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Comets are considered a potential source of inner solar system volatiles, butthe timing of this delivery relative to that of Earth's accretion is stillpoorly understood. Measurements of xenon isotopes in comet67P/Churyumov-Gerasimenko revealed that comets partly contributed to theEarth's atmosphere. However, there is no conclusive evidence of a significantcometary component in the Earth's mantle. These geochemical constraints wouldfavour a contribution of comets mainly occurring after the last stages ofEarth's formation. Here, we evaluate whether dynamical simulations satisfythese constraints in the context of an Early Instability model. We performdynamical simulations of the solar system, calculate the probability ofcollision between comets and Earth analogs component embryos through time andestimate the total cometary mass accreted in Earth analogs as a function oftime. While our results are in excellent agreement with geochemicalconstraints, we also demonstrate that the contribution of comets on Earth mighthave been delayed with respect to the timing of the instability, due to astochastic component of the bombardment. More importantly, we show that it ispossible that enough cometary mass has been brought to Earth after it hadfinished forming so that the xenon constraint is not necessarily in conflictwith an Early Instability scenario. However, it appears very likely that a fewcomets were delivered to Earth early in its accretion history, thuscontributing to the mantle's budget. Finally, we compare the delivery ofcometary material on Earth to Venus and Mars. These results emphasize thestochastic nature of the cometary bombardment in the inner solar system.

A race against the clock: Constraining the timing of cometary bombardment relative to Earth's growth