D/H Ratio in the Interiors of Rocky Protoplanets Accreting in the Solar Nebula

The deuterium/hydrogen (D/H) ratio of primordial water partitioned into a planetary interior seems to be different on Earth and Mars. Water from volcanic rocks originating from Earth's deep mantle has a low D/H ratio with high ³He/⁴He ratios, implying that it was inherited partially from the solar nebula. In contrast, the D/H ratio of water in the Martian meteorites considered to represent the mantle does not trend toward that of the solar nebula. These differences may be owing to differences in the types of atmospheric structures formed on protoplanets accreting in the solar nebula. Using a 1D radiative-equilibrium model, we analyze the thermal structure of a hybrid-type protoatmosphere in which the solar nebula component dominates the upper layer while a degassed component dominates the lower layer. Our analysis implies Mars-sized protoplanets maintain a hybrid-type protoatmosphere and the D/H ratio of the lower atmosphere resembles that of the building blocks. Conversely, when the mass is larger than Mars-sized, the compositional stratification is collapsed by convective mixing of the solar nebula component with the degassed component, and the D/H ratio approaches that of the solar nebula. This tendency becomes stronger when the planetary mass is larger. If water vapor is distributed through a magma ocean into the planetary interior, Mars-sized protoplanets are likely to reflect the D/H ratios of the building blocks, while larger protoplanets are likely to have acquired a solar-nebula-like D/H ratio.