The Existing Frameworks of Delivery of Major Volatiles and the Feasibility of Mars-Mass Planetary Embryos as the Major Volatile Contributors to Bulk Silicate Earth.

The presence of major volatile elements-carbon, hydrogen, nitrogen, and sulfur-on Earth is critical for establishing life. The origin of these life-essential volatile elements (LEVEs) on Earth has been studied for many years. Here, we present a brief compilation of the prevailing ideas regarding volatile delivery to Earth and evaluate their origins, strengths, and weaknesses. Motivated by the fact that one model of LEVE delivery is via a giant impactor to Earth, we subsequently present a geochemical model aimed at understanding the possible volatile inventory and fractionation between the core, the silicate magma ocean (MO), and the atmosphere of a Mars-mass embryo. We looked at various end-member accretion scenarios of the embryo and their influence on the embryo's LEVE budget and the LEVE ratios. We varied various chemical (initial concentration of volatiles in the undifferentiated bodies and the oxygen fugacity [fO₂] of geochemical fractionation) and physical parameters (silicate-mass fraction of the accreting bodies, MO depth) to observe their effects on the absolute and relative LEVE budgets of the embryo. Our results show that an oxidizing condition (logf O₂ ≥ IW-1 [Iron-Wüstite]) is critical in establishing the relative LEVE budget of the embryo's MO, closer to that of present-day bulk silicate Earth. Furthermore, the accretion of larger bodies to form the Mars-mass embryo results in the closest match of the LEVE ratios to that of the present-day bulk silicate Earth (BSE). However, the absolute LEVE budget of the MO of Mars-mass embryo is depleted by at least 1-2 orders of magnitude compared with the BSE under all model calculation scenarios. In contrast, the CI-chondrite-normalized LEVE budget of the embryos's core, in many of the scenarios, especially from the reduced (e.g., IW-2) bodies, overlaps or exceeds the present-day BSE estimate. We argue that for a Mars-mass, differentiated embryo, the cores provide a better prospect for LEVE delivery to proto-Earth, through core breakups and subsequent mixing in the MO or solid mantle. Future studies need to better assess whether the fractional retention of core materials in the silicate reservoir can match the present-day BSE LEVE budgets and how such a process compares with the LEVE delivery via less-processed primitive asteroids.