Modeling the Historical Flux of Planetary Impactors

Abstract

The impact cratering record of the Moon and the terrestrial planets provides important clues about the formation and evolution of the Solar System. Especially intriguing is the epoch ≃3.8-3.9 Gyr ago (Ga), known as the Late Heavy Bombardment (LHB), when the youngest lunar basins such as Imbrium and Orientale formed. The LHB was suggested to originate from a slowly declining impactor flux or from a late dynamical instability. Here we develop a model for the historical flux of large asteroid impacts and discuss how it depends on various parameters, including the time and nature of the planetary migration/instability. We find that the asteroid impact flux dropped by 1 to 2 orders of magnitude during the first 1 Gyr and remained relatively unchanged over the last 3 Gyr. The early impacts were produced by asteroids whose orbits became excited during the planetary migration/instability, and by those originating from the inner extension of the main belt (E-belt; semimajor axis 1.6 < a < 2.1 au). The inner main belt dominated the asteroid impact record after ∼3-4 Ga. The profiles obtained for the early and late versions of the planetary instability initially differ, but end up being similar after ∼3 Ga. Thus, the time of the instability can only be determined by considering the cratering and other constraints during the first ≃1.5 Gyr of the Solar System history. Our absolute calibration of the impact flux indicates that asteroids were probably not responsible for the LHB, independently of whether the instability happened early or late, because the calibrated flux is not large enough to explain Imbrium/Orientale and a significant share of large lunar craters. Comets and leftovers of the terrestrial planet formation provided additional, and probably dominant source of impacts during early epochs. The cometary impacts occur during a narrow interval after the instability and would not be recorded on surfaces if the planetary instability happened early.-2 -

Dynamics of Planetary Systems within Star Clusters: Aspects of the Solar System’s Early Evolution

“…In order to evaluate this expression, we have to specify the architecture of the giant planets. In this regard, it is crucial to note that the orbits of the giant planets almost certainly experienced significant divergent migration early in the solar system's lifetime, owing to a transient dynamical instability that ensued due to their interactions with a ∼ 20 M ⊕ primordial disk of planetesimals extending from ∼ 15 AU to Neptune's present-day orbit (Tsiganis et al 2005;Nesvorný & Morbidelli 2012). This means that during the epoch relevant to cluster-induced dynamics, the orbital configuration of the giant planets was likely more tightly packed than today's solar system.…”

“…This is because the outward ejection of ∼ 20 M ⊕ of planetesimals that occurred during the instability was the last major expulsion of icy material into the trans-Neptunian region, and had this event occurred while the cluster was still present, the Oort cloud would have been rendered unbound by passing stars. Consequently, for the calculation at hand, we adopt a compact multi-resonant configuration for the giant planets where Jupiter and Saturn, as well as Uranus and Neptune are locked into 3:2 mean motion resonances while Saturn and Uranus are entrained into a 4:3 resonance, which has been previously shown to adequately serve as an initial condition for the Nice model instability (although we also note that the specific choice of resonance indexes does not affect our results on a qualitative level; Batygin & Brown 2010;Nesvorný & Morbidelli 2012). The planetesimal disk is modeled as a series of 20 concentric rings, equally spaced between 15 and 35 AU, each containing 1 M ⊕ of material.…”

“…In particular, assembly of planets is expected to unfold within a geometrically thin disk of gas and dust that dissipates over the course of the first 1 − 10 Myr of the host star's lifetime (Armitage 2011). In contrast, the subsequent dynamical evolution can transpire over much longer timescales, spanning hundreds of Myr (Tsiganis et al 2005;Nesvorný & Morbidelli 2012), or even several Gyr (Davies et al 2014;Laskar & Gastineau 2009;Batygin et al 2015). Moreover, while the process of planet assembly is primarily controlled by local physics taking place within protoplanetary disks (Lambrechts et al 2014), dynamical evolution that ensues after a newborn planetary system emerges from its natal nebula arXiv:2002.05656v1 [astro-ph.EP] 13 Feb 2020 can be strongly influenced by its external environment (see Hernández et al 2007;Malmberg et al 2007 and references therein).…”

Linking the formation and fate of exo-Kuiper belts within Solar system analogues

“…On the other hand, a prior inwards migration of the giant planets can explain Jupiter's composition and the asymmetry of its Trojan populations (Öberg & Wordsworth 2019; Pirani et al 2019). Additionally, the giant planets likely underwent at least one gravitational instability which has re-ordered the planets at some, still-uncertain, previous epoch (Thommes et al 2002;Tsiganis et al 2005;Morbidelli et al 2007;Nesvorný & Morbidelli 2012;Morbidelli et al 2018). These instabilities have also shaped the Kuiper Belt and scattered disc.…”

Delivery of Water and Volatiles to the Terrestrial Planets and the Moon

“…For comets from the trans-Neptunian belt (with initial distances in a range of 20-30 AU), Nesvorný et al (2017) found that the probabilities of collisions with Venus, the Earth, Mars, and the Moon are 3.7 × 10 -7 , 5.0 × 10 -7 , 9.1 × 10 -8 , and 2.6 × 10 -8 , respectively. For asteroids with initially large semimajor axes, from 1.6 to 3.3 AU, these probabilities were (1.2-1.5) × 10 -2 , (1.0-1.1) × 10 -2 , (3.6-3.9) × 10 -3 , (4.4-5.3) × 10 -4 , respectively.…”