What is Neptune's D/H ratio really telling us about its water abundance ?

Abstract

We investigate the deep water abundance of Neptune using a simple 2-component (core + envelope) toy model. The free parameters of the model are the total mass of heavy elements in the planet (Z), the mass fraction of Z in the envelope (f_env), and the D/H ratio of the accreted building blocks (D/H_build ). We systematically search the allowed parameter space on a grid and constrain it using Neptune's bulk carbon abundance, D/H ratio, and interior structure models. Assuming solar C/O ratio and cometary D/H for the accreted building blocks forming the planet, we can fit all of median ~ 7%), and the rest the constraints if less than ~ 15% of Z is in the envelope (f_env is locked in a solid core. This model predicts a maximum bulk oxygen abundance in Neptune of 65 \times solar value. If we assume a C/O of 0.17, corresponding to clathrate-hydrates building blocks, we predict a maximum oxygen abundance of 200 times solar value with a median value of ~ 140. Thus, both cases lead to an oxygen abundance significantly lower than the preferred value of Cavali\'e et al. (2017) (~ 540 times solar), inferred from model dependent deep CO observations. Such high water abundances are excluded by our simple but robust model. We attribute this discrepancy to our imperfect understanding of either the interior structure of Neptune or the chemistry of the primordial protosolar nebula.