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

Abstract

Comets are considered a potential source of inner solar system volatiles, but the timing of this delivery relative to that of Earth’s accretion is still poorly understood. Measurements of xenon isotopes in comet 67P/Churyumov-Gerasimenko revealed that comets partly contributed to the Earth’s atmosphere. However, there is no conclusive evidence of a significant cometary component in the Earth’s mantle. These geochemical constraints would favor a contribution of comets mainly occurring after the last stages of Earth’s formation. Here, we evaluate whether dynamical simulations satisfy these constraints in the context of an Early Instability model. We perform N-body simulations of the solar system’s dynamic evolution considering an outer disk of 10000 comets, different initial conditions for the inner solar system and assuming that the giant planet instability happened within ≃10 million years of the start of planet formation. Out of 30 simulations, 13 meet a list of criteria for success. We calculate the probability of collision between comets and Earth analogs component embryos through time and estimate the total cometary mass accreted in Earth analogs as a function of time. We determine that the cumulative cometary mass accreted by the Earth analogs during the first 100 million years ranges between 4 ×1021 and 5 ×1024 g. While our results are in excellent agreement with geochemical constraints, we also demonstrate that the contribution of comets on Earth might have been delayed with respect to the timing of the instability, due to a stochastic component of the bombardment. More importantly, we show that it is possible that enough cometary mass has been brought to Earth after it had finished forming so that the xenon constraint is not necessarily in conflict with an Early Instability scenario. Indeed, 2 of the 13 successful simulations provide enough comet delivery after a last giant impact happening later than 30 Myr. However, it appears very likely that a few comets were delivered to Earth early in its accretion history, thus contributing to the mantle’s budget. Finally, we compare the delivery of cometary material on Earth to Venus and Mars. These results emphasize the stochastic nature of the cometary bombardment in the inner solar system.