Prof. Dr. Kai Wünnemann

Leibniz Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, D-10115 Berlin, Tel.: +49 30-20938857

A4 Summary

Large impact basins are the most prominent landforms on the Moon. Basin-forming impactors delivered the vast majority of matter during the late-accretion phase of the early Earth-Moon system and, thus, may have significantly contributed to the volatile inventory of the terrestrial planets and the Moon. The lunar basins are the oldest and only remaining physical traces of this heavy bombardment. Additionally, they represent important time markers of the early history of the Earth-Moon system. We wish to (1) obtain a full inventory of lunar impact basins; (2) study their formation and subsequent modifications; (3) study the distribution of ejected material and impact melt; (4) study the mixing of ejecta in the megaregolith by impact gardening and estimate its fractional persistence at the lunar surface; and, finally, (5) estimate the energy (and mass) of the impactors as well as understand the character of the lunar environment at the time the basins formed. The project will also significantly contribute to the petrological and geochemical evaluation and radiometric dating of critical lunar samples by providing constraints on the correlation of Apollo specimens to large basin-forming impact events and their ejecting and emplacement history.

C2 Summary

Giant impacts on Earth heavily influenced core formation and may have contributed to late accretion of material, but exactly to what extent the present day geochemical signature of Earth’s mantle reflects the processes of core formation and late accretion and how much of the delivered material was incorporated into the core remains unclear. To better understand these processes it is key to comprehend how the material delivered by giant impacts was dispersed upon impact, and how the metal (or sulphide) settled in a global or partial magma ocean.

The first issue¾knowing the distribution of the impactor material¾is important because the way in which the impact-delivered metal interacts with the ambient magma strongly depends on the size–frequency distribution of particles or clumps the impactor is dispersed into. The second issue¾the settling of impactor material¾ also strongly depends on the thermodynamic convection state of the magma ocean. Thus, understanding how the magma ocean interacted with the sinking metal particles is crucial for understanding core formation. For example, we must understand how both of these factors ¾ particle size and convection state of the ambient magma¾affected the material’s sinking speed: while metal particles that sink quickly leave little time for chemical equilibration, those that sink sufficiently slowly could allow time for equilibration between metal and silicate materials, potentially shaping the chemical signatures we see in Earth’s mantle today.

Therefore, we propose to investigate, by means of numerical experiments, how impactors and, in case of differentiated bodies, their cores were dispersed during the penetration into a magma ocean, how the size–frequency distribution of droplets, chunks, or large clumps depends on impact parameters (impact velocity, relative size to the depth of the magma ocean, rheology of the impactor and target) and how the flow style of the magma ocean affects the interaction of metal and silicates. By combining impact modelling with sophisticated models of the magma ocean that account for solidification processes, we expect to gain a better understanding of the changing reaction of the magma ocean to impacts and the changing nature of the sedimentation during its evolution.

C4 Summary

The Moon-forming impact is thought to be the last giant collision event that Earth experienced, marking the end of Earth’s accretion process and setting the initial conditions for the subsequent thermochemical evolution of the Earth and the Moon. For both bodies, an early magma ocean is thought to have existed, as a consequence of giant impacts leading to widespread melting. As the geological record of the early evolution history is lacking on Earth only the Moon probably shows relicts of the crystallization of a global or partial magma ocean. The goal of this subproject is to understand how these magma oceans formed and crystallized and how they influenced the thermochemical and tectonic evolution of Earth and Moon. To this end, we aim to constrain the range of plausible thermal conditions after a giant collision that may have resulted in the formation of a global or partial magma ocean on Earth. We will employ well-established numerical models, and we will further develop and adjust them to allow us to simulate giant collisions as big as the Moon-forming event. Using the thermal setting from impact modelling as our input for subsequent detailed fluid and thermodynamical modelling, we intend to quantify dynamics, crystallization, and degassing of a magma ocean in order to constrain the resulting density and temperature profiles and volatile distribution. We will use these models to derive parameterizations, with the help of which we will study the thermochemical evolution of the Moon after its formation, including the phase in which the lunar magma ocean evolved. With these models for both the Earth and the Moon, we will define the thermochemical condition at the time of the late veneer on both bodies. In the end, this subproject aims at providing a model that is consistent with the geochemical signature of the Earth and the Moon.