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 _ { \text { 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 the constraints if less than \sim 15 % of Z is in the envelope ( f _ { env } ^ { median } \sim 7 % ) , and the rest 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 \sim 140 .
Thus , both cases lead to an oxygen abundance significantly lower than the preferred value of [ \citeauthoryear Cavalié et al.2017 ] ( \sim 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 .