Speaker: Professor Miki Nakajima 

University of Rochester NY

 

Abstract: The Apollo lunar samples reveal that Earth and Moon have strikingly similar isotopic ratios, suggesting that these bodies may share the same source materials. This leads to the giant impact hypothesis, suggesting the Moon formed from a disk that was generated by an impact between Earth and a Mars-sized object. This disk would have had high temperature (~ 4000 K), and its silicate vapor mass fraction would have been ~20 wt %. However, impact simulations indicate that this model does not mix the two bodies well, making it challenging to explain the similarity assuming that the impactor was isotopically different from the proto-Earth. In contrast, more energetic impact models that would produce higher vapor mass fractions (~80-90 wt%) could mix the two bodies, naturally solving the problem. However, these recent energetic models also tend to mix the Earth's mantle, which appears to be inconsistent with geochemical observations that suggest Earth was never completely mixed. In contrast, the canonical impact is not energetic enough to mix the mantle, suggesting that it may remain a good model candidate for the lunar origin.

In addition to the mantle state, we propose here that the vapor mass fraction of the Moon-forming disk offers an additional constraint. As a partially vaporized Moon-forming disk cools, silicate droplets condense and form lunar seeds, which would eventually become the Moon. During the Moon’s accretion, lunar seeds can fall onto Earth due to a strong headwind originating from the silicate vapor, making it challenging to form a large Moon. This would be especially the case for vapor-rich disks that would be produced by the recent models. However, this issue could be avoided if lunar seeds could grow rapidly enough that they avoid this inspiraling process. Quick growth can be achieved by streaming instability, which is a clump formation process caused by a spontaneous concentration and gravitational collapse of the particles. Here, we investigate whether the streaming instability can operate in the Moon-forming disk using the numerical code Athena.  

Moreover, we will also discuss our new scaling law for the geometry of an impact-induced magma ocean based on smoothed particle hydrodynamics simulations. We find that the equilibrium pressures computed in our model could be much larger than the conventional models, that would affect the metal-silicate equilibration process and resulting chemistry of the planets.

 

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