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

Abstract

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. Published by Elsevier B.V.