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 three sources: planetesimals left over from primary accretion, asteroids from the hypothetical E-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 . 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; 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.

Supply of phosphate to early Earth by photogeochemistry after meteoritic weathering

“…Previous calculations of the meteoritic delivery of phosphorus to Earth during late accretion gave similar values to those we report here. 45 , 46 Until very recently, 38 , 47 however, there existed no quantitative estimate for late accretion that used cratering data and highly siderophile element abundances combined with dynamics to show enstatite chondrites were a principle source of late accretion to Hadean Earth. Clearly it cannot be expected that the deposition of phosphorus resulting from late accretion occurred evenly over the surface of Earth, hence we provide the flux in kg/year.…”

Vertical angular momentum constraint on lunar formation and orbital history

“…Apart from the dynamical evolution, is also susceptible to small changes due to collisional processes after the Moon-forming giant impact, e.g., the stochastic late-accretional impacts on Earth that are proposed to explain the presence of highly siderophile elements in the terrestrial mantle ( 22 – 26 ). These impacts can change the Earth’s rotational AM by up to 4%, with the Earth rotating with a period between 6 and 8 h ( 22 ).…”

“…A reexamination of those results shows that, even with all these additional effects, Lz was still conserved to about 3.5% ( e is the present-day value, Me is Earth's mass; L p s = 0.345Lr and L p z = 0.339Lr ). Apart from the dynamical evolution, Lz is also susceptible to small changes due to collisional processes after the Moonforming giant impact, e.g., the stochastic late-accretional impacts on Earth that are proposed to explain the presence of highly siderophile elements in the terrestrial mantle (22)(23)(24)(25)(26). These impacts can change the Earth's rotational AM by up to 4%, with the Earth rotating with a period between 6 and 8 h (22).…”

Lightning strikes as a major facilitator of prebiotic phosphorus reduction on early Earth

“…Schreibersite is a common accessory mineral within some classes of meteorites 7 and is also found in some highly reduced glasses formed by lightning strikes called fulgurites 14 – 16 . The Earth likely experienced a monotonic decline in impactors from the moon forming impact at ~4.5 Ga to present 17 , 18 , providing potentially 10 5–7 kg of reduced phosphorus annually throughout the Hadean and early Archean 11 . Consequently, it has generally been assumed that other schreibersite sources are trivial 13 .…”