Fig. 4. Core hydrogen contents (in ppm, left) and mantle H₂O contents (in wt %, right) during the course of accretion of Earth and Mars.

The base model considers heterogeneous planetary accretion, first of EH chondrites (to 85 and 75% accretion of Earth and Mars, respectively), and later of CI chondrites (the final 15 and 25% of Earth and Mars, respectively). Planetary building block compositions are detailed elsewhere (11, 12, 16). Low H content in planetary cores is attributed to the presence of Si (fig. S5).