Contribution of the Moon-forming Impactor to the Volatile Inventory in the Bulk Silicate Earth

Abstract

The timing and mechanism by which the present-day inventory of life-essential volatiles hydrogen-carbon-nitrogen-sulfur (H-C-N-S) in the bulk silicate Earth (BSE) was established are debated. In this study we have modeled the equilibrium partitioning of H-C-N-S between core, magma ocean (MO), and atmosphere to determine whether the Moon-forming impactor (MFI) was the primary source of volatiles in the BSE. Our findings suggest that the MFI’s core and MO-degassed atmosphere were its primary H-C-N-S reservoirs. Since the MFI likely lost its MO-degassed atmosphere before the giant impact, most of the BSE’s volatiles must come from the small fraction of the MFI’s core which reequilibrated with Earth’s post-impact MO. This implies a high H-C-N-S inventory in the MFI (up to 50% of volatile-rich carbonaceous chondrites) to establish the BSE’s volatile budget. Although isotopic compositions of nonvolatile elements do not rule out the possibility of substantial volatile-rich, carbonaceous material accretion, MFI’s collisional growth from thermally metamorphosed/differentiated planetesimals makes it improbable that it contained ∼50% carbonaceous chondrite equivalent of H-C-N during its differentiation. Therefore, the MFI was unlikely the primary source of volatiles in the BSE. A significant portion of the BSE’s volatile inventory (especially H and C) likely predates the Moon-forming event. To prevent loss to space and segregation into Earth’s core, volatile-bearing materials must be delivered during the final accretion event(s) preceding the Moon-forming event. The substantial size of the proto-Earth at this stage, combined with limited metal-silicate equilibration during the Moon-forming event, facilitated the retention of these volatiles within the BSE.