Stanford University
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Miki Nakajima, University of Rochester, Origin of the Earth and Moon

When:
Thursday, Nov 14, 2019 12:00 PM
Where:
Mitchell 350/372
Audience:
Faculty/Staff, Students, Alumni/Friends
Sponsor:
Geophysics Department

Miki Nakajima

School of Arts and Sciences, University of Rochester

Origin of the Earth and Moon

Host: Sonia Tikoo

The Apollo lunar samples reveal that the 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, our giant impact simulations using the smoothed particle hydrodynamics (SPH) method show that these recent energetic models tend to mix the Earth's mantle, which does not appear to be consistent with geochemical observations that suggest that Earth was never completely mixed. Additionally, these recent models may face an additional challenge. Our hydrodynamic simulations using the code Athena suggest that a vapor-rich Moon-forming disk may not be able to form a large Moon because growing lunar seeds can fall onto Earth in a very short timescale due to a strong headwind from the vapor.  This effect is negligible for the standard model because the vapor-mass fraction of the disk is small, making a weak headwind. Thus, our study indicates that the standard giant hypothesis remains a strong candidate for the lunar origin.

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