Oxygen Isotope Constraints on the Importance of Photochemical Processing in Protoplanetary Disks

TL;DR

The paper tests whether photochemical processing in protoplanetary disks can explain Solar System oxygen isotope variations or if these signals are inherited from the natal molecular cloud. Using the CANDY model to evolve CO and H$_2$O isotopologues in a dynamic disk with diffusion and dust growth, the authors assess the potential to shift rocky material isotopes toward Earth-like values. They find that, across a wide parameter space, the heavy-oxygen reservoir generated in the disk remains too diluted by the initial water inventory to match terrestrial $\delta^{18}$O, suggesting solar-nebula photoprocessing is not the primary driver of the observed isotopic evolution. The results imply molecular-cloud inheritance dominates and that disk photochemistry primarily affects gas and ice phases rather than the solids that form planets, with implications for interpreting disk observations and planetary isotopic signatures.

Abstract

Observations have revealed evidence of photochemical processing in protoplanetary disks. This processing occurs in the photon dominated layer, the optically thin regions of the disk high above the disk midplane. It remains unclear, however, how much this photochemical processing impacts the compositions of the planets and their building blocks within the disk. Here we use the oxygen isotopic compositions of Solar System solids, which has been attributed to photochemistry in the solar nebula, to quantitatively evaluate whether this processing could have produced the conditions needed to provide the diversity of compositions seen in the Solar System. We do this by modeling the chemical evolution while fine dust grows into the building blocks of the planets. We find that the oxygen isotopic evolution cannot be attributed to processing in the solar nebula and must instead be inherited from the parent molecular cloud. Further, our results indicate that the observed photochemical processing in protoplanetary disks does not significantly impact the compositions of planets that form within.

Figures (12)

• Figure 1: Illustration of the reaction network used in this study. Parameters for determining reaction rates are given in Table 1.
• Figure 2: Oxygen isotopic evolution in the no growth model described in the text. Top: Oxygen isotopic composition of H$_{2}$O and CO as a function of height in our model disk at various times throughout the simulation with no dust growth. The $\delta^{17}$O values evolve in essentially identical way to $\delta^{18}$O, so only the latter are shown here. The vertical dotted line represents the starting isotopic compositions of the two species. The $\delta^{18}$O of CO goes to -$\infty$ as all CO is destroyed at the very surface of the disk. Bottom: Column averaged isotopic evolution of the H$_{2}$O and CO over time in the model.
• Figure 3: Same as Figure 2, but for the dust growth case. The grey line in the bottom panel reflects the cumulative isotopic evolution of the pebbles that grow in the disk.
• Figure 4: Comparison of the evolving water reservoirs contained in pebbles and mantles of dust grains to the reservoir needed to mix with a initially solar mix of silicates to produce the oxygen isotope ratios observed in Solar System rocks. The mixing ratio is defined as the ratio of the water to total hydrogen atoms present, $X_{\mathrm{H_{2}O}}$=$n_{\mathrm{H_{2}O}}$/$n_{H}$
• Figure 5: Isotopic evolution of the CO and H$_{2}$O reservoirs in the disk (left) and pebbles (right) for various rates of dust growth.