Impact bombardment chronology of the terrestrial planets from 4.5 Ga to 3.5 Ga

Abstract

Subsequent to the Moon's formation, late accretion to the terrestrial planets strongly modified the physical and chemical nature of their silicate crusts and mantles. Here, we combine dynamical N-body and Monte Carlo simulations to determine impact probabilities, impact velocities, and expected mass augmentation onto the terrestrial planets from four sources: planetesimals left over from primary accretion, asteroids from the hypothetical E-belt, the main asteroid belt, and comets arriving from the outer Solar System. We present new estimates of the amount of cometary material striking the terrestrial planets in an early (ca. 4480 Ma) episode of planetesimal-driven giant planet migration (Mojzsis et al., 2019). We conclude that the Moon and Mars suffer proportionally higher cometary accretion than Venus and Earth. We further conclude that the background mass addition from small leftover planetesimals to Earth and Mars is far less than independent estimates based on their respective mantle abundances of highly-siderophile elements and terrestrial tungsten isotopes. This supports the theory that both planets were struck by single large bodies that delivered most of their terminal mass augmentation since primary accretion, rather than a throng of smaller impactors. Our calculated lunar, martian and mercurian chronologies use the impacts recorded onto the planets from dynamical simulations rather than relying on the decline of the population as a whole. We present fits to the impact chronologies valid from 4500 Ma to ca. 3700 Ma by which time the low number of planetesimals remaining in the dynamical simulations causes the impact rate to drop artificially. The lunar timeline obtained from these dynamical simulations using nominal values for the masses of each contributing reservoir is at odds with both the calibrated Neukum (Neukum et al., 2001) and Werner (Werner et al., 2014; Werner, 2019) chronologies. For Mars, the match with its calibrated Werner chronology is no better; by increasing the mass of the E-belt by a factor of four the dynamical lunar and martian chronologies are in line with that of Werner (2019) and match constraints from the current population of Hungaria asteroids. Yet, neither of our dynamical timelines fit well with that of Neukum. The dynamical lunar and martian chronologies are also different from each other. Consequently, the usual extrapolation of such chronologies from one planetary body to the other is technically inappropriate.