The Origin of the Moon: The Movie


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Abstract from Kokubo et al. (2000)

We investigate the evolution of a circumterrestrial disk of debris generated by a giant impact on the earth and the characteristics of the moon accreted from the disk by using high-resolution N-body simulation. We find that in most cases the disk evolution results in the formation of a single large moon on a nearly non-inclined circular orbit outside the Roche limit, which is consistent with the previous work by Ida et al. (1997). The efficiency of incorporation of disk material into a moon is 10--55%, which increases with the initial specific angular momentum of the disk. These results hardly depend on the initial condition of the disk as long as the mass of the disk is 2 to 4 times the present lunar mass and most mass of the disk exists inside the Roche limit. The time scale of the disk evolution is determined mainly by the surface density of the disk. The evolution of the disk is summarized as follows: The disk contracts through the mutual collisions of disk particles. Gravitational instability takes place and particle clumps grow inside the Roche limit. The clumps are elongated by the Kepler shear, which forms spiral arms (non-axisymmetric structure). Particles are transfered to the outside of the Roche limit due to the gravitational torque exerted by the spiral arms. When a spiral arm is extended beyond the Roche limit, the tip of the spiral arm collapses to form a small moonlet. The rapid accretion of these small moonlets forms a lunar seed. The seed exclusively grows by sweeping up particles transfered over the Roche limit. When the moon becomes large enough to gravitationally dominate the disk, it pushes the rest of the inner disk to the earth. The formation time scale of the moon is of the order of a month.


The Movie Files

When you play the movie, you may need the gamma correction of the display.

Frames from the Movie

t=0TK
The initial protolunar disk.
t=3TK
Spiral-arm structure forms as a result of the gravitational instability of the disk.
t=15TK
A lunar seed forms outside the Roche limit.
t=30TK
The seed exclusively grows by accreting material transfered beyond the Roche limit.
t=50TK
The lunar formation is almost completed.
t=300TK
The accreted moon.


References

Ida, S., R. M. Canup, and G. R. Stewart 1997.
Lunar Accretion from an impact-generated disk. Nature 389, 353-357.
Kokubo, E., J. Makino, and S. Ida 2000.
Evolution of a circumterrestrial disk and formation of a single moon. Icarus 148 , 419-436.
Kokubo, E. R. M. Canup, and S. Ida 1999.
Lunar accretion from an impact-generated disk. submitted to Origin of the Earth and Moon.
Miura, H., E. Kokubo, and K. Ichinose 1998.
The origin of the moon. In Proceedings of Computer Visualization Symposium '98, 49-52.


Copyright EK1999

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