Timing of Late Accretion

Project area A will study the late accretion history of the terrestrial planets. The focus is on the Moon, because its record of early bombardment has been used to calibrate the cratering record of other solar system objects. ...

The main goal is to test different models of the late accretion flux from the time of the giant impact to about 3.5 Ga and to construct a model of the impact history of the Moon during this time interval that is consistent with isotopic ages, the observed cratering record, geochemical data and celestial dynamics.

At a later stage, we may study the impact history of other objects in the inner solar system. The interdisciplinary approach will use new isotopic age data obtained on ancient lunar impactites, new crater count and size frequency data, updated inventories of lunar basins and their deposits and modelling of the formation of lunar basins, craters and their deposits.

The main hypothesis of project area A postulates that the overall decrease of the accretion flux in the inner solar system between 4.5 and 3.5 Ga shows multiple spikes, which reflect accretion of large objects in short time intervals. A modest increase of the accretion rate between 4.2 and 3.8 Ga may reflect a delivery of different populations of planetesimals.

Further important hypotheses and questions of Project area A are:

  1. Distributions of isotopic ages between 4.5 and 3.5 Ga in impactites from the Moon and other inner solar system objects reflect a combination of factors: sampling bias, chronometer response to impact events and accretion of large planetesimals. All A projects.
  2. How did the formation of lunar impact deposits bias the crater chronology and how did the cooling history of the Moon affect the impact record? All A projects (C4).
  3. Is there evidence for a weak increase of the accretion flux between 4.2 and 3.5 Ga and does it mark a change in impactor populations and their compositions? A3, A1, A2 (B1, B3).

We will use a multi-tiered approach to improve our understanding of the significance of the ages and their abundance distributions in lunar impactites. Geochronometers with high closure temperatures (Lu-Hf, Sm-Nd, U-Pb zircon) will be applied to ancient lunar impactites (A1) in order to obtain more data on such systems, which, as indicated by recent U-Pb work on zircons, tend to yield significantly older ages than the Ar-Ar-plagioclase chronometer. Different types of simple and complex lithologies of impactites will be studied to evaluate resetting by secondary heating and the role of early and late (< 3,5 Ga) impact events.

The goal is also to correlate specific HSE, SVE and lithophile element compositions of impact rocks with ages of samples and impact deposits. Crater counting will be performed using improved imaging data from recent lunar remote sensing missions (A2, A3) to obtain a stratigraphic age for the South Pole-Aitken basin and for impact deposits in the vicinity of the Apollo landing sites and some of the large basins (Nectaris, Serenitatis, Imbrium). Subprojects A2, A3 and A4 will use new quantitative criteria and statistical models to improve relative age estimates of ancient surface units on the lunar nearside.

The work proposed in A3 will improve the lunar production function of craters and will evaluate if it was different during specific time intervals, which would suggest different impactor populations. If the production function was not constant, this would influence relative ages of basins obtained by crater counting. The new data will be used as parameters in models of the cratering and late mass accretion history of the Moon (A4) and we expect feedback from results of thermal evolution models of the lunar magma ocean (C4). Ideally, models of the lunar impact history and modification of impact deposits will allow calculation of probabilities that certain lunar impact deposits may be related to specific basin forming events.

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