Physics Colloquium: Origin of moons in the solar system and beyond

Physics Colloquium
Origin of moons in the solar system and beyond

Dr. Miki Nakajima
Assistant Professor of Earth and Environmental Sciences and Physics and Astronomy
University of Rochester

Abstract:
The Apollo lunar samples reveal that Earth and the Moon have strikingly similar isotopic ratios, suggesting that these bodies may share the same source materials. This leads to the "standard" 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. In contrast, recent studies suggest that more energetic impact models that produce higher vapor mass fractions (~80-90 wt%) could mix the two bodies, naturally solving the problem. However, these energetic models may have a challenge during the Moon accretion phase. Our analyses suggest that km-sized moonlets experience a strong gas drag from the vapor portion of the disk and fall onto Earth in a very short timescale. This problem could be avoided if large moonlets (>1000 km) form very quickly by the process called streaming instability. We investigate this possibility by conducting numerical simulations with the code called Athena. We will discuss implications of this study for moons in the solar system and extrasolar systems (exomoons). We will also briefly mention our ongoing work on terrestrial craters (Vredefort and Sudbury impact basins).

Speaker Bio:
Miki Nakajima is an assistant professor in the Department of Earth and Environmental Sciences and in the Department of Physics and Astronomy at the University of Rochester. Her research interest includes origin of Earth and the Moon, the Martian moons, the early Earth and lunar environments, and the impact history in the solar system. In particular, she investigates various impact processes and considers their geological, geophysical, and geochemical implications based on theoretical and numerical modeling. Prior to moving to Rochester in 2018, Nakajima was a postdoctoral fellow at the Carnegie Institution for Science. She received her Ph.D. in planetary science at the California Institute of Technology. She is currently a member of the Planetary Science and Astrobiology Decadal Survey.

Intended Audience:
Beginners, undergraduates, graduates, experts. Those with interest in the topic.

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