Evidence for Distinct C/O Ratios in Uranus and Neptune from Disequilibrium Chemistry

Uranus and Neptune are the most distant and the least explored planets within our solar system. To this day, the formation history of these ice giants remains uncertain. A better understanding of their deep atmospheric composition helps to constrain where and how these planets formed within the protoplanetary disk.  Remote sensing can only retrieve the atmospheric composition down to the few bar level in the millimeter to infrared domain, and an entry probe as part of the Uranus Flagship mission may only measure the atmospheric composition down to ~10-20 bars. Some species are measured in this upper-tropospheric region at abundances several order of magnitude above what is predicted by thermochemical equilibrium. Atmospheric models are thus needed to interpret how the measured abundances of such species reflect the deeper atmospheric composition. 

Using a thermochemical & diffusion model (Cavalie et al., 2024), we aim to take advantage of such disequilibrium species to further constrain Uranus's and Neptune's deep atmospheric compositions. We have updated the model to consider the meridional variability of several parameters in the troposphere, including a formulation for the eddy diffusivity coefficient, Kzz, based on convection under rotation laboratory experiments. We provide a range of plausible deep O/H ratios for both planets, as well as assess the impact of uncertainty propagation in a large chemical network on such ratios. We compute C/O ratios as a function of latitude, and compare them with a protoplanetary disk model results (e.g. Schneeberger et al. 2023) that tracks the time-dependent radial transport of solids through the disk. Our results suggest a different C/O between Uranus and Neptune and thus may support a different formation pathway for the ice giants.