance of Uranus and Neptune, and they demonstrate the importance of accounting for meridional variability and chemical kinetic uncertainties. This work provides a framework for selecting the latitude entry of a future Uranus Orbiter and Probe mission.

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