Origin of the Earth and Moon

The Moon likely formed by a collision between Earth and an impactor.

This impact formed a hot and partially vaporized disk around Earth from which our Moon accreted. The details of the impact, such as the impactor size and velocity, have been actively debated. Here, we suggest that the impact can be constrained by the state of the Earth’s mantle; short-lived isotopes of the Earth’s mantle suggest that the Earth’s mantle has never been mixed even by the Moon forming impact. Another potential clue arises from the Earth’s core. According to recent geophysical and dynamical studies, the Earth’s core may have developed a compositionally stratified structure during its formation, which would have suppressed an early dynamo of the Earth. However, this appears to be inconsistent with paleomagnetic measurements that indicate the presence of an early dynamo. One of the potential solutions to these problems is that the Moon-forming impact mixed the core and reset the stratification without completely mixing the mantle. To understand the initial states of the Earth’s mantle and core, we simulate impact processes using a scheme called smoothed particle hydrodynamics (SPH).

Our simulations show that the mantle heterogeneity could have survived the giant impact while the core could have dynamically mixed if the impactor was about the size of Mars and the impact velocity was similar to the escape velocity. More energetic impact models are not favorable because they would have mixed the mantle. Moreover, we will discuss how much volatiles would have been lost from the Moon-forming disk by hydrodynamic escape.

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