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From collisions to impacts – the late growth history of Terrestrial Planets

collision event between two celestial bodies

The formation history of our solar system, in particular of the terrestrial or Earth-like planets (Mercury, Venus, Mars, and Earth) is shaped by collisions of almost planet-sized objects, so-called planetary embryos, and the bombardment by smaller objects, so-called planetesimals or asteroids. The last major collision event in the early history is considered to be the encounter of a Mars-sized planetary embryo with proto-Earth about 4.5 billion years ago, which presumably resulted in the formation of the Moon. The heavily cratered landscapes on the Moon testify to the violent subsequent bombardment history of the planets with decreasing frequency of impacts with time. The intensity of the bombardment is thought to have remained approximately constant since 3.8 billion years ago.  For the time span in between (4.5 – 3.8 billion years), which is often called the “Late Accretion Phase”, when the last but crucial bit of materials were added by impacts to the final mass of the planets, relatively little is known. This is in particular true for Earth as hardly any remnants in terms of rock samples are preserved from this crucial evolution period.  To gain deeper insights into the late growth history and composition of Earth, Mars, Venus, Mercury, and the Moon scientists from Museum für Naturkunde Berlin are collaborating with other experts from universities and research institutions in Berlin and Münster. The transregional collaborative research centre, TRR170, Late Accretion into Terrestrial Planets, is funded by the German Science Foundation (DFG) since January 2016.

The multi-disciplinary research initiative is based on sample materials from the Moon and meteorites, remote sensing data from space missions, and numerical modelling of geodynamic, atmospheric and impact processes. The latter is addressed by MfN scientist. The overarching goals of the project are:

  • Test different assumptions on how the frequency of impacts changed with time
  • Determine the composition of late-accreted material and compare it to other planetary materials
  • Evaluate the consequences of the physical and chemical evolution that occurred in the early Earth and Moon, starting from the giant impact and the formation of the Moon until about 3.8 billion years ago
  • Compare the physical and chemical evolution of the Earth and the Moon with the record of late accretion on Mars, Mercury and asteroids in the main asteroid belt

The project is subdivided into three research areas with a strong involvement of MfN researchers in area A and C:

  • A - Timing
    Timing, in particular the dating of cratering events, is essential for our understanding of the various processes that contributed to the formation of the Earth and other terrestrial planets. By using theoretical and experimental methods, such as counting craters on the surface of the Moon or Mars, the researchers aim at revealing the chronology of the formation of the crust, the outer shell of planetary bodies. Researchers at MfN focus on developing a model to reconstruct the entire bombardment history of the Moon and its implications.
  • B - Chemical Budget
    Another objective is to determine the chemical composition and origin of the materials accreted late by impacts. These studies will provide an important groundwork for answering central questions for example about the origin of water. The research also focusses on understanding how the accreted material has changed over time and how the composition of the planets has been affected by impacts.
  • C - Geodynamical Implications
    In addition, the geodynamic implications of late accretion are addressed by numerical modelling of the impact processes, the impact-induced formation, cooling, and crystallisation of magma oceans, and the interaction with proto-atmospheres. MfN researchers focus in particular on the fate of material delivered by impacts and to what extent giant impacts, such as the Moon-forming event, caused wide-spread melting of the crust and upper mantle on Earth resulting in a global or least partial magma ocean.  

The results of the collaborative research project will contribute considerably to a better understanding of the early evolution of the terrestrial planets paving the way for habitable conditions on Earth.