Origin and abundances of H₂O in the terrestrial planets, Moon, and asteroids

The presence of H₂O within differentiated terrestrial bodies in the inner Solar System is well established; however, the source(s) of this H₂O and the time of its arrival to the inner Solar System is an area of active study. At present, the prevailing model for the origin of inner Solar System H₂O calls upon carbonaceous chondrites as the source. This is largely based on reported observations that H- and N-isotopic compositions of differentiated planetary bodies are largely the same and within a range of values that overlaps with carbonaceous chondrites as opposed to comets or the Sun. In this contribution, we evaluate the efficacy of this model and other models for the origin of inner Solar System H₂O by considering geochronological constraints on early Solar System history, constraints on primary building blocks of differentiated bodies based on nucleosynthetic isotope anomalies, and constraints from dynamical models of planet formation. In addition to H- and N-isotopic data, these constraints indicate that an interstellar source of H₂O was present in the inner Solar System within the first 4 Ma of CAI formation. Furthermore, the most H₂O-rich carbonaceous chondrites are unlikely to be the source of H₂O for the earliest-formed differentiated bodies based on their minimally overlapping primary accretion windows and the separation of their respective isotopic reservoirs by Jupiter in the timespan of about 1–4 Ma after CAI formation. The presence of deuterium-rich, non-nebular H₂O sources in the inner Solar System prior to the formation of carbonaceous chondrites or comets implies early contributions of interstellar ices to both the inner and outer Solar System portions of the protoplanetary disk. Evidence for this interstellar ice component in the inner Solar System may be preserved in LL chondrites and in the mantle of Mars. In contrast to the earlier-formed bodies within the inner Solar System, Earth's protracted accretion window may have facilitated incorporation of H₂O in its interior from both the inner and outer Solar System, helping the Earth to become a habitable planet.