The Earth and the Earth-like planets in the inner solar system were formed by collisions of protoplanets. The last giant collision of Earth with a Mars-sized body resulted in the formation of the Moon. How the Earth developed from a fireball covered by a magma ocean thousands of kilometers thick, to a habitable planet with liquid water on the surface and an atmosphere and biosphere remains enigmatic. It is assumed that the period immediately after the formation of the Moon 4.5 to about 3.8 billion years ago, when the last large basins on the Moon were formed by large impacts of cosmic bodies, is of particular significance. Cooling, crystallization, and segregation of matter in the Earth’s interior and the intensive bombardment by cosmic bodies (impacts), which provided additional matter, make this “late growth phase” especially important for the further evolution of Earth. Since almost no remnants in terms of a geological record from this period have survived on Earth, the Moon is all the more significant. It serves as a kind of archive of the frequent impacts that must have taken place on Earth in the same period.

The group of researchers has published a quantitative model in the journal Nature that can explain some important data and observations of lunar research. The results provide an explanation as to why the Moon, after its formation, accumulated a much smaller mass of cosmic material in its mantle and crust than the Earth over the same period. By simulating asteroid impacts, the authors have determined that in the case of the Moon with its relatively small mass, most of the material of impacting bodies does not remain in the Moon, while in the case of the more massive Earth, most of the impact material is added to the Earth. Furthermore, the model explains the mass of the material that is incorporated into the Moon’s mantle and crust by impacts after the formation of the Moon. Based on these observations, it follows that the assumed exponential decrease of the bombardment rate of the Moon after its formation is plausible. If the Moon was formed 4.5 billion years ago, the results indicate that it would have recorded about 200 impacts leading to large basins with a diameter of more than 300 kilometers. In fact, there are only about 40 to 50 such impact basins. The authors explain this by the fact that a soft and thin crust did not allow for preserving craters or basin structures during the existence of the magma ocean and that the impacting bodies dipped directly into the Moon’s mantle.

Contributing to the international research team were scientists from Freie Universität Berlin and the Museum für Naturkunde Berlin, along with others from Macau University of Science and Technology (China), the Planetary Science Institute (Tucson, USA), the University of Nice (France), and the University of California, Davis. Meng-Hua Zhu from Macau University of Science and Technology is first author.

The research work is part of the Collaborative Research Center TRR 170 (Berlin-Münster) funded by the German Research Foundation (DFG), which investigates the late growth phase of the Earth-like planets. The new results also align with other new studies from the Collaborative Research Center that rely on the idea that the magma ocean had to cool for at least 100 million years. The oldest rocks of the Moon’s crust were dated at just under 4.4 billion years, suggesting the Moon is likely to be about 4.5 billion years old.